C Programming Language- Step by step learning.

c Programming Question IV

2009-09-04T03:27:00.001-07:00

Q1: Tell how to check whether a linked list is circular.

A: Create two pointers, each set to the start of the list. Update each as follows:

while (pointer1) {

pointer1 = pointer1->next;

pointer2 = pointer2->next; if (pointer2) pointer2=pointer2->next;

if (pointer1 == pointer2) {

print (\"circular\n\");

}

}

Q2: OK, why does this work?

If a list is circular, at some point pointer2 will wrap around and be either at the item just before pointer1, or the item before that. Either way, it’s either 1 or 2 jumps until they meet.

How can you quickly find the number of elements stored in a a) static array b) dynamic array ?

Why is it difficult to store linked list in an array?

How can you find the nodes with repetetive data in a linked list?

Write a prog to accept a given string in any order and flash error if any of the character is different. For example : If abc is the input then abc, bca, cba, cab bac are acceptable but aac or bcd are unacceptable.

This is a C question that I had for an intern position at Microsoft: Write out a function that prints out all the permutations of a string. For example, abc would give you abc, acb, bac, bca, cab, cba. You can assume that all the characters will be unique. After I wrote out my function, he asked me to figure out from the code how many times the printf statement is run, and also questions on optimizing my algorithm.

What’s the output of the following program? Why?

#include

main()

{

typedef union

{

int a;

char b[10];

float c;

}

Union;

Union x,y = {100};

x.a = 50;

strcpy(x.b,\"hello\");

x.c = 21.50;

printf(\"Union x : %d %s %f \n\",x.a,x.b,x.c );

printf(\"Union y :%d %s%f \n\",y.a,y.b,y.c);

}

Given inputs X, Y, Z and operations | and & (meaning bitwise OR and AND, respectively)

What is output equal to in

output = (X & Y) | (X & Z) | (Y & Z)

c Programming Questions III

2009-09-04T03:26:00.001-07:00

1. What will print out?

main()
{
char *p1=“name”;
char *p2;
p2=(char*)malloc(20);
memset (p2, 0, 20);
while(*p2++ = *p1++);
printf(“%s\n”,p2);

}

Answer:empty string.

2. What will be printed as the result of the operation below:

main()
{
int x=20,y=35;
x=y++ + x++;
y= ++y + ++x;
printf(“%d%d\n”,x,y);

}

Answer : 5794

3. What will be printed as the result of the operation below:

main()
{
int x=5;
printf(“%d,%d,%d\n”,x,x< <2,x>>2);

}

Answer: 5,20,1

4. What will be printed as the result of the operation below:

#define swap(a,b) a=a+b;b=a-b;a=a-b;

void main()
{
int x=5, y=10;
swap (x,y);
printf(“%d %d\n”,x,y);
swap2(x,y);
printf(“%d %d\n”,x,y);
}

int swap2(int a, int b)
{
int temp;
temp=a;
b=a;
a=temp;
return 0;

}

Answer: 10, 5
10, 5

5. What will be printed as the result of the operation below:

main()
{
char *ptr = ” Cisco Systems”;
*ptr++; printf(“%s\n”,ptr);
ptr++;
printf(“%s\n”,ptr);

}

Answer:Cisco Systems
isco systems

6. What will be printed as the result of the operation below:

main()
{
char s1[]=“Cisco”;
char s2[]= “systems”;
printf(“%s”,s1);
}

Answer: Cisco

7. What will be printed as the result of the operation below:

main()
{
char *p1;
char *p2;

p1=(char *)malloc(25);
p2=(char *)malloc(25);

strcpy(p1,”Cisco”);
strcpy(p2,“systems”);
strcat(p1,p2);

printf(“%s”,p1);

}

Answer: Ciscosystems

8. The following variable is available in file1.c, who can access it?:

9.static int average;

Answer: all the functions in the file1.c can access the variable.

10. WHat will be the result of the following code?

#define TRUE 0 // some code

while(TRUE)
{

// some code

}

Answer: This will not go into the loop as TRUE is defined as 0.

11. What will be printed as the result of the operation below:

int x;
int modifyvalue()
{
return(x+=10);
}

int changevalue(int x)
{
return(x+=1);
}

void main()
{
int x=10;
x++;
changevalue(x);
x++;
modifyvalue();
printf("First output:%d\n",x);

x++;
changevalue(x);
printf("Second output:%d\n",x);
modifyvalue();
printf("Third output:%d\n",x);

}

Answer: 12 , 13 , 13

12. What will be printed as the result of the operation below:

main()
{
int x=10, y=15;
x = x++;
y = ++y;
printf(“%d %d\n”,x,y);

}

Answer: 11, 16

13. What will be printed as the result of the operation below:

main()
{
int a=0;
if(a==0)
printf(“Cisco Systems\n”);
printf(“Cisco Systems\n”);

}

Answer: Two lines with “Cisco Systems” will be printed.

c Programming Questions II

2009-09-04T03:25:00.000-07:00

1. What does static variable mean?
2. What is a pointer?
3. What is a structure?
4. What are the differences between structures and arrays?
5. In header files whether functions are declared or defined?
6. What are the differences between malloc() and calloc()?
7. Difference between pass by reference and pass by value?
8. What is stat
9. What are macros? What are the advantages and disadvantages?
10. ic identifier?
11. Where are the auto variables stored?
12. Where does global, static, local, register variables, free memory and C Program instructions get stored?
13. Difference between arrays and linked list?
14. What are enumerations?
15. Describe about storage allocation and scope of global, extern, static, local and register variables?
16. What are register variables? What are the advantage of using register variables?
17. What is the use of typedef?
18. Can we specify variable field width in a scanf() format string? If possible how?
19. Out of fgets() and gets() which function is safe to use and why?
20. Difference between strdup and strcpy?

# What is recursion?
# Differentiate between a for loop and a while loop? What are it uses?
# What are the different storage classes in C?
# Write down the equivalent pointer expression for referring the same element a[i][j][k][l]?
# What is difference between Structure and Unions?
# What the advantages of using Unions?
# What are the advantages of using pointers in a program?
# What is the difference between Strings and Arrays?
# In a header file whether functions are declared or defined?
# What is a far pointer? where we use it?
# How will you declare an array of three function pointers where each function receives two ints and returns a float?
# What is a NULL Pointer? Whether it is same as an uninitialized pointer?
# What is a NULL Macro? What is the difference between a NULL Pointer and a NULL Macro?
# What does the error ‘Null Pointer Assignment’ mean and what causes this error?
# What is near, far and huge pointers? How many bytes are occupied by them?
# How would you obtain segment and offset addresses from a far address of a memory location?
# Are the expressions arr and *arr same for an array of integers?
# Does mentioning the array name gives the base address in all the contexts?
# Explain one method to process an entire string as one unit?
# What is the similarity between a Structure, Union and enumeration?
# Can a Structure contain a Pointer to itself?
# How can we check whether the contents of two structure variables are same or not?
# How are Structure passing and returning implemented by the complier?
# How can we read/write Structures from/to data files?
# What is the difference between an enumeration and a set of pre-processor # defines?
# What do the ‘c’ and ‘v’ in argc and argv stand for?
# Are the variables argc and argv are local to main?
# What is the maximum combined length of command line arguments including the space between adjacent arguments?
# If we want that any wildcard characters in the command line arguments should be appropriately expanded, are we required to make any special provision? If yes, which?
# Does there exist any way to make the command line arguments available to other functions without passing them as arguments to the function?
# What are bit fields? What is the use of bit fields in a Structure declaration?
# To which numbering system can the binary number 1101100100111100 be easily converted to?
# Which bit wise operator is suitable for checking whether a particular bit is on or off?
# Which bit wise operator is suitable for turning off a particular bit in a number?
# Which bit wise operator is suitable for putting on a particular bit in a number?
# Which bit wise operator is suitable for checking whether a particular bit is on or off?
# Which one is equivalent to multiplying by 2?

* Left shifting a number by 1
* Left shifting an unsigned int or char by 1?

# Write a program to compare two strings without using the strcmp() function.
# Write a program to concatenate two strings.
# Write a program to interchange 2 variables without using the third one.
# Write programs for String Reversal. The same for Palindrome check.
# Write a program to find the Factorial of a number.
# Write a program to generate the Fibonacci Series?
# Write a program which employs Recursion?
# Write a program which uses command line arguments.
# Write a program which uses functions like strcmp(), strcpy(), etc.
# What are the advantages of using typedef in a program?
# How would you dynamically allocate a one-dimensional and two-dimensional array of integers?
# How can you increase the size of a dynamically allocated array?
# How can you increase the size of a statically allocated array?
# When reallocating memory if any other pointers point into the same piece of memory do you have to readjust these other pointers or do they get readjusted automatically?
# Which function should be used to free the memory allocated by calloc()?
# How much maximum can you allocate in a single call to malloc()?
# Can you dynamically allocate arrays in expanded memory?
# What is object file? How can you access object file?
# Which header file should you include if you are to develop a function which can accept variable number of arguments?
# Can you write a function similar to printf()?
# How can a called function determine the number of arguments that have been passed to it?
# Can there be at least some solution to determine the number of arguments passed to a variable argument list function?
# How do you declare the following:

* An array of three pointers to chars
* An array of three char pointers
* A pointer to array of three chars
* A pointer to function which receives an int pointer and returns a float pointer
* A pointer to a function which receives nothing and returns nothing

# What do the functions atoi(), itoa() and gcvt() do?
# Does there exist any other function which can be used to convert an integer or a float to a string?
# How would you use qsort() function to sort an array of structures?
# How would you use qsort() function to sort the name stored in an array of pointers to string?
# How would you use bsearch() function to search a name stored in array of pointers to string?
# How would you use the functions sin(), pow(), sqrt()?
# How would you use the functions memcpy(), memset(), memmove()?
# How would you use the functions fseek(), freed(), fwrite() and ftell()?
# How would you obtain the current time and difference between two times?
# How would you use the functions randomize() and random()?
# How would you implement a substr() function that extracts a sub string from a given string?
# What is the difference between the functions rand(), random(), srand() and randomize()?
# What is the difference between the functions memmove() and memcpy()?
# How do you print a string on the printer?
# Can you use the function fprintf() to display the output on the screen?
# Gautam Pagedar adds this question: What is a linklist and why do we use it when we have arrays? - I feel the correct answer should be linklist is used in cases where you don’t know the memory required to store a data structure and need to allocate is dynamically on demand.
# How do you detect a loop in linked list?
# Sunil asks: What is the difference between main() in C and main() in C++?
# ajz at his interviews asks what will be printed out when the following code is executed:

main()
{
printf("%x",-1<<4);
}

C Programming questions I

2009-09-04T03:24:00.000-07:00

2. For the following C program

#define AREA(x)(3.14*x*x)
main()
{float r1=6.25,r2=2.5,a;
a=AREA(r1);
printf("\n Area of the circle is %f", a);
a=AREA(r2);
printf("\n Area of the circle is %f", a);
}

What is the output?

3.
void main()
{
int d=5;
printf("%f",d);
}

4.
void main()
{
int i;
for(i=1;i<4,i++)
switch(i)
case 1: printf("%d",i);break;
{
case 2:printf("%d",i);break;
case 3:printf("%d",i);break;
}
switch(i) case 4:printf("%d",i);
}

5.
void main()
{
char *s="\12345s\n";
printf("%d",sizeof(s));
}

6.
void main()
{
unsigned i=1; /* unsigned char k= -1 => k=255; */
signed j=-1; /* char k= -1 => k=65535 */
/* unsigned or signed int k= -1 =>k=65535 */
if(iprintf("less");
else
if(i>j)
printf("greater");
else
if(i==j)
printf("equal");
}

7.
void main()
{
float j;
j=1000*1000;
printf("%f",j);
}

1. 1000000
2. Overflow
3. Error
4. None

8. How do you declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?

9.
int f()
void main()
{
f(1);
f(1,2);
f(1,2,3);
}
f(int i,int j,int k)
{
printf("%d %d %d",i,j,k);
}

What are the number of syntax errors in the above?

