CHAPTER 3: Functions
1. What is the purpose of main() function?
In C, program execution starts from the main() function. Every C program must contain a main() function. The main function may contain any number of statements. These statements are executed sequentially in the order which they are written.
The main function can in-turn call other functions. When main calls a function, it passes the execution control to that function. The function returns control to main when a return statement is executed or when end of function is reached.
In C, the function prototype of the 'main' is one of the following:
int main(); //main with no arguments
int main(int argc, char *argv[]); //main with arguments
The parameters argc and argv respectively give the number and value of the program's command-line arguments.
Example:
#include <stdio.h>
/* program section begins here */
int main() {
// opening brace - program execution starts here
printf("Welcome to the world of C");
return 0;
}/
/ closing brace - program terminates here
Output:