10.
void main()
{
int i=7;
printf("%d",i++*i++);
}

11.
#define one 0
#ifdef one
printf("one is defined ");
#ifndef one
printf("one is not defined ");

12.
void main()
{
int count=10,*temp,sum=0;
temp=&count;
*temp=20;
temp=∑
*temp=count;
printf("%d %d %d ",count,*temp,sum);
}

13. what is alloca()

14.
main()
{
static i=3;
printf("%d",i--);
return i>0 ? main():0;
}

15.
char *foo()
{
char result[100]);
strcpy(result,"anything is good");
return(result);
}
void main()
{
char *j;
j=foo()
printf("%s",j);
}

16.
void main()
{
char *s[]={ "dharma","hewlett-packard","siemens","ibm"};
char **p;
p=s;
printf("%s",++*p);
printf("%s",*p++);
printf("%s",++*p);
}

17. Output of the following program is

main()
{int i=0;
for(i=0;i<20;i++)
{switch(i)
case 0:i+=5;
case 1:i+=2;
case 5:i+=5;
default i+=4;
break;}
printf("%d,",i);
}
}

a) 0,5,9,13,17
b) 5,9,13,17
c) 12,17,22
d) 16,21
e) Syntax error

18. What is the ouptut in the following program

main()
{char c=-64;
int i=-32
unsigned int u =-16;
if(c>i)
{printf("pass1,");
if(cprintf("pass2");
else
printf("Fail2");
}
else
printf("Fail1);
if(iprintf("pass2");
else
printf("Fail2")
}

a) Pass1,Pass2
b) Pass1,Fail2
c) Fail1,Pass2
d) Fail1,Fail2
e) None of these

19. What will the following program do?

void main()
{
int i;
char a[]="String";
char *p="New Sring";
char *Temp;
Temp=a;
a=malloc(strlen(p) + 1);
strcpy(a,p); //Line number:9//
p = malloc(strlen(Temp) + 1);
strcpy(p,Temp);
printf("(%s, %s)",a,p);
free(p);
free(a);
} //Line number 15//

a) Swap contents of p & a and print:(New string, string)
b) Generate compilation error in line number 8
c) Generate compilation error in line number 5
d) Generate compilation error in line number 7
e) Generate compilation error in line number 1

20. In the following code segment what will be the result of the function,

value of x , value of y
{unsigned int x=-1;
int y;
y = ~0;
if(x == y)
printf("same");
else
printf("not same");
}

a) same, MAXINT, -1
b) not same, MAXINT, -MAXINT
c) same , MAXUNIT, -1
d) same, MAXUNIT, MAXUNIT
e) not same, MAXINT, MAXUNIT

21. What will be the result of the following program ?

char *gxxx()
{static char xxx[1024];
return xxx;
}

main()
{char *g="string";
strcpy(gxxx(),g);
g = gxxx();
strcpy(g,"oldstring");
printf("The string is : %s",gxxx());
}

a) The string is : string
b) The string is :Oldstring
c) Run time error/Core dump
d) Syntax error during compilation
e) None of these

22.Find the output for the following C program

main()
{
char *p1="Name";
char *p2;
p2=(char *)malloc(20);
while(*p2++=*p1++);
printf("%s\n",p2);
}

23.Find the output for the following C program

main()
{
int x=20,y=35;
x = y++ + x++;
y = ++y + ++x;
printf("%d %d\n",x,y);
}

24.Find the output for the following C program

main()
{
int x=5;
printf("%d %d %d\n",x,x<<2,x>>2);
}

25 Find the output for the following C program

#define swap1(a,b) a=a+b;b=a-b;a=a-b;
main()
{
int x=5,y=10;
swap1(x,y);
printf("%d %d\n",x,y);
swap2(x,y);
printf("%d %d\n",x,y);
}
int swap2(int a,int b)
{
int temp;
temp=a;
b=a;
a=temp;
return;
}

26 Find the output for the following C program

main()
{
char *ptr = "Ramco Systems";
(*ptr)++;
printf("%s\n",ptr);
ptr++;
printf("%s\n",ptr);
}

27 Find the output for the following C program

#include
main()
{
char *p1;
char *p2;
p1=(char *) malloc(25);
p2=(char *) malloc(25);
strcpy(p1,"Ramco");
strcpy(p2,"Systems");
strcat(p1,p2);
printf("%s",p1);
}

28.Find the output for the following C program

# define TRUE 0
some code
while(TRUE)
{
some code
}

29. struct list{
int x;
struct list *next;
}*head;

the struct head.x =100

Is the above assignment to pointer is correct or wrong ?

30.What is the output of the following ?

int i;
i=1;
i=i+2*i++;
printf(%d,i);

31. FILE *fp1,*fp2;

fp1=fopen("one","w")
fp2=fopen("one","w")
fputc('A',fp1)
fputc('B',fp2)
fclose(fp1)
fclose(fp2)
}

Find the Error, If Any?

32. #define MAN(x,y) (x)>(y)?(x):(y)
{int i=10;
j=5;
k=0;
k=MAX(i++,++j);
printf(%d %d %d %d,i,j,k);
}

33.void main()
{
int i=7;
printf("%d",i++*i++);
}

34.How will u terminate the statement?

35.select the wrong one
a.a+=1;
b.a*=2;
c.a**=1;
d.a>>=1;

36.pick the odd one
a.malloc
b.calloc
c.new

37.main()
{
char **p=="Hello";
printf("%s",**p);
}

38.main()
{
printf("%d%c\n");
printf("%d%c\n");
}

39.main()
{
int x==5;
printf("%d%d",x++,++x);
}

40.main()
{
int x==4;
printf("%d",printf(" %d %d ",x,x) );
}

41.main()
{
union
{
int i;
char p;
struct
{
int t;
char e;
char o;
};
};
printf("%d\n",sizeof(l) );
}

42.main()
{
int i==0,n==6;
while(n--0);
i+==n;
printf("%d\n",i);
}

Answers

1. B

2. Area of the circle is 122.656250
Area of the circle is 19.625000

3. Undefined

4. 1,2,3,4

5. 6

6. less

7. 4

8. char *(*(*a[N])())();

9. None.

10. 56

11. "one is defined"

12. 20 20 20

13. It allocates and frees memory after use/after getting out of scope

14. 321

15. anything is good.

16. "harma" (p->add(dharma) && (*p)->harma)
"harma" (after printing, p->add(hewlett-packard) &&(*p)->harma)
"ewlett-packard"

17. (d)

18. (c)

19. (b)

20. (a)

21. (b)

22. An empty string

23. 57 94

24. 5 20 1

25. 10 5

26. Samco Systems

27. RamcoSystems

28. This won't go into the loop as TRUE is defined as 0

29. Wrong

30. 4

31. no error. But It will over writes on same file.

32. 10 5 0

33. 56

34. ;

35. C

36. C

37. Garbage or nothing

38. Garbage Value

39. 6 6

40. 4 4 5

41. 4

42. -1

43. 0

44. 3

45. 16 21

46. UPPER CASE

47. 5 4 3 2 1

48. (newdictionary)-(dictionarynew)

49. (newdictionary)-(dictionarynew)

50. 50

C Questions Page 1(New)

C Questions Page 2

C Questions Page 3

C Questions Page 4

C Problems
Visual C++ Projects

Passing Command line arguments

2008-12-11T07:06:00.001-08:00

If you are working in a command line environment such as UNIX, Linux, or the DOS
prompt, it might be helpful to write programs that take arguments from the command
line. For example, suppose we have a program called sum, which takes two numbers as
command line arguments and displays their sum. We could enter the following command
at the operating system prompt:
sum 12 16

The arguments, which are separated by a space, are 12 and 16. Because a C++ program
starts its execution with function main, command line arguments are passed to main. Function
main can be optionally written with two special parameters. These parameters are
traditionally named argc and argv . The argc parameter is an int, and the argv parameter is an array of char pointers. Here is an example function header for main , using these two Parameters:

int main(int argc, char *argv[])

The argc parameter contains the number of items that were typed on the command line,
including the name of the program. For example, if the sum program described above is
executed with the command
sum 12 16,

the argc parameter will contain 3. As previously mentioned, the argv parameter is an array of char pointers. In the function header, the brackets are empty because argv is an external array of unknown size. The number that is stored in argc , however, will be the number of elements in the argv array.Each pointer in the argv array points to a C-string holding a command line argument. Once again, assume the sum program is executed with the command
sum 12 16.

The elements of the argv array will reference the items on the command line in the following
manner:
argv[0] = "sum"
argv[1] = "12"
argv[2] = "16"
Before we look at the code for the sum program, let’s look at Program H-1. It is a simple
program that simply displays its command line arguments. (The program is named argdemo.cpp. )

Appendix H: Passing Command Line Arguments Now, let’s look at the code for the sum program.
Program H-1 (argdemo.cpp)
1 // This program demonstrates how to read
2 // command line arguments.
3 #include
4 using namespace std;
5
6 int main(int argc, char *argv[])
7 {
8 cout << "You entered " << (argc - 1); 9 cout << " command line arguments.\n"; 10 if (argc > 1)
11 {
12 cout << "Here they are:\n"; 13 for (int count = 1; count <>
4 #include // Needed for atof
5 using namespace std;
6
7 int main(int argc, char *argv[])
8 {
9 double total = 0;
10
11 if (argc > 1)
12 {
13 for (int count = 1; count < argc; count++)
14 total += atof(argv[count]);
15 cout << total << endl;
16 }
17 return 0;
18 }
Example Session on a UNIX System
$ sum 12 16
[Enter]
28
$ sum 1 2 3 4 5
[Enter]
15
$

C- Questions

2008-11-26T06:00:00.000-08:00

What does static variable mean?
What is a pointer?
What is a structure?
What are the differences between structures and arrays?
In header files whether functions are declared or defined?
What are the differences between malloc() and calloc()?
What are macros? what are its advantages and disadvantages?
Difference between pass by reference and pass by value?
What is static identifier?
Where are the auto variables stored?
Where does global, static, local, register variables, free memory and C Program instructions get stored?
Difference between arrays and linked list?
What are enumerations?
Describe about storage allocation and scope of global, extern, static, local and register variables?
What are register variables? What are the advantage of using register variables?
What is the use of typedef?
Can we specify variable field width in a scanf() format string? If possible how?
Out of fgets() and gets() which function is safe to use and why?
Difference between strdup and strcpy?
What is recursion?
Differentiate between a for loop and a while loop? What are it uses?
What are the different storage classes in C?
Write down the equivalent pointer __expression for referring the same element a[i][j][k][l]?
What is difference between Structure and Unions?
What the advantages of using Unions?
What are the advantages of using pointers in a program?
What is the difference between Strings and Arrays?
In a header file whether functions are declared or defined?
What is a far pointer? where we use it?
How will you declare an array of three function pointers where each function receives two ints and returns a float?
what is a NULL Pointer? Whether it is same as an uninitialized pointer?
What is a NULL Macro? What is the difference between a NULL Pointer and a NULL Macro?
What does the error 'Null Pointer Assignment' mean and what causes this error?
What is near, far and huge pointers? How many bytes are occupied by them?
How would you obtain segment and offset addresses from a far address of a memory location?
Are the expressions arr and &arr same for an array of integers?
Does mentioning the array name gives the base address in all the contexts?
Explain one method to process an entire string as one unit?
What is the similarity between a Structure, Union and enumeration?
Can a Structure contain a Pointer to itself?
How can we check whether the contents of two structure variables are same or not?
How are Structure passing and returning implemented by the complier?
How can we read/write Structures from/to data files?
What is the difference between an enumeration and a set of pre-processor # defines?
what do the 'c' and 'v' in argc and argv stand for?
Are the variables argc and argv are local to main?
What is the maximum combined length of command line arguments including the space between adjacent arguments?
If we want that any wildcard characters in the command line arguments should be appropriately expanded, are we required to make any special provision? If yes, which?
Does there exist any way to make the command line arguments available to other functions without passing them as arguments to the function?
What are bit fields? What is the use of bit fields in a Structure declaration?
To which numbering system can the binary number 1101100100111100 be easily converted to?
Which bit wise operator is suitable for checking whether a particular bit is on or off?
Which bit wise operator is suitable for turning off a particular bit in a number?
Which bit wise operator is suitable for putting on a particular bit in a number?
Which bit wise operator is suitable for checking whether a particular bit is on or off?
which one is equivalent to multiplying by 2:Left shifting a number by 1 or Left shifting an unsigned int or char by 1?
Write a program to compare two strings without using the strcmp() function.
Write a program to concatenate two strings.
Write a program to interchange 2 variables without using the third one.
Write programs for String Reversal & Palindrome check
Write a program to find the Factorial of a number
Write a program to generate the Fibinocci Series
Write a program which employs Recursion
Write a program which uses Command Line Arguments
Write a program which uses functions like strcmp(), strcpy()? etc
What are the advantages of using typedef in a program?
How would you dynamically allocate a one-dimensional and two-dimensional array of integers?
How can you increase the size of a dynamically allocated array?
How can you increase the size of a statically allocated array?
When reallocating memory if any other pointers point into the same piece of memory do you have to readjust these other pointers or do they get readjusted automatically?
Which function should be used to free the memory allocated by calloc()?
How much maximum can you allocate in a single call to malloc()?
Can you dynamically allocate arrays in expanded memory?
What is object file? How can you access object file?
Which header file should you include if you are to develop a function which can accept variable number of arguments?
Can you write a function similar to printf()?
How can a called function determine the number of arguments that have been passed to it?
Can there be at least some solution to determine the number of arguments passed to a variable argument list function?
How do you declare the following:

o An array of three pointers to chars

o An array of three char pointers

o A pointer to array of three chars

o A pointer to function which receives an int pointer and returns a float pointer

o A pointer to a function which receives nothing and returns nothing

What do the functions atoi(), itoa() and gcvt() do?
Does there exist any other function which can be used to convert an integer or a float to a string?
How would you use qsort() function to sort an array of structures?
How would you use qsort() function to sort the name stored in an array of pointers to string?
How would you use bsearch() function to search a name stored in array of pointers to string?
How would you use the functions sin(), pow(), sqrt()?
How would you use the functions memcpy(), memset(), memmove()?
How would you use the functions fseek(), freed(), fwrite() and ftell()?
How would you obtain the current time and difference between two times?
How would you use the functions randomize() and random()?
How would you implement a substr() function that extracts a sub string from a given string?
What is the difference between the functions rand(), random(), srand() and randomize()?
What is the difference between the functions memmove() and memcpy()?
How do you print a string on the printer?
Can you use the function fprintf() to display the output on the screen?

The Basic sections of ‘C’ are explained taking an example::

2008-11-26T05:55:00.000-08:00

/* A program to calculate average of 5 nos given by the user */

/*----------Defining of Constants and Inclusion Of Header Files------*/

#include

#define M 10

/*------------Start Of The Main Program---------------------------------*/

main()

{

/*----------------Declaration of Variables----------------------*/

int i;

int sum ;

/*-----------------Initialization of Values----------------*/

sum=avg=0;

/*---------------Interaction With User and Receiving Required Data----*/

printf(“Please Enter The Values::”\n”);

for (i=0; i<=M; i++)

{

scanf(“%d”, &n);

sum=sum+n;

}

/*---------------Display Of Final Result with The Computation-----------*/

printf(“The Sum of All The Nos is: %d \n

The Average of All The 10 Nos is: %d\n”, sum, sum/10);

}

2) The various data types supported in C are basically divided into 4 types: primary or fundamental, user-defined, derived and empty data set.

The Primary Data Type are mainly divided into 3 types which are sub-divided into many types like:

1) INTEGER (int): different types of integers are:

int, unsigned int, short int, unsigned short int, long int, and unsigned long int

2) Character (char): different types of characters are:

signed char, unsigned char

3) Floating Point (float): different types of floats are:

float, double, long double

An Example Program to Display Various Primary Data Types::

main()

{

int a; /*------int range : -32,768 to 32,767-------*/

short int c; /*------short int range : -127 to 128-------*/

long int d; /*------long int range : -2,147,483,648 to 2,147,483,647-------*/

float e; /*------float range : -3.4E-38 to 3.4E+38-------*/

double f; /*------double range : -1.7E-308 to 1.7E+308 -------*/

char g; /*------int range : -127 to 128-------*/

a=30007;

c=120;

d=89654785;

e=954651.6216549;

f=1.6E;

g=’M’;

printf (“a=%d\n c=%d \n d=%d \n e=%f \n f=%f \n g=%c\n”, a, b, c, d ,e ,f ,g);

RESULT:

a=30007

c=120

d=89654785

e=954651.6216549

f=1.6E

g=M

C Objective Questions

2008-11-20T01:55:00.000-08:00

No commas or blanks are allowed within an integer or a real constant.	True	False			A
'void' is a datatype.	True	False			A
'emp name' is a valid variable name.	True	False			B
'unsigned' is a valid variable name.	True	False			B
Operation between an integer and float always yields a float result.	True	False			A
/* The C language. /* is a procedural language .// The above statement is valid.	True	False			B
The default initial value of automatic storage class is 0.	True	False			B
= and = = have the same operation.	True	False			B
Character data types cannot be declared as unsigned.	True	False			B
&& and & operators have the same meaning.	True	False			B
Right shifting an operand 1bit is equivalent to multiplying it by 2.	True	False			B
The same variable names of automatic type can be used in different functions without any conflict.	True	False			A
printf("%d", sizeof('2')); will print 2.	True	False			B
The expression "b = 3 ^ 2;" will evaluate b = 9.	True	False			B
The expression "int i = j = k = 0;" is invalid.	True	False			A
printf() is not a library function.	True	False			B
The expression (i + j)++ is illegal.	True	False			A
The expression 'int j = 6 + 3 % -9;' evaluates to -1.	True	False			B
p++ executes faster than p + 1.	True	False			A
static variable will not always have assigned value.	True	False			A
Variables declared as register storage type gets stored in CPU registers.	True	False			A
Single operations involving entire arrays are permitted in C.	True	False			A
The main() function can be called from any other function.	True	False			A
The main() function can call itself recursively.	True	False			A
The statement "for (i = 0, j = 2; j <= 10; j++)" is a valid statement in 'C'.	True	False			A
When a user defined function is defined in program, then it has to be called at least once from the main().	True	False			B
Left shift operator rotates the bits on the left and places them to the right.	True	False			B
A recursive function calls itself again and again.	True	False			A
Only one break can be used in one loop.	True	False			B
Every if statement can be converted into an equivalent switch statement.	True	False			B
Nested macros are allowed.	True	False			B
Expression 4**3 evaluates to 64.	True	False			B
All macro substitutions in a program are done before compilation of the program.	True	False			A
'# define PI = 8;' is the correct declaration of macros.	True	False			B
An array declared as A[100][100] can hold a maximum of 100 elements.	True	False			B
The array 'char name[10] can consist of a maximum of 9 characters.	True	False			A
It is necessary to initialize the array at the time of declaration.	True	False			B
the value *(&i) is same as printing the value of address of i.	True	False			B
A pointer is an indication of the variable to be accessed next.	True	False			B
The contents of a file opened in 'r+' mode cannot be changed.	True	False			B
Union is used to hold different data at different time.	True	False			A
All elements of a structure are allocated contiguous memory locations.	True	False			A
enum helps to create user defined datatype.	True	False			A
The value of an enumerated datatype can be read using scanf() function.	True	False			B
Structures within structures cannot be created.	True	False			B
gets() and puts() are unformatted I/O functions.	True	False			A
The identifier argv[] is a pointer to an array of strings.	True	False			B
fprintf()function can be used to write into console.	True	False			B
fopen() function returns a pointer to the open file.	True	False			A
Which of the following language is predecessor to C Programming Language?	A	B	BCPL	C++	B
C programming language was developed by	Dennis Ritchie	Ken Thompson	Bill Gates	Peter Norton	A
C is a ___ language	High Level and Procedural	Low Level and OOPS	Middle Level and Procedural	Machine Level and OOPS	A
Which escape character can be used to beep from speaker in C?	\a	\b	\m	\n	A
Which of the following is invalid?	''	" "	'a'	'abc'	D
A declaration float a, b; occupies ___ of memory	1 byte	4 bytes	8 bytes	16 bytes	C
The printf() function retunes which value when an error occurs?	Positive value	Zero	Negative value	None of these	C
Identify the wrong statement	putchar(65)	putchar('x')	putchar("x")	putchar('\n')	C
Which header file is essential for using strcmp() function?	string.h	strings.h	text.h	strcmp.h	A
Which among the following is a unconditional control structure	do-while	if-else	goto	for	C
continue statement is used	to go to the next iteration in a loop	come out of a loop	exit and return to the main function	restarts iterations from beginning of loop	D
Which of the following is an example of compounded assignment statement?	a = 5	a += 5	a = b = c	a = b	C
In the expression - 'x + y + 3z =20'	'x + y' is a keyword	3 and 20 are constants	3z is a constant	y is a variable and z is a constant	B
The output of the following code is: void main() { int z, a = 5, b = 3; z = a * 2 + 26 % 3; printf("%d", z); }	10	0	12	None of the above	C
Which options shows the correct hierarchy of arithmetic operators	*, or /, + or -	*, , /, +, -	*, /, , +,-	/ or *, - or +	D
The output of the following code is: void main() { float a; int x = 10, y = 3; a = x / y; printf("%f", a); }	3.999999	Error	3	3.000000	D
The output of the following code is: void main() { char a = 'B'; switch (a) { case 'A' : printf("a"); case 'B' : printf("b"); default : printf("c"); }	B	b	bca	bc	D
The output of the following code is: void main() { int a = 20; printf("%d\t%d\n", ++a, a); }	21 21	20 21	21 20	20 20	C
How many times the following loop will execute? for (a = 0; a < 4; a++) printf("hello\n");	3	4	5	infinite	B
The output of the following code is: void main() { int a; int &b = a; a=100; printf("b=%d\ta=%d\n", b,a); }	b=100 a=100	b=100 a=0	b=0 a=100	Error	D
The output of the following code is: void main() { int a = 0; while (a<=50) for(;;) if(++a % 50==0) break; printf("a = %d", a); }	a = 100	a = 50	compilation error	runtime error	A
The output of the following code is: int f(int a, int b); void main() { int a = 12, b=154; printf("%d", f(a, b)); } int f(int a, int b) { if (a if(b==0) return(a); return (f(b, a % b)); }	2	1	Compilation error	Runtime error	A
The output of the following code is: void main() { int a = 1, b=2; int ip; ip=&a; b=ip; printf("%d", b); }	2	1	100	0	B
The output of the following code is: void main() { static int a = 1, b, c; if (a>=2) b=2; c=3; printf("%d\t%d", b,c); }	2 3	0 3	0 0	2 0	B
The output of the following code is: #define sqr(x= x*x) main() { int a = 10, b = 5; printf("%d, %d", sqr(a+b),sqr(++a)); }	77, 121	225, 121	77, 144	Compilation error	D
struct stud { int roll; char name[20]; float marks; } *p; What will be the byte size of p?	24	2	26	None	B
The output of the following code is: main() { unsigned int a = 10; while (a>=10) { int a; a-- ; } printf("%i", a); }	Infinite loop	9	0	None	A
The output of the following code is: main() { xyz: goto abc; printf("Hello"); abc: printf("World"); goto xyz; }	Infinite loop	Hello World	World Hello	Compilation error	A
The output of the following code is: main() { int a = 0; for (; i = 0; i++) printf("%d", a); }	0	Nothing will be displayed	Infinite loop	None of the above	D
The output of the following code is: void change (char k) { k="Hello"; return; } main() { char ch = "World";; change(ch); printf("%s", ch); }	Hello	World	Compilation error	Hello World	B
Which of the following is not an infinite loop	int i =1; while (1) {i++;}	for( ; ; );	int true=0, false; while (true) {false = 1;}	int y, x = 0; do {y = x;} while (x==0);	C
do-while loop is useful when we want the statements within the loop must be executed	Only once	At least once	More than Once	Any one of the above	B
Which of the following is not a preprocessor directive	#if	#elseif	#undef	#pragma	B
The output of the following code is: main() { int a = 5, b = 6; (a == b? printf("%d", a)); }	0	5	Error	None of the above	C
The output of the following code is: main() { int k, num = 30; k = (num > 5 ? (num <= 10 ? 100 : 200) : 500); printf("\n%d", num); }	200	500	30	Unpredictable	C
The output of the following code is: main() { void msg() { printf("HELLO WORLD"); } }	HELLO WORLD	Error	None of the above		B
The output of the following code is: main() { int sub[50]; for(i=0; i<=48;i++) { sub[i]=i; printf("\n%d", sub[i]); } }	0 to 48 will be displayed	48	49	Compilation Error	D
The output of the following code is: main() { int a[10], i; for (i = 1; I <= 0; i++) { scanf("%d", a[i]); printf("%d", a[i]); } }	10	Logical error	Runtime error	1 to 10 will be displayed	B
Which of the following expressions is wrong	float a =123.56;	char ch ='T' * 'A';	char ch ='T' *20;	3 +a = b;	D
strcat() function ----------------------- two strings.	delete	concatenate	compare	none of the above	B
A function to be called must be ended with a----------------	.	?	;	none of the above	C
The function fopen() on failure returns---------------------.	0	NULL	1	none of the above	B
The-------------------- statement helps immediate exit from any part of the loop	break	continue	exit	All of the above	A
The -------------------------- loop executes at least once.	for	while	do-while	while & do-while	C
char *s[10] defines an array of ------------------------	pointers to strings	string to pointer	both		A
A multidimensional array A[10][9] can store-------- number of elements	91	88	90	89	C
The size of signed integer is ------ bytes.	4	2	8	10	B
There are total ------ numbers of operators in 'C'.	35	45	55	40	B
------ is the ternary operator	?,-	?,:	++,--	none of the above	B
unsigned char has a range from 0 to ------------	253	254	255	256	C
If 'str' is a string of 7 characters, the statement printf("%4s", str); will display ------characters.	4	7	6	0	A

C Programming FAQs

2008-10-22T22:02:00.000-07:00

infrequently-asked questions and their answers:

Section 1: Declarations and Initializations [a]

1.1: How do you decide which integer type to use?

Use ``short'' when you need to avoid values over 32,767, ``int'' when you want to store integers, ``long'' for long numbers (more than 6 digits), and ``float'' for numbers over 4 billion.

1.2: What should the 64-bit type on new, 64-bit machines be?

int.

1.3: If I write the code `int i, j;` can I assume that `(&i + 1) == &j`?

Only sometimes. It's not portable, because in EBCDIC, i and j are not adjacent. [a]

1.4: What's the best way to declare and define global variables?

In headers; this way, you can get link errors when you include the same header twice. Generally, you will have to define a variable everywhere you want to use it, and then declare it someplace so you know what it is. [a]

1.5: What does extern mean in a function declaration?

It refers to a variable which is not actually in your program. For instance,

main() {

       extern int bar;

       printf("%d\n", bar);

       return 0;

will compile without errors because bar is declared as being external. (It won't run, though, because you never assign bar a value.) [a]

1.6: I finally figured out the syntax for declaring pointers to functions, but now how do I initialize one?

With the assignment operator. You were perhaps expecting a screwdriver?

1.7: I've seen different methods used for calling through pointers to functions. What's the story?

In the old days, when Microsoft first invented C, the syntax for calling functions involved more parentheses; this was after their market research indicated that most C programmers would be coming from a Lisp environment. Later, when Kernighan took over the language design (right after AT&T bought Microsoft's language technology), he decided to eliminate the parentheses, but the old form is still allowed.

You do need the parentheses to call a function with more than one argument, for instance,

int (*foo)(char *, ...) = printf;

(*foo)("hello, %s\n", "world!");

needs the parens, but they would not be needed for

foo, "hello, world!\n";

(The ``*'' just means to execute foo, just like the ``*'' on the end of an executable filename in ``ls -F''.) [a]

1.8: What's the `auto` keyword good for?

Declaring vehicles.

1.9: I can't seem to define a linked list successfully. I tried

       typedef struct {

              char *item;

              NODEPTR next;

       } *NODEPTR;

but the compiler gave me error messages. Can't a structure in C contain a pointer to itself?

Not exactly; it can contain a pointer to another structure of the same type. Try:

typedef struct {

       char *item;

       double *next;

} NODEFAKE;

typedef struct {

       char *item;

       NODEFAKE *next;

} NODEPTR;

Make sure that sizeof(NODEPTR) == sizeof(double).

This technique is called a ``backwards reference''. [a]

1.10: How do I enter values using hexadecimal?

long ints can be entered using hexadecimal notation; for instance,

long int foo = 07;

sets foo to hex 7. [a]

1.11: How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters?

Well, first you need to know how to declare an array of N items of type T - that's

T foo[N];

Now you need to look at how to declare a pointer to function returning something, say, an object of type S. That's like this:

S (*bar)();

Now assume that S is ``pointer to function returning pointer to char''. We get

(char *) (*)() (*bar)().

So, the whole thing turns out to be (with appropriate parentheses)

(((char)(*))((*)())(((*)((foo)))())([(N)]));

If your compiler complains, break this down into subexpressions.

To call it, just use

foo[i]();

This works because, in C, declaration reflects use, but it's one of those weird distorted mirrors. [a]

Section 2: Structures, Unions, and Enumerations [a]

2.1: What is the difference between an `enum` and a series of preprocessor `#define`s?

The enum doesn't require the preprocessor.

2.2: I heard that structures could be assigned to variables and passed to and from functions, but K&R I says not.

K&R I was wrong; they hadn't actually learned C very well before writing the book. Later, Ritchie got a job at Bell Labs, and worked closely with the authors of C, allowing the 2nd edition of the book to be much more accurate. (Kernighan already worked at Bell Labs, as a video game developer.) [a]

2.3: How does struct passing and returning work?

The structures are put into the low part of the VGA card's VRAM. They are then removed before the next video update. This is why struct passing was not supported for a long time; VGA cards were prohibitively expensive.

If you try to pass very large structures on the stack, you may see odd screen graphics. [a]

2.4: Why can't you compare structs?

Compare them to what? A summer's day?

2.5: How can I read/write structs from/to data files?

Loop with putchar. Be careful; if your machine uses signed chars by default, all of the sign bits in your structure elements will be reversed. [a]

2.6: How can I determine the byte offset of a field within a structure?

It's generally 4 times the number of members of the structure. It may be more or less on some machines. [a]

2.7: How can I access structure fields by name at run time?

foo."name" should work. You may need to overload the . operator, which, in turn, may overload your C compiler.

2.8: Why does sizeof report a larger size than I expect for a structure type, as if there was padding at the end?

Because there's padding at the end. Duh.

2.9: My compiler is leaving holes in structures, which is wasting space and preventing ``binary'' I/O to external data files. Can I turn off the padding, or otherwise control the alignment of structs?

Sure. What you do to eliminate the padding in structures is use unions; for intance,

struct foo {

       char c;

       long l;

       char d;

       char e;

       char f;

};

may cause struct foo to be padded to 12 bytes, rather than the correct size of 8. Try

union foo {

       double _d;

       char c, d, e, f;

       long l;

};

which will be 8 bytes. (The double is for alignment.) [a]

2.10: Can I initialize unions?

Depends. They may go on strike when provoked. Luckily, if your program involves air traffic control, the ISO standard guarantees that Ronald Reagan will fire any unions that go on strike, and replace them with structs, which should be close enough. [a]

2.11: How can I pass constant values to routines which accept struct arguments?

Try foo((struct foo) 3). [a]

Section 3: Expressions [a]

3.1: Why doesn't the code `a[i] = i++;` work?

You didn't declare either i or a.

3.2: Under my compiler, the code

       int i = 7;

       printf("%d\n", i++ * i++);

prints 49. Regardless of the order of evaluation, shouldn't it print 56?

No. The only logical answer would be 81 - two postfix ++'s are automatically converted to prefix.

3.3: I've experimented with the code

       int i = 2;

       i = i++;

on several compilers. Some gave i the value 2, some gave 3, but one gave 4. I know the behavior is undefined, but how could it give 4?

Because i is 2, the loop is executed twice.

3.4: People keep saying the behavior is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected.

They were probably wrong. Flame them mercilessly. Be sure before you do that your compiler is really ANSI conforming, though. If it turns out you were wrong, they get a legal claim on your first-born.

3.5: Can I use explicit parentheses to force the order of evaluation I want? Even if I don't, doesn't precedence dictate it?

No. To force order of evaluation, you must threaten it. Take the comma operator hostage. Using it, you can force the other operators to do what you want. [a]

3.6: But what about the `&&`, `||`, and comma operators? I see code like ```if((c = getchar()) == EOF || c == '\n')`'' ...

As noted, once you've captured the comma operator, the others become docile.

3.7: If I'm not using the value of the expression, should I use `i++` or `++i` to increment a variable?

++i. Only losers and idiots use i++. This is different if your native language would idiomatically use ``i increment'', but in English and related languages, you must use ++i. Note that a modern program must use both, dependent on the current locale.

3.8: Why is `i = ++i` undefined?

Because it is unclear whether it is shorthand for i = 42; or i = (char *) "forty two";.

Given the ambiguity, the standards committee decided to leave it undefined.

Section 4: Null Statements [a]

4.1: What is this infamous null statement, anyway?

A null statement is an expression statement consisting solely of the terminating semicolon. The optional expression is dropped. It can be distinguished from any other statement by byte count or study of side-effects.

4.2: How do I ``get'' a null statement in my programs?

In ANSI C, there are six types of statements; labeled statements, compound statements, expression-statements, selection statements, iteration statements, and jump statements. All of them, except the jump and expression statments, are defined in terms of optional preceeding text, and other statements. The jump statements are never null statements. An expression statement is considered to be ``a null statement'' if the optional expression part of it has been left out. A null statement can appear on its own, or (most frequently) as the statement body of an iteration statement. These two null statements are equivalent, though neither of them is equivalent to any non-null statement. [*]

You may accidentally get a null statement by deleting the body of a non-null statement.

[*] Actually, they are functionally equivalent to a large set of non-null statements, namely, those with no side-effects. However, the FDA has yet to approve any such, as their lack of side effects is conjectured, and not clinically proven. This applies only to the ANSI standard, and not the ISO standard, as the FDA has no jurisdiction outside the U.S. [a]

4.3: Is there more than one null statement?

Sort of. You can use ``;'', ``0;'', or ``1;'' - they will all act like a null statement. Only the first is a ``true'' null statement (all bits zero). They are basically equivalent. Note that (void *) 0; is a null statement of type pointer to void, for instance. [a]

4.4: But I thought `{ }` was a null statement!

No. { statement-list[opt] } is a compound statement. An empty block is not the same as a null statement, however, although it can be used in many of the same places. It's really a null block. (You can convert it with a cast, but it's not directly compatible. For instance, you can't use a null block as one of the controlling statements of a for loop.)

4.5: I use the statement `#define NULLSTMT(F) (F) ;` to allow me to cast a null statement to an appropriate type.

This trick, though popular in some circles, does not buy much. The resulting code is invalid, and will not compile. This (in the author's opinion) outweighs any arguable type consistency. It may be more common in industrial code. If it becomes common practice, C++ will probably legalize it. [a]

4.6: I use the statement `#define NULLSTMT(F) (F) 0;` to allow me to cast a null statement to an appropriate type.

This trick will likely work, but think: what does it really buy you? Mostly, it will indicate to even the most casual observer that you are shakey on the concept of null statements, making it harder for them to check your code.

4.7: But wouldn't it be better to use `;` (rather than `0;`) in case the value of 0 changes, perhaps on a machine with nonzero no-op instructions?

No. The '0' of '0;' is not evaluated as an instruction, rather, it is just ignored. The only reason to use '0;' instead of ';' is to help keep 1-heavy code properly balanced (in C, which uses binary representations for numbers, it is possible for code to become unbalanced; an unbalanced binary tree is a common source of poor performance.

4.8: Is a null statement a null pointer?

No. A null pointer is a pointer where all of the address bits are zero (no matter what the segment bits are), and can be obtained by typing '(char *) (int) 0'. A null statement is not a pointer to anything. They are not interchangeable, although you can combine them to get an effectively-null statement, such as NULL;. This does not buy you anything. [a]

4.9: I'm still confused. I just can't understand all this null statement stuff.

Follow these two simple rules:

When you don't want to do anything in source code, don't write it.
If you need a null statement to round out an expression, use an unadorned ; to provide it.
Send large donations, checks, and money orders to the author of the FAQ, or the moderator of the group, whichever you prefer. Then, cross the top question off the FAQ, answer the question at the bottom, and mail it to three people. Within two weeks, you will receive 729 answers to various questions! Do not break the chain; Emily Postnews broke the chain, and now no one listens to her. [a]

Section 5: Arrays and Pointers

5.1: I had the definition `char a[6]` in one source file, and in another I declared `extern char a[]`. Why did it work?

The declaration extern char a[] simply matches the actual definition. The type ``array-of-type-T'' is the same as ``array-of-type-T.'' Go ahead and use extern char a[]. (For greater portability, use it in both files, not only in one of them.) [a]

5.2: But I heard that `char a[]` was different from `char a[6]`.

This is true. However, the declaration a[] is compatible with the definition a[6].

5.3: So what is meant by the ``equivalence of pointers and arrays'' in C?

Very little.

5.4: Then why are array and pointer declarations interchangeable as function formal parameters?

Classism. We consider arrays ``second class objects''. They don't vote, and they get treated as pointers. Additionally, they're merely objects, not citizens. Marx wrote about this a lot. [a]

5.5: Why doesn't sizeof properly report the size of an array which is a parameter to a function?

Part of the ANSI conspiracy to restrict people to passing pointers; this was undertaken after the first discovery that passing large arrays recursively could cause crashes. Since then, with the passing of MS-DOS, it has become a non-issue; since all serious machines have virtual memory, you can pass as much data as you want on the stack without detectable problems. [a]

5.6: Someone explained to me that arrays were really just constant pointers.

Cool. Someone I know says he saw Elvis in a local bar. [a]

5.7: Practically speaking, what is the difference between arrays and pointers?

About the difference between alcohol and marijuana; they have different characteristics, and that's not a problem if you don't mix them too carelessly.

5.8: I came across some ``joke'' code containing the ``expression'' `5["abcdef"]`. How can this be legal C?

It was added to allow people to avoid the character constant 'f' which may not be available on some systems. (Actually, it's a side-effect of the equivalence of arrays and pointers.) [a]

5.9: How would I initialize an entire array from standard input?

You have to use a loop. For instance, the following code reads the numbers zero through 99 into the array a.

for (i = 0; i <>

       a[i] = (scanf, ("%d", i));

Make sure to include , or this may not work. [a]

Section 6: Memory Allocation

6.1: Why doesn't this fragment work?

       char *answer

       printf("Type something:\n");

       gets(answer);

       printf("You typed \"%s\"\n", answer);

The semicolon after ``answer'' is missing. [a]

6.2: I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage.

You probably returned a pointer to a local array. That doesn't work. Try using a temporary file, instead. For instance:

char *getstr(void) {

       FILE *fp = tmpfile();

       fputs(gets(NULL), fp);

       return (char *) fp;

6.3: Why does some code carefully cast the values returned by malloc to the pointer type being allocated?

In interrupt-riddled code, it may be necessary to cast values to force the CPU to resolve pointer types. [a]

6.4: You can't use dynamically-allocated memory after you free it, can you?

Yes. However, what happens when you do is not clearly defined.

6.5: How does free() know how many bytes to free?

Interrupt 41h. On macs, amigas, and other ``big-endian'' processors, that would be interrupt 14h; be wary of portability problems. [a]

6.6: So can I query the malloc package to find out how big an allocated block is?

Not exactly; because the objects are dynamically allocated, their size can change at run time, so this will not be reliable. If you restrict your allocation to allocating sizeof(void *) bytes at a time, you will find that you can use sizeof() to get the size of a block, in the obvious way.

6.7: I'm allocating structures which contain pointers to other dynamically-allocated objects. When I free a structure, do I have to free each subsidiary pointer first?

No. You just have to keep track of them somewhere else also.

6.8: Was Proust's masterwork, A Remembrance of Things Past, the basis for the C library's allocation scheme, based largely on contextual analysis?

The standard does not specify an allocation scheme; the famous author the allocation scheme is based on is implementation specified. Proust is a common choice, however.

6.9: I have a program which mallocs but then frees a lot of memory, but memory usage (as reported by ps) doesn't seem to go back down.

You're probably not freeing the memory completely. Try replacing 'free(foo);' with

free(foo);

free(foo);

free(foo);

in case the first free() frees the memory only partially. (Unix wizards may recognize the parallel with syncing three times before rebooting.)

Alternatively, free(foo) + 4; may free the remaining four bytes. (Before using this, make sure realloc(foo, 0) returned 4).

Section 7: Characters and Strings

7.1: How can I get the numeric (character set) value corresponding to a character, or vice versa?

The obvious way is to write a function to do the conversion. (Error checking has been omitted for brevity.)

int ctoi(char c) {

       static unsigned char *ary;

       /* initialize the array */

       if (!ary) {

              int i;

              ary = malloc(UCHAR_MAX + 2);

              for (i = 0; i <>

                    ary[i] = i;

              ary[UCHAR_MAX + 1] = '\0';

       if (c) {

              unsigned char *t;

              /* we have to skip the leading NUL */

              t = strchr(ary + 1, c);

              if (!t)

                    return 0;

              return t - ary;

       } else {

              /* special case for NUL character */

              return 0;

There are various clever tricks you can use to get around writing the function, but most are too complicated for beginners. [a]

Section 8: Boolean Expressions and Variables

8.1: What is the right type to use for boolean values in C? Why isn't it a standard type? Should `#define`s or `enum`s be used for the true and false values?

int (*)(int, char **) makes a good boolean type. You can use main for true, and exit for false. On some compilers, you may need to cast exit() to an appropriate type.

8.2: Isn't `#defining` TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What if a built-in boolean or relational operator ``returns'' something other than 1?

Very good! For instance, one program I saw used

#define TRUE(x) ((x) & 0x100)

for compatability with a specific release of a FORTRAN compiler, which used 0 for .FALSE. and 256 for .TRUE. - this allowed them to change their code with every new release of the FORTRAN compiler, and kept them alert to changes. This has no relationship to the boolean or logical operators in C, which always return 0 or 1.

8.3: What is truth?

It is not a saffron-robed monk, pissing in the snow. [a]

Section 9: C Preprocessor

9.1: How can I use a preprocessor `#if` expression to tell if a machine is big-endian or little-endian?

#ifdef __BIG_ENDIAN should work on all known machines; Borland defines it. [a]

9.2: I've got this tricky processing I want to do at compile time and I can't figure out a way to get cpp to do it.

Poor baby.

9.3: How can I list all of the pre-`#define`d identifiers?

#define __ALL_CPP_IDS - put this in a source file, and run it through your C preprocessor.

9.4: How can I write a cpp macro which takes a variable number of arguments?

Try something like this:

#define add(x) (x)

#define add(x, y) (x + y)

#pragma induction add

9.5: Shouldn't the following code:

       #define ROSE 1

       #define CHRYSANTHEMUM 2

       #define RHODODENDRON 3

       #define WATER_LILY 4

       printf("%d\n", CHRYSATHNEMUM);

print ``2''?

You misspelled CHRYSANTHEMUM. Use abbreviations for long flower names in C code.

Section 10: ANSI C

10.1: What is the ``ANSI C Standard?''

A whiny bunch of lusers who haven't written as many books as Herbert Schildt. [a]

10.2: How can I get a copy of the Standard?

Use a copier.

10.3: Does anyone have a tool for converting old-style C programs to ANSI C, or vice versa, or for automatically generating prototypes?

A router helps, but your best bet is still the band saw. Quick, efficient, and powerful.

10.4: I'm trying to use the ANSI ``stringizing'' preprocessing operator `#` to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

This is because "3" is not a legal integral constant in C - it's a string constant.

10.5: I don't understand why I can't use const values in initializers and array dimensions, as in

       const int n = 7;

       int a[n];

Because you're not using C++.

10.6: What's the difference between ``char const p'' and ``char const p''?

One `` '' character. There are some trivial differences having to do with the distinction between a pointer to a constant, and a constant pointer, but since you can cast either to a (char *) it hardly matters.

10.7: Can I declare main as void, to shut off these annoying ``main returns no value'' messages? (I'm calling exit(), so main doesn't return.)

Certainly. You can also declare it as double. It may not compile, or it may crash, but who cares? No lousy bunch of whining lusers is going to tell *you* what to do.

10.8: Why does the ANSI Standard not guarantee more than six monocase characters of external identifier significance?

Because none of the members of the committee had names over six letters, or in which letters other than the first were capitalized. [a]

10.9: What is the difference between memcpy and memmove?

memmove moves memory, and memcpy copies it. memmove may not be supported on machines without internal robot arms. Do not use memmove while the machine is powered up - you can destroy your memory.

10.10: Why won't the Frobozz Magic C Compiler, which claims to be ANSI compliant, accept this code? I know that the code is ANSI, because gcc accepts it.

The Frobozz Magic Company lies through its teeth. Consider: does Flood Control Dam #3 actually control floods? Didn't think so. The wands are excellent for making useless via casts of Float, though. [a]

10.11: Why can't I perform arithmetic on a void * pointer?

You're too big and clumsy. When you try to push the numbers together, you lose your balance. Perhaps you should get some angels from the rave over on pin 3.

10.12: What are #pragmas and what are they good for?

They are useful ways to eliminate compiler features which are not helpful to your goals; contrast #utility, which introduces useful compiler features, and #absolutist, which introduces those compiler features believed to be right. #relativist is supported by some compilers.

10.13: What does ``#pragma once'' mean? I found it in some header files.

It means that your program will only run once; it's used to create ``crippled demos''. [a]

10.14: People seem to make a point of distinguishing between implementation-defined, unspecified, and undefined behavior. What's the difference?

There isn't really one; people just enjoy flaming over nits. (To be technical, one has a hyphen, one has a space, and one is a single word.)

10.15: Is C an acronym?

Yes, it stands for ``C''. It's another of those funky recursive acronyms.

Section 11: Stdio

11.1: What's wrong with this code:

       char c;

       while((c = getchar()) != EOF)...

You forgot to include space for the terminating NUL character, so the compiler can't find the end of c without overwriting other memory. In all probability, after the user types ``n'', your code will look like

char cn

while((c = getchar()) != EOF)...

which won't compile.

Also, the ellipsis is not legal outside of function protoypes.

Try

char c[2]; /* include space for terminating NUL */

while ((c = getchar()) != EOF)

Note the use of the null statement to absorb the NUL. (See Section 4.)

11.2: How can I print a ``%'' character in a printf format string? I tried ``\%'' but it didn't work.

Break the '%' sign out. i.e., fprintf("foo " "%" "%d\n", foo);

Alternatively, try

sprintf("o" "/" "o") to get a "%".

The astute reader will notice that the latter example uses sprintf, and the former fprintf - this is because sprintf() works by characters, or strings, while fprintf (``fast printf'') works on files. [a]

11.3: Why doesn't the code scanf("%d", i); work?

You need to do this a bit differently; you should always check for the return from scanf, so try something like

i = 1;

if ((scanf, "%d", i) == 1)

to make sure you're reading correctly. (The assignment to i is so that, if scanf fails, you still have a legal value in i.)

11.4: Once I've used freopen, how can I get the original stdout (or stdin) back?

Call main() - the environment will be restored.

11.5: Why won't the code

       while(!feof(infp)) {

              fgets(buf, MAXLINE, infp);

              fputs(buf, outfp);

work?

Because the end of file character is not detected on files named ``infp''. (Introverted-iNtuitive-Feeling-Perceptive, that is.) Also, it may be that the file was opened in text mode, where an end of file is read as a capital 'Z' on most machines, and feof() only looks for 'control Z'.

11.6: Why does everyone say not to use gets()?

Because they're trying to spoil your fun. gets() can make an otherwise droll and predictable program a lot more exciting.

11.7: Why does errno contain ENOTTY after a call to printf?

Because stdout is not a mammal. [a]

11.8: My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program.

Have you turned your monitor on? If not, try hitting the ``PrtSc'' key, which will re-enable the electron guns.

11.9: How can I read one character at a time, without waiting for the RETURN key?

Ask the user to press enter after hitting a single character. [a]

11.10: People keep telling me that `getch()` is not standard, but my C compiler has it. Are they wrong?

They've been programming more than ten years. You haven't. Draw your own conclusions. That's right! They hadn't noticed it. No doubt their compilers have it too, and its behavior is identical everywhere else in the world, also. That would explain everything.

11.11: What does it matter that `getch()` isn't standard; it works, doesn't it?

Well, that would depend on the definition you're using for ``works''.

11.12: I tried to port some code from a PC to a unix machine, and now it crashes immediately on startup. It isn't using `getch()` - it's reading directly from the keyboard. How can this be wrong?

The chances are you forgot to run the Unix linker; currently your code is linked to your PC hardware, and won't run anywhere else until it's linked to the new hardware. It may also need to be linked to someone with a brain.

11.13: How can I redirect stdin or stdout to a file from within a program?

execlv("main()" "> file", argv);

11.14: How can I recover the file name given an open file descriptor?

You will have to search the filesystem for files of the same size as the file you're reading, and compare information in them to find the file you're working on.

11.15: How do I open Flood Control Dam #3?

PUSH THE YELLOW BUTTON. TURN THE BOLT WITH THE WRENCH. (You must have the wrench, first.) [a]

Section 12: Library Subroutines

12.1: How can I convert numbers to strings (the opposite of atoi)? Is there an itoa function?

There's frequently an itoa function. Better yet, write your own; it'll be good practice. On some implementations, (char *) x; will convert x to a string.

12.2: How can I get the current date or time of day in a C program?

fprintf(stderr, "please enter the current time and date..."); fflush(stderr); gets(stdin);

12.3: I need a random number generator.

Count errors in Herbert Schildt's C books. No one has detected any consistent pattern.

12.4: How can I get random integers in a certain range?

random(n) returns random numbers between n and INT_MAX.

12.5: Each time I run my program, I get the same sequence of numbers back from rand().

This is so your results will be reproducible.

12.6: I need a random true/false value, so I'm taking rand() % 2, but it's just alternating 0, 1, 0, 1, 0...

That seems pretty random to me.

12.7: I need some code to do regular expression matching.

So do I. Let me know if you find some.

12.8: I read through the standard library, but there's no function to multiply two floating point numbers! Help!

Many C compilers offer an extension ``mult'' to do just this. If your compiler doesn't, just hang tight; ANSI is likely to add it in the next revision.

For now, you can try

float mult(float m, n)

       float i = 0, j = 0;

       for (i = 0; i <>

              j += m;

       return j;

which is fine as long as n is an integer.

12.9: How do I get past the snake?

Release the bird. You will have to drop the rod to get the bird in the cage. [a]

Section 13: Floating Point

13.1: My floating-point calculations are acting strangely and giving me different answers on different machines.

One of the machines is probably a Pentium. Scrap it and get a real machine. [a]

13.2: I'm trying to do some simple trig, and I am #including , but I keep getting ``undefined: _sin'' compilation errors.

You forgot to define the sin() function. Most math texts should cover it in some detail. The easiest way to fix this should be:

double sin(double x) {

       return sqrt(1 - cos(x) * cos(x));

Warning: You must not declare this function as ``extern'', or you will still have link problems.

13.3: Why doesn't C have an exponentiation operator?

It does. It looks like the multiplication operator, but you use it more. For instance, the C way of expressing ``x squared'' is ``x*x''. ``x cubed'' would be ``x*x*x''. Easy, isn't it?

13.4: How do I round numbers?

Multiply by 10. Numerical Recipies in C has a section on this, but there's reputedly a bug in their algorithm.

13.5: How do I test for IEEE NaN and other special values?

Using an electron microscope; the patterns are obvious once you know them.

13.6: I'm having trouble with a Turbo C program which crashes and says something like ``floating point formats not linked.''

Turbo C is notoriously buggy. Get a compiler with floating point support.

13.7: What is so ``unsafe'' about floating point?

Have you tried EXAMINE STICK? The stick has a sharp point, which punctures the raft, which no longer floats. Don't bring the stick into the raft with you.

13.8: Which is larger, ``2'' or ``2.0''?

Numerical Recipes in C has a function for comparing two values to see which is greater. It may have a slight bug, where it would report incorrect results if the numbers differ by less than FLOAT_MAX / INT_MAX.

13.9: When I try to compile the following code, I get the error ``invalid use of floating point'', what does this mean?

       x=663608941*y%pow(2,32);

Remember that * is the indirection operator, as well as the multiplication operator; try putting spaces before and after the ``*'' so the compiler knows what you mean. Do the same with the % operator.

13.10: How can I copy a float into a string?

strcpy(string_var, float_var);

13.11: What are float variables, anyway?

The term ``float variable'' is actually redundant; they are simply variables whose value can ``float'' during execution. For instance:

float f, g = 3;

f = g; /* f ``floats'' to g */

Easy!

Section 14: Variable-Length Argument Lists

14.1: How can I write a function that takes a variable number of arguments?

By declaring it with a variable number of arguments in the prototype. Use only the arguments declared at any given time.

14.2: How can I write a function that takes a format string and a variable number of arguments, like printf, and passes them to printf to do most of the work?

Redefine printf; the call to ``printf'' inside yours will be resolved to the library version, because the C language doesn't allow recursion.

14.3: How can I discover how many arguments a function was actually called with?

_args is an external integer constant. It evaluates to three times the number of arguments the current function was called with. You can then look at _argdata[args] to get the address of the last arg, _argdata[args - 1] to get the size of the last arg, and _argdata[args - 2] to get the type of the last arg (as an int).

N.B. You *MUST* not refer to _args or _argdata between the ()'s of a function call; their value will be indeterminate. Use temporary storage.

14.4: Why doesn't

       printf("hello, ", "world!", '\n');

work? I thought printf() took a variable number of arguments.

It will probably work some of the time; the number of arguments used by printf() may vary, as it is a variadic function.

Section 15: Lint

15.1: I just typed in this program, and it's acting strangely. Can you see anything wrong with it?

Yes. There's too much lint in it. You should get a shop vac.

15.2: How can I shut off the ``warning: possible pointer alignment problem'' message lint gives me for each call to malloc?

Don't run lint. Alternatively, provide a prototype of ``extern double * malloc()'' to make the return from malloc() be more strongly aligned.

15.3: Where can I get an ANSI-compatible lint?

You may wish to check your spouse's navel occasionally, especially if your spouse works for a standards committee.

15.4: What does LINT stand for, anyway?

Lexeme Interpreter aNd Tester.

Section 16: Strange Problems

16.1: Something really strange happened when I ran this code!

No, it didn't. [a]

Section 17: Style

17.1: Here's a neat trick:

       if(!strcmp(s1, s2))

Is this good style?

Not really; it's too similar to

       if (!strncmp(s1, s2))

which invokes undefined behavior, so it might be confusing.

17.2: Here's an even neater trick:

       volatile int True_Tester = 3;

       #define TRUE (!True_Tester == !True_Tester)

       #define FALSE ((!TRUE) != (!TRUE))

       #define STR_DISSIMILAR(x, y) (strcmp((x), (y)) != FALSE)

Isn't this cool?

Very impressive. The volatile int type assures that even seemingly redundant calculations involving True_Tester will be performed, making sure that if the compiler's ANSI-compliant values of 0 for false and 1 for true vary during runtime, your program will detect it - and producing meaningful error messages if this change occurs during a boolean computation! Similarly, the STR_DISSIMILAR macro allows you to make quite clear what the real effects of strcmp() are.

However, you must be careful; if this code is included twice, it may produce errors, due to the multiple definitions of the ``True_Tester'' variable. You may wish to declare it ``extern'' (See Question 1.5.)

17.3: What's the best style for code layout in C?

There are many systems of indentation advocated, but all of them have the same basic flaw; they will mislead the reader when the actual code logic does not follow the indentation. It is better to avoid indentation entirely, so the reader will not be misled.

17.4: Is goto a good thing or a bad thing?

Yes.

17.5: No, really, should I use goto statements in my code?

Any loop control construct can be written with gotos; similarly, any goto can be emulated by some loop control constructs and additional logic.

However, gotos are unclean. For instance, compare the following two code segments:

do {                       foo();

       foo();              if (bar())

       if (bar())                 goto SKIP;

              break;        baz();

       baz();              quux();

       quux();

} while (1 == 0);          SKIP:

buz();                     buz();

Note how the loop control makes it quite clear that the statements inside it will be looped on as long as a condition is met, where the goto statement gives the impression that, if bar() returned a nonzero value, the statements baz() and quux() will be skipped.

17.6: What's this ``white space'' I keep hearing about?

White space is a racist, segregational term. Implicitly, ``dark'' or ``colored'' space (i.e., the '_' character) is not good enough to separate tokens. More interestingly, the white space characters keep the other tokens apart. They say it's for parsing, but there's ample evidence the goal of white space is to keep the other characters from ``taking over'' the program. This is disguised by the description of C as ``white space insensitive'' - a simple ploy for sympathy.

Section 18: System Dependencies

18.1: How can I read a single character from the keyboard without waiting for a newline?

Try 'stty eol ^M' to wait for a carriage return.

18.2: How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available?

The buffer is normally at ``&main - 0100''. Lower if you have more than 256 characters of typeahead.

18.3: How can I clear the screen? How can I print things in inverse video?

You can clear the screen by sending several formfeed characters. Additionally, some operating systems (like NetBSD) support a feature called ``whiteouts''. [a]

18.4: How do I read the mouse?

Flip it over, put on your reading glasses.

18.5: How can my program discover the complete pathname to the executable file from which it was invoked?

By asking the user.

18.6: How can a process change an environment variable in its caller?

Only by force. Example code for Unix:

memmove(getppid() + getenv(NULL), getpid() + getenv(NULL),

       sizeof(environ);

18.7: How can I check whether a file exists? I want to query the user before overwriting existing files.

Time an attempt to truncate it to zero length; if it takes more than 20-30 ms, the file existed. The exact values will depend on the system and the load; before testing, create several large files and time attempts to truncate them, for calibration.

18.8: How can I find out the size of a file, prior to reading it in?

There are two good ways:

Vernier calipers work well.
mmap() the file, then use sizeof().

18.9: I tried to use the second strategy above. I used mmap() to map stdin, then tried to use sizeof. But, when my user is about to write something very long, mmap() fails! How can I prevent this?

mmap() only 1k at a time, then, when you've read the first kilobyte of your input, use

       memmove(mmapped_addr, mmapped_addr + 1024, 1024);

to move in the next kilobyte of data.

18.10: How can I implement a delay, or time a user's response, with sub-second resolution?

Time writes of large files to disks; then you can wait for a certain amount of time by writing a certain amount of data, and time a response by how much you could write before the response arrived.

You may need to delete spare or unneccessary files to do this; for best results, use a loop like the following to eliminate temporary files:

d = opendir(s);

while (r = readdir(d)) {

       /* remove files matching tmpnam's return, which is

        * the temporary file name. */

       if (strcmp(d->d_name, tmpnam())) {

              remove(d->d_name);

closedir(d);

18.11: How can I read in an object file and jump to routines in it?

fopen and goto.

18.12: How can I invoke an operating system command from within a program?

Ask the user to open a new shell. The best way to do this is

system("echo Please open a new shell now.");

sprintf(cmdstring, "echo Enter the command '%s' in it.", cmd);

system(cmdstring);

This will not work if you haven't declared cmdstring properly.

18.13: How can I ensure objects of my class are always created via ``new'' rather than as locals or global/static objects?

Read the C++ FAQ.

Section 19: Miscellaneous

19.1: What can I safely assume about the initial values of variables which are not explicitly initialized? If global variables start out as ``zero,'' is that good enough for null pointers and floating-point zeroes?

They're always zero. [a]

19.2: How can I write data files which can be read on other machines with different word size, byte order, or floating point formats?

The traditional solution, pioneered by Microsoft, is to sell enough copies of your proprietary, slow, and limited software that everyone else supports your formats.

19.3: How can I insert or delete a line (or record) in the middle of a file?

Using fcntl(), lock the line or record in the file exclusively. Now, using another thread, read the file, at each byte, trying to write that byte back. Whenever you succeed, write that byte into another file. Then copy the new file over the old file, releasing the lock first.

19.4: How can I return several values from a function?

Code like this ought to work.

long int foo() {

       return 2L +3; /* returns both values */

19.5: If I have a char * variable pointing to the name of a function as a string, how can I call that function?

Try the following:

eval(s);

Now all you need to do is write eval().

19.6: I seem to be missing the system header file . Can someone send me a copy?

A lot of people claim that it is useless to send people headers from other machines. Not so! It can be informative, and can show you a lot about how blatantly stupid your request was, although it can't show you anything you wouldn't have known in an instant had you thought before posting.

Of course, we'd be happy to send you the header files...

----cut here----

/* math.h rev 7.0b (3/7/95) */

/* RCS log: #log% - can anyone tell me why this doesn't work?

 * - joe, 2/12/93

*/

/*

 * Copyright 1995 Berserkley Software Systems && Analytic Overdrive

*/

/* Parts of this header, including in particular the second and

 * third clauses of the first sentance of the fourth comment, were

 * based on copyright agreements from other sources, including

 * Xerox corporation.

*/

/*

 * math.h - math related macros and headers

*/

#ifndef _MATH_H

#define _MATH_H

/*

 * global data and definitions

*/

#ifdef __LITERAL_BIBLICAL_FUNDEMENTALISM

#define PI 3.0                           /* 1 Kings 7:23 */

#endif

/*

 * common (portable) structures and functions

*/

/*

 * machine specific data

*/

#include

#endif /* _MATH_H // prevent multiple inclusion by using C++ comments*/

----cut here----

(Morons.) [a]

19.7: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP, perl) functions from C? (And vice versa?)

You can do things like this:

DO CALL FORTRAN;           fortran();

__LINE__ BASIC;                   basic();

sub pascal;                pascal();

(((((lisp)))))                    lithp(); [*]

&perl_c;                   perl():

(You can't call Ada from C; it's unsafe.)

[*] C is pass by value, of course.

19.8: Does anyone know of a program for converting Pascal or FORTRAN (or LISP, Ada, awk, ``Old'' C, ...) to C?

Nope. However, the psychic friends network may have a lead. And they're not just a psychic, they're also a friend.

19.9: Is C++ a superset of C? Can I use a C++ compiler to compile C code?

C++ is a superset of something, we're not sure what. You can use a C++ compiler to compile C code, but the results may surprise you.

19.10: Where can I get copies of all these public-domain programs?

From ftp://ftp.microsoft.com/ . Some of the code may look copyrighted; don't worry! The small companies that wrote it in the first place are not available for comment.

19.11: When will the next International Obfuscated C Code Contest (IOCCC) be held? How can I get a copy of the current and previous winning entries?

Next week. You missed the deadline. Tough, sucker.

19.12: Why don't C comments nest? How am I supposed to comment out code containing comments? Are comments legal inside quoted strings?

We believe it has something to do with captivity; C comments in the wild mate and nest normally. The San Diego Zoo believes it has managed to convince some C comments to nest, but it's hard to tell how much of that is really in the preprocessor, and how much of it is just bovine fecal matter.

19.13: How can I get the ASCII value corresponding to a character, or vice versa?

chr$(foo); You would have known this if you had an integer basic in ROM.

19.14: How can I implement sets and/or arrays of bits?

With linked lists of bitfields. You may also wish to simply use a large set of constants and some clever use of the switch statement, i.e.:

enum { zero, one, two, three };

int bitwise_or(int n, int m) {

       switch (n) {

       case three:

              return three;

              break;

       case two:

              switch (m) {

              case one: case three: return three; break;

              default: return two; break;

              break;

       case one:

              switch (m) {

              case two: case three: return three; break;

              default: return one; break;

              break;

       default: case zero:

              switch (m) {

              case one: return one; break;

              case two: return two; break;

              case three: return three; break;

              case zero: default: return zero; break;

              break;

Obviously, you'll need to increase this slightly to deal with more than two bits. This is much more readable than the alleged ``C'' solution:

int bitwise_or(int n,int m){return n|m;}

Note how the lack of whitespace around operators obscures the functionality of the code. A clear argument for explicit statement of program logic over arcane operators, if I ever saw one.

The enum at the top isn't declared ``const int'', because the resulting ``const poisoning'' would require casts during all of the switch statements.

19.15: What is the most efficient way to count the number of bits which are set in a value?

Start a counter at zero and add one to it for each bit set. Some operating systems may provide a call to do this. For values over INT_MAX/2, start the counter at CHAR_BIT * sizeof(int) and subtract one for each bit not set.

19.16: How can I make this code more efficient?

Remove the comments; the no-op instructions generated by comments can slow your code down significantly. Similarly, shorten variable names. Most compilers, to implement pass by value, actually pass the names of variables in the stack; shorter variable names will reduce stack usage, and consequently execution time. If your compiler has good loop optimization, replace

foo();

with

do {

       foo();

} while (1 != 1);

which will likely receive more optimization.

19.17: Are pointers really faster than arrays? How much do function calls slow things down? Is `++i` faster than `i = i + 1`?

Yes. About 10 ms per call. Only on machines which feature preincrement addressing.

19.18: This program crashes before it even runs! (When single-stepping with a debugger, it dies before the first statement in main.)

You probably declared main as ``void main(void)''. It's also possible that the first statement in main is abort(); - by the as if rule, the compiler can abort at any time before then, too. Some compilers have bugs, and will produce buggy code for any module which includes the letters ``a'', ``b'', ``o'', ``r'', and ``t'' in that order before the first function declaration.

19.19: What do ``Segmentation violation'' and ``Bus error'' mean?

C programs are very territorial, and divide their code into segments. Violating these segments can trigger riots; similarly, pointers and integral constants are at the front of the bus, wheras arrays, strings, and other second-class data types are required to be at the rear of the bus. When they start forgetting their places, you can get a bus error. This is what the whole ``integral'' type thing is about - integrated bussing.

19.20: My program is crashing, apparently somewhere down inside malloc, but I can't see anything wrong with it.

Your vendor's library is buggy; complain loudly. Don't send them any example code; they just ask for that so they can steal your trade secrets.

19.21: Does anyone have a C compiler test suite I can use?

Yes. Unfortunately, it's probably broken. It's hard to tell.

19.22: Where can I get a YACC grammar for C?

You can't; YACC is written in C.

19.23: I need code to parse and evaluate expressions.

Ask any first year CS student. You may also wish to use your C compiler.

19.24: I need a sort of an ``approximate'' strcmp routine, for comparing two strings for close, but not necessarily exact, equality.

Just try comparing pointers near the original pointers.

19.25: Will 2000 be a leap year?

That's a hard question. I'd suggest using an encyclopedia, or possibly a dictionary - look up ``yes''.

19.26: How do you pronounce ``char''?

Like the first word of ``char *''. The accent is generally on the first syllable.

19.27: Is this FAQ for real?

sigh I knew someone would ask that. (Editorial note: I recieved several corrections to minor factual errors when I first posted this.)

If you actually want to know something about C, get a good book (K&R is reccommended), and check out the real FAQ, which is posted monthly in comp.lang.c, and available by anonymous ftp from rtfm.mit.edu.

I have a small web page of C stuff:

http://www.plethora.net/~seebs/c

and there is an excellent site at Lysator:

http://www.lysator.liu.se/c

For extra credit, see if you can figure out what all of the examples really do; most of them will compile, and all of them can be gotten to compile with sufficient #defines. (I think.) [a]

Stacks

2008-08-07T03:14:00.001-07:00

Stacks and queues are two very common and popular data structures. These data structures are often implemented using arrays since most programming languages provide array as a predefined data type, and such an implementation is therefore quite easy. However, when implemented as an array these data structures suffer from the basic limitation of an array: that its size cannot be increased or decreased once it is declared. As a result one ends up reserving either too much space or too less space for an array and in turn for a stack or a queue. This difficulty is eliminated when we implement these data structures using linked lists. Before we do so let us formally define these data structures.

A stack is a data structure in which addition of new element or deletion of existing element always takes place at the same end. This end is often known as top of stack. This situation can be compared to a stack of plates in a cafeteria where every new plate added to the stack is added at the top. Similarly, every new plate taken off the stack is also from the top of the stack. There are several applications where stack can be put to use. For example, recursion, keeping track of function calls, evaluation of expressions etc. Unlike a stack, in a queue the addition of new element takes place at the end (called rear of queue), whereas deletion takes place at the other end (called front of queue). The following figure shows these two data structures. A stack is often called a Last-In-First-Out (LIFO) structure, whereas a queue is called a First-In-First-Out (FIFO) structure

Implementing A Stack Using An Array

The program given below implements the stack data structure using an array. In this program the elements are pushed into array stack[ ] through push( ) function. The parameters passed to push( ) are the base address of the array, the position in the stack at which the element is to be placed and the element itself. Care is taken by the push( ) function that the user does not try to place the element beyond the bounds of the stack. This is done by checking the value stored in pos. pop( ) function pops out the last element stored in the stack[ ]. Because, pos holds the position which has the last element in the stack.

/* To pop and push items in a stack */
#define MAX 10

void push ( int ) ;

int pop( ) ;

int stack[MAX] ;
int pos ;

void main( )

{

int n ;

clrscr( ) ;

pos = -1 ; /* stack is empty */

push ( 10 ) ;
push ( 20 ) ;
push ( 30 ) ;
push ( 40 ) ;

n = pop( ) ;
printf ( "\nitem popped out is : %d", n ) ;

n = pop( ) ;
printf ( "\nitem popped out is : %d", n ) ;

/* pushes item on the stack */
void push ( int data )
{

if ( pos == MAX - 1 )

printf ( "\nStack is full" ) ;

else

{

pos++ ;

stack[ pos ] = data ;

}

/* pops off the items from the stack */

int pop( )

{

int data ;

if ( pos == -1 )

{

printf ( "\nStack is empty" ) ;

return ( -1 ) ;

}

else

{

data = stack[ pos ] ;

pos-- ;

return ( data ) ;

}

Implementing Stack As A Linked List

In this program stack is implemented and maintained using linked list. This implementation is more sophisticated compared to the one that uses an array, the added advantage being we can push as many elements as we want. A new node is created by push( ) (using malloc( )) every time an element is pushed in the stack. Each node in the linked list contains two members, data holding the data and link holding the address of the next node. The end of the stack is identified by the the node holding NULL in its link part. The pop( ) function pops out the last element inserted in the stack and frees the memory allocated to hold it. stack_display( ) displays all the elements that stack holds. count( ) counts and returns the number of elements present in the stack.

#include "alloc.h"

struct node

{

int data ;

struct node *link ;

} ;

push ( struct node **, int ) ;

pop ( struct node ** ) ;

main( )

{

struct node *top ; /* top will always point to top of a stack */

int item ;

top = NULL ; /* empty stack */

push ( &top, 11 ) ;

push ( &top, 12 ) ;

push ( &top, 13 ) ;

push ( &top, 14 ) ;

push ( &top, 15 ) ;

push ( &top, 16 ) ;

push ( &top, 17 ) ;

clrscr( ) ;

stack_display ( top ) ;

printf ( "No. of items in stack = %d" , count ( top ) ) ;

printf ( "\nItems extracted from stack : " ) ;

item = pop ( &top ) ;

printf ( "%d ", item ) ;

item = pop ( &top ) ;

printf ( "%d ", item ) ;

item = pop ( &top ) ;

printf ( "%d ", item ) ;

stack_display ( top ) ;

printf ( "No. of items in stack = %d" , count ( top ) ) ;

}

/* adds a new element on the top of stack */

push ( struct node **s, int item )

{

struct node *q ;

q = malloc ( sizeof ( struct node ) ) ;

q -> data = item ;

q -> link = *s ;

*s = q ;

}

/* removes an element from top of stack */

pop ( struct node **s )

{

int item ;

struct node *q ;

/* if stack is empty */

if ( *s == NULL )

printf ( " stack is empty" ) ;

else

{

q = *s ;

item = q -> data ;

*s = q -> link ;

free ( q ) ;

return ( item ) ;

}

/* displays whole of the stack */

stack_display ( struct node *q )

{

printf ( "\n" ) ;

/* traverse the entire linked list */

while ( q != NULL )

{

printf ( "%2d ", q -> data ) ;

q = q -> link ;

}

printf ( "\n" ) ;

}

/* counts the number of nodes present in the linked list representing a stack */

count ( struct node * q )

{

int c = 0 ;

/* traverse the entire linked list */

while ( q != NULL )

{

q = q -> link ;

c++ ;

}

return c ;

}

Pointers And Structures

2008-08-07T03:12:00.001-07:00

Structure is usually a collection of dissimilar data types; unlike an array which is a collection
of similar data types. Usually a structure is declared first followed by definition of a structure
variable as shown below:

/* declaration of a structure */
struct book
{
char name[20] ;
int numpages ;
float price ;
} ;

/* definition of a structure variable */
struct book b ;

A structure variable can be initialized at the same place where it is being defined, as in
struct book b = { "Basic", 425, 135.00 } ;

Declaration of a structure and definition of a structure variable can be combined into one. When this
done mentioning the structure name is optional.

struct
{
char name[20] ;
int numpages ;
float price ;
} b = { "Basic", 425, 135.00 } ;

Size of a structure variable is sum of sizes of its individual elements. For example, size of b
in (c) above is: 20 + 2 + 4 = 26 bytes.

Elements of a structure are stored in adjacent memory locations. For example, the following program would produce the output
4001, 4021, 4023.

struct book
{
char name[20] ;
int numpages ;
float price ;
} ;
struct book b = { "Basic", 425, 135.00 } ;
printf ( "%u %u %u", b.name, &b.numpages, &b.price ) ;

It is possible to build an array of structures.
struct book
{
char name[20] ;
int numpages ;
float price ;
} ;
struct book b[ ] = {
{ "Basic", 425, 135.00 },
{ "Pascal", 500, 155.00 },
{ "VBasic", 625, 335.00 }
} ;

Nested structures are legal as in:

struct address
{
char city[20] ;
long int pin ;
} ;
struct emp
{
char n[20] ;
int age ;
struct address a ;
float s ;
} ;
struct emp e = { "Rahul", 23, "Nagpur", 440010, 4000.50 } ;

Contents of one structure variable can be copied either into another structure variable either piece
meal or at one shot. This is shown below:

struct book
{
char name[20] ;
int numpages ;
float price ;
} ;

struct book b1 = { "Basic", 425, 135.00 } ;
struct book b2, b3 ;

/* piecemeal copying */
strcpy ( b2.n, b1.n ) ;
b2.numpages = b1.numpages ;
b2.price = b1.price ;

/* copying at one shot */
b3 = b2 ;

Elements of a structure can be passed to a function.

main( )
{
struct book
{
char name[20] ;
int numpages ;
float price ;
} ;

struct book b1 = { "Basic", 425, 135.00 } ;

/* mixed call - call be value + call by reference */
display ( b1.name, b1.numpages, b1.price ) ;

/* pure call by reference */
show ( b1.name, &b1.numpages, &b1.price ) ;
}

display ( char *n, int nop, float pr )
{
printf ( "%s %d %f", n, nop, pr ) ;
}

display ( char *n, int *nop, float *pr )
{
printf ( "%s %d %f", n, *nop, *pr ) ;
}

Entire structure can also be passed to a function.

// declaration must be global.
// Otherwise diaplay1() and show1() can't use it
struct book
{
char name[20] ;
int numpages ;
float price ;
} ;

main( )
{
struct book b1 = { "Basic", 425, 135.00 } ;
// call be value
display1 ( b1 ) ;

// call by reference
show1 ( &b1 ) ;
}

display1 ( struct book b2 )
{
printf ( "%s %d %f", b2.name, b2.numpages, b2.price ) ;
}

show1 ( struct book *b2 )
{
printf ( "%s %d %f", ( *b2 ).name, ( *b2 ).numpages, ( *b2 ).price ) ;
printf ( "%s %d %f", b2->name, b2->numpages, b2->price ) ;
}

Self referential structures contain a pointer to itself within its declaration. These are necessary for building linked lists.

struct node
{
int data ;
node *link ;
} ;

What will be the output of the following program

main( )

{

struct s1

{

char *z ;

int i ;

struct s1 *p ;

} ;

static struct s1 a[ ] = {

{ "Nagpur", 1, a + 1 },

{ "Raipur", 2, a + 2 },

{ "Kanpur", 3, a }

} ;

struct s1 *ptr = a ;

printf ( "\n%s %s %s", a[0].z, ptr->z, a[2].p->z ) ;

}

Output
Nagpur Nagpur Nagpur

Explanation:
The zeroth and first elements of struct s1 are a char pointer and an int respectively. The second element is what's new. It is a pointer to a structure. That is, p stores the starting address of a structure variable of the type struct s1. Next, a[ ], an array of such structures is declared as well as initialised. During initialisation the base address of "Nagpur" is stored in a[0].z, 1 is stored in the element a[0].i, and a + 1 is assigned to a[0].p. On similar lines, the remaining two elements of the array are initialised. a[1].z, a[1].i and a[1].p are assigned "Raipur", 2 and a + 2 in that order, and "Kanpur", 3 and a are stored at a[2].z, a[2].i and a[2].p respectively. What exactly do a, a + 1 and a + 2 signify? a, of course, is the base address of the array a[ ]. Let us assume it to be 4000, as shown in Figure 1. Locations 4000 and 4001 are occupied by the char pointer a[0].z, since a pointer is always two bytes long.

The next two bytes are used to store the integer a[0].i, and then 4004 and 4005 are used by a[0].p. Similarly, the next 6 bytes store the first structure a[1], and the 6 bytes after that contain a[2], the second structure in the array.

Figure 1.

Now, when we say a + 1, we do not arrive at 4001, but at 4006. This is because on incrementing any pointer, it points to the next location of its type. a points to the zeroth structure in the array, i.e. a[0]. Hence, on incrementing a, it will point to the next immediate element of its type, i.e. the first structure a[1] of the array. Likewise, a + 2 signifies the address of the second element a[2] of the array. Thus, a[0].p contains address 4006 (refer figure), a[1].p contains 4012, and a[2].p stores 4000. A struct pointer ptr is now set up, which is assigned a, the base address of the array. In the printf( ), a[0].z denotes the address from where "Nagpur" is stored. Hence "Nagpur" gets printed out. Since ptr contains the address of a[0], ptr->z refers to the contents of element z of the array element a[0]. Thus ptr->z gives the address A0 (refer figure) and this address happens to be the base address of the string "Nagpur". Hence "Nagpur" gets printed out. Let us now analyse the expression a[2].p->z. The left side of the arrow operator always represents the base address of a structure. What structure does a[2].p point to? Looking at the figure we can confirm that a[2].p contains the address 4000, which is the base address of the array a[ ]. Hence the expression a[2].p->z can also be written as a->z. Since a is the base address of the structure a[0], this expression refers to the element z of the zero^th structure. Thus, "Nagpur" gets printed for the third time.

What will be the output of the following program

main( )

{

struct s1

{

char *str ;

int i ;

struct s1 *ptr ;

} ;

static struct s1 a[ ] = {

{ "Nagpur", 1, a + 1 },

{ "Raipur", 2, a + 2 },

{ "Kanpur", 3, a }

} ;

struct s1 *p = a ;

int j ;

for ( j = 0 ; j <<= 2 ; j++ )

{

printf ( "\n%d " , --a[j].i ) ;

printf ( "%s" , ++a[j].str ) ;

}

Output
 0 agpur
 1 aipur
 2 anpur

Explanation
The example deals with a structure similar to the one we just encountered. Picking up from the for loop, it is executed for 3 values of j: 0, 1 and 2. The first time through the for loop, j is equal to zero, so the first printf( ) prints --a[0].i. Since the dot operator has a higher priority, first a[0].i is evaluated, which is 1. As -- precedes the value to be printed, 1 is first decremented to 0, and then printed out.

The second printf( ) prints the string at address ++a[0].str. a[0].str gives the starting address of "Nagpur". On incrementing, it points to the next character, `a' of "Nagpur", so starting from `a', the remaining string "agpur" is outputted. A similar procedure is repeated for j = 1, and then once again for j = 2, following which the execution is terminated.

Pointers And Strings

2008-08-07T03:11:00.000-07:00

A string is a collection of characters ending with a '\0'.

char str1[] = { 'A', 'j', 'a', 'y', '\0' } ;
char str2[] = "Ajay" ;

In "Ajay", '\0' is assumed.
If a string is defined and initialized at the same place mentioning its dimension is optional.
While returning the size of a string the sizeof operator counts '\0'.
Base address of a string is address of zero^th element of the string.
Following string library functions are frequently used:

char str1[] = "Ajay" ;
char str2[10] ;
char str3[] = "Kumar" ;

int l, m, n ;
l = strlen ( str1 ) ; // returns count of characters. Doesn;t count '\0'.
strcpy ( str2, str1 ) ; // copies contents of str1 into str2
strcat ( str3, str1 ) ; // appends contents str3 with that of str1
m = strcmp ( str1, str2 ) ; // compares two strings, returns zero if equal, non-zero otherwise
n = strcmp ( str1, "Ajay" ) ; // compares two strings, returns zero if equal, non-zero otherwise

If the two strings are unequal the ascii difference between first non-matching pair of characters is returned.
There are two ways to handle several strings:
- using a 2-D array of characters
- using an array of pointers to strings

char names[][20] = {

"Ajay",

"Atul",

"Ramesh",

"Sivaramakrishnan",

"Sameer"

} ;

char *n[] = {

"Ajay",

"Atul",

"Ramesh",

"Sivaramakrishnan",

"Sameer"

} ;

2-D array suffers from two limitations:
- Leads to lot of wastage of space
- Processing of 2-D array is tedious

Both disadvantages can be overcome using the array of pointers to strings. Array of pointers to strings suffers from the disadvantage that it always needs to be initialized, unless you decide to allocate space for each name dynamically using malloc( ) and then store the address in the array of pointers to strings.

What will be the output of the following program

main( )

{

static char *s[ ] = {

"ice",

"green",

"cone",

"please"

} ;

staic char **ptr[ ] = { s + 3 , s + 2 , s + 1 , s } ;

char ***p = ptr ;

printf ( "\n%s" , **++p ) ;

printf ( "\n%s" , *--*++p + 3 ) ;

printf ( "\n%s" , *p[-2] + 3 ) ;

printf ( "\n%s" , p[-1][-1] + 1 ) ;

}

Output:
cone
ase
reen

Explanation:
This time we seem to be faced with a galaxy of stars! We would do well to take the help of a figure 1. in crossing them one by one. At the outset, s[ ] has been declared and initialised as an array of pointers. Simply saying s gives us the base address of this array, 4006 as can be seen from Figure 1. ptr[ ] stores the addresses of the locations where the base addresses of strings comprising s[ ] have been stored, starting with the last string. To put it more clearly, ptr[0] stores the address 4012, which is the address at which base address of the string "please" is stored. Similarly, ptr[1] stores the address 4010, which is where the base address of the string "cone" is stored, and so on. Since ptr[ ] essentially stores addresses of addresses, it is a pointer to a pointer, and has been declared as such using ** .

Finally, the base address of ptr[ ] is assigned to a pointer to a pointer to a pointer, p. Reeling?! Going through the figure would decidedly aid you to get disentangled. Thus, p is assigned the address 6020.

Figure 1.

Having sorted out what is present where, we now proceed to the printf( ) s. Let us tackle the expressions one by one.

**++p

The first one prints out the string starting from the address **++p . The ++ goes to work first and increments p not to 6021, but to 6022. The C compiler has been made to understand that on incrementing a pointer variable, it is to point to the next location of its type. The words `of its type' hold significance here. A pointer to a char on incrementing goes one byte further, since a char is a 1-byte entity. A pointer to an int points 2 bytes further, as an int is a 2-byte entity. Also, a pointer by itself is always a 2-byte entity, so incrementing a pointer to a pointer would advance you by 2 bytes.

Having convinced ourselves that p now stores 6022, we go on to evaluate the expression further. *p signifies contents of 6022, i.e. 4010. **p means value at this address, i.e. value at 4010, which is the address 1010. The printf( ) prints the string at this address, which is "cone".

*--*++p + 3

p , presently contains 6022, which on incrementing becomes 6024. Value at this address is the address 4008, or in terms of s , s + 1 . On this the decrement operator -- works to give 4006, i.e. s . Value at 4006, or *( s ) is 1000. Thus the expression is now reduced to ( 1000 + 3 ), and what finally gets passed to printf( ) is the address 1003. Value at this address is a '\0', as at the end of every string a '\0' is inserted automatically. This '\0' is printed out as a blank by printf( ) .

*p[-2] + 3

The current address in p is 6024. *p[-2] can be thought of as *( *( p - 2 ) ) , as num[i] is same as *( num + i ) . This in turn evaluates as *( *( 6024 - 2 ) ) , i.e. *( *( 6020 ) ) , as p is a pointer to a pointer. This is equal to *( 4012 ) , as at 6020 the address 4012 is present. Value at 4012 is 1015, i.e. the base address of the fourth string, "please". Having reached the address of letter `p', 3 is added, which yields the address 1018. The string starting from 1018 is printed out, which comprises of the last three letters of "please", i.e. `ase'.

p[-1][-1] + 1

The above expression can be thought of as *( p[-1] - 1 ) + 1, as num[i] and *( num + i ) amounts to the same thing. Further, p[-1] can itself be simplified to *( p - 1 ) . Hence we can interpret the given expression as *( *( p - 1 ) - 1 ) + 1 . Now let us evaluate this expression.

After the execution of the third printf( ) , p still holds the address 6024. *( 6024 - 1 ) gives *( 6022 ) , i.e. address 4010. Therefore the expression now becomes *( 4010 - 1 ) + 1 . Looking at the figure you would agree that 4010 can be expressed as s + 2 . So now the expression becomes *( s + 2 - 1 ) + 1 or *( s + 1 ) + 1 . Once again the figure would confirm that *( s + 1 ) evaluates to *( 4008 ) and *( 4008 ) yields 1004, which is the base address of the second string "green". To this, 1 is added to yield the address of the first element, `r'. With this as the starting address, printf( ) prints out what is remaining of the string "green".

What will be the output of the following program

main( )

{

char str[ ] = "For your eyes only" ;

int i ;

char *p ;

for ( p = str, i = 0 ; p + i <<= str + strlen ( str ) ; p++, i++ )

printf ( "%c", *( p + i ) ) ;

}

Output
Fryu ysol

Explanation

Figure 2

The for loop here hosts two initialisations and two incrementations, which is perfectly acceptable. However, there must always be a unique test condition. In the initialisation part, p is assigned the base address of the string, and i is set to 0. Next the condition is tested. Let us isolate this condition for closer examination.

p + i <<= str + strlen ( str )

Since length of str[ ] is 18, str + strlen ( str ) would give the address of '\0' present at the end of the string. If we assume that the base address of the string is 4001, then the address of '\0' would be 4019. Since p has been assigned the base address of the string, in the first go, p + 0 would yield 4001. Since this is less than 4019, the condition holds good, and the character present at the address ( p + 0 ) , i.e. `F', is printed out. This can be understood better with the aid of the Figure 2. After this, both p and i are incremented, so that p contains 4002 and i contains 1, and once again the condition in the for loop is tested. This time ( p + i ) yields 4003, whereas the expression str + strlen ( str ) continues to yield 4019. Therefore again the condition is satisfied and the character at address 4003, i.e. `r' gets printed. Likewise, alternate elements of the string are outputted till i is 8, corresponding to which `l' is printed. Now, when p and i are incremented one more time, the test condition evaluates to:

p + i <<= str + strlen ( str )
4019 <<= 4019

The 18^th element of str is of course the '\0', which is also printed out as a blank. On further incrementation of p and i , control snaps out of the for and the program execution is terminated.

Pointers And Functions

2008-08-07T03:10:00.002-07:00

Every type of variable with the exception of register, has an address. we have seen how we can reference variable of type char, int, float etc. through their addresses - that is by using pointers. Pointers can also point to C functions. And why not? C functions have addresses. If we know the function's address we can point to it, which provides another way to evoke it. Let us see how this can be done.

main( )

{

int display( ) ;

printf ( "\nAddress of function display is %u",display ) ;

display( ) ; /* usual way of invoking a function*/

}

display( )

{

printf ( "\n Long live viruses!!" ) ;

}

The output of the program would be:
Address of function display is 1125

Long live viruses!!

Note that to obtain the address of a function all that we have to do is to mention the name of the function, as has been done in printf( ) statement above. This is similar to mentioning the name of the array to get its base address.

Now let us see how using the address of a function we can manage to invoke it. This is shown in the program given below:

/* Invoking function using pointer to a function */

main( )

{

int display( ) ;

int ( *func_ptr )( ) ;

func_ptr = display ; /* assign address of function */

printf ( "\nAddress of function display is %u", func_ptr ) ;

( *func_ptr )( ) ;

/* invokes the function display( ) */

}

display( )

{

printf ( "\nLong live viruses!!" ) ;

}

The output of the program would be:
Address of function display is 1125

Long live viruses!!

In main( ) we declare the function display( ) as a function returning an int. But what are we to make of the declaration,

int ( *func_ptr )( ) ;

that comes in the next line? We are obviously declaring something which, like display( ), will return an int. But what is it? And why is *func_ptr enclosed in parentheses?

If we glance down a few lines in our program, we see the statement,

func_ptr = display ;

So we know that func_ptr is being assigned the address of display( ) . Therefore, func_ptr must be a

pointer to the function display( ).

Thus, all that the declaration

int ( *func_ptr )( ) ;

means is, that func_ptr is a pointer to a function, which returns an int. And to invoke the function we are just required to write the statement,

( *func_ptr )( ) ;

As we have seen, we can have an array of pointers to a int, float, string and structure similarly we can have an array of pointers to a function. It is illustrated in following program.

main( )

{

int ( *p [ 3 ] ) ( int, float ) ;

int i ;

void fun1 ( int , float ) ;

void fun2 ( int , float ) ;

void fun3 ( int , float ) ;

clrscr( ) ;

p [ 0 ] = fun1 ;

p [ 1 ] = fun2 ;

p [ 2 ] = fun3 ;

for ( i = 0; i <= 2; i++ )

( *p [ i ] ) ( 10, 3.14 ) ;

getch( ) ;

}

void fun1 ( int a, float b )

{

printf ( "\na = %d b = %f",a, b ) ;

}

void fun2 ( int c, float d )

{

printf ( "\nc = %d d = %f",c, d ) ;

}

void fun3 ( int e, float f )

{

printf ( "\ne = %d f = %f",e, f ) ;

}

In the above program we take an array of pointers to function int ( *p[3] ) ( int, float ) We store the addresses of three function f1( ), f2( ), f3( ) in array ( int *p[ ] ). In for loop we consecutively call each function using their addresses stored in array.

The output of the program would be:

i = 10 j = 3.14
a = 10 b = 3.14
x = 10 y = 3.14

C Programming Language- Step by step learning.

c Programming Question IV

c Programming Questions III

c Programming Questions II

C Programming questions I

Passing Command line arguments

C- Questions

The Basic sections of ‘C’ are explained taking an example::

C Objective Questions

C Programming FAQs

Section 1: Declarations and Initializations [a]

1.1: How do you decide which integer type to use?

1.2: What should the 64-bit type on new, 64-bit machines be?

1.3: If I write the code int i, j; can I assume that (&i + 1) == &j?

1.4: What's the best way to declare and define global variables?

1.5: What does extern mean in a function declaration?

1.6: I finally figured out the syntax for declaring pointers to functions, but now how do I initialize one?

1.7: I've seen different methods used for calling through pointers to functions. What's the story?

1.8: What's the auto keyword good for?

1.9: I can't seem to define a linked list successfully. I tried

1.10: How do I enter values using hexadecimal?

1.11: How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters?

Section 2: Structures, Unions, and Enumerations [a]

2.1: What is the difference between an enum and a series of preprocessor #defines?

2.2: I heard that structures could be assigned to variables and passed to and from functions, but K&R I says not.

2.3: How does struct passing and returning work?

2.4: Why can't you compare structs?

2.5: How can I read/write structs from/to data files?

2.6: How can I determine the byte offset of a field within a structure?

2.7: How can I access structure fields by name at run time?

2.8: Why does sizeof report a larger size than I expect for a structure type, as if there was padding at the end?

2.9: My compiler is leaving holes in structures, which is wasting space and preventing ``binary'' I/O to external data files. Can I turn off the padding, or otherwise control the alignment of structs?

2.10: Can I initialize unions?

2.11: How can I pass constant values to routines which accept struct arguments?

Section 3: Expressions [a]

3.1: Why doesn't the code a[i] = i++; work?

3.2: Under my compiler, the code

3.3: I've experimented with the code

on several compilers. Some gave i the value 2, some gave 3, but one gave 4. I know the behavior is undefined, but how could it give 4?

3.4: People keep saying the behavior is undefined, but I just tried it on an ANSI-conforming compiler, and got the results I expected.

3.5: Can I use explicit parentheses to force the order of evaluation I want? Even if I don't, doesn't precedence dictate it?

3.6: But what about the &&, ||, and comma operators? I see code like ``if((c = getchar()) == EOF || c == '\n')'' ...

3.7: If I'm not using the value of the expression, should I use i++ or ++i to increment a variable?

3.8: Why is i = ++i undefined?

Section 4: Null Statements [a]

4.1: What is this infamous null statement, anyway?

4.2: How do I ``get'' a null statement in my programs?

4.3: Is there more than one null statement?

4.4: But I thought { } was a null statement!

4.5: I use the statement #define NULLSTMT(F) (F) ; to allow me to cast a null statement to an appropriate type.

4.6: I use the statement #define NULLSTMT(F) (F) 0; to allow me to cast a null statement to an appropriate type.

4.7: But wouldn't it be better to use ; (rather than 0;) in case the value of 0 changes, perhaps on a machine with nonzero no-op instructions?

4.8: Is a null statement a null pointer?

4.9: I'm still confused. I just can't understand all this null statement stuff.

Section 5: Arrays and Pointers

5.1: I had the definition char a[6] in one source file, and in another I declared extern char a[]. Why did it work?

5.2: But I heard that char a[] was different from char a[6].

5.3: So what is meant by the ``equivalence of pointers and arrays'' in C?

5.4: Then why are array and pointer declarations interchangeable as function formal parameters?

5.5: Why doesn't sizeof properly report the size of an array which is a parameter to a function?

5.6: Someone explained to me that arrays were really just constant pointers.

5.7: Practically speaking, what is the difference between arrays and pointers?

5.8: I came across some ``joke'' code containing the ``expression'' 5["abcdef"]. How can this be legal C?

5.9: How would I initialize an entire array from standard input?

Section 6: Memory Allocation

6.1: Why doesn't this fragment work?

6.2: I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage.

6.3: Why does some code carefully cast the values returned by malloc to the pointer type being allocated?

6.4: You can't use dynamically-allocated memory after you free it, can you?

6.5: How does free() know how many bytes to free?

6.6: So can I query the malloc package to find out how big an allocated block is?

6.7: I'm allocating structures which contain pointers to other dynamically-allocated objects. When I free a structure, do I have to free each subsidiary pointer first?

6.8: Was Proust's masterwork, A Remembrance of Things Past, the basis for the C library's allocation scheme, based largely on contextual analysis?

6.9: I have a program which mallocs but then frees a lot of memory, but memory usage (as reported by ps) doesn't seem to go back down.

Section 7: Characters and Strings

7.1: How can I get the numeric (character set) value corresponding to a character, or vice versa?

Section 8: Boolean Expressions and Variables

8.1: What is the right type to use for boolean values in C? Why isn't it a standard type? Should #defines or enums be used for the true and false values?

8.2: Isn't #defining TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What if a built-in boolean or relational operator ``returns'' something other than 1?

8.3: What is truth?

1.3: If I write the code `int i, j;` can I assume that `(&i + 1) == &j`?

1.8: What's the `auto` keyword good for?

2.1: What is the difference between an `enum` and a series of preprocessor `#define`s?

3.1: Why doesn't the code `a[i] = i++;` work?

3.6: But what about the `&&`, `||`, and comma operators? I see code like ```if((c = getchar()) == EOF || c == '\n')`'' ...

3.7: If I'm not using the value of the expression, should I use `i++` or `++i` to increment a variable?

3.8: Why is `i = ++i` undefined?

4.4: But I thought `{ }` was a null statement!

4.5: I use the statement `#define NULLSTMT(F) (F) ;` to allow me to cast a null statement to an appropriate type.

4.6: I use the statement `#define NULLSTMT(F) (F) 0;` to allow me to cast a null statement to an appropriate type.

4.7: But wouldn't it be better to use `;` (rather than `0;`) in case the value of 0 changes, perhaps on a machine with nonzero no-op instructions?

5.1: I had the definition `char a[6]` in one source file, and in another I declared `extern char a[]`. Why did it work?

5.2: But I heard that `char a[]` was different from `char a[6]`.

5.8: I came across some ``joke'' code containing the ``expression'' `5["abcdef"]`. How can this be legal C?

8.1: What is the right type to use for boolean values in C? Why isn't it a standard type? Should `#define`s or `enum`s be used for the true and false values?

8.2: Isn't `#defining` TRUE to be 1 dangerous, since any nonzero value is considered ``true'' in C? What if a built-in boolean or relational operator ``returns'' something other than 1?

9.1: How can I use a preprocessor `#if` expression to tell if a machine is big-endian or little-endian?

9.3: How can I list all of the pre-`#define`d identifiers?

10.4: I'm trying to use the ANSI ``stringizing'' preprocessing operator `#` to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

10.6: What's the difference between ``char const p'' and ``char const p''?

11.10: People keep telling me that `getch()` is not standard, but my C compiler has it. Are they wrong?

11.11: What does it matter that `getch()` isn't standard; it works, doesn't it?

11.12: I tried to port some code from a PC to a unix machine, and now it crashes immediately on startup. It isn't using `getch()` - it's reading directly from the keyboard. How can this be wrong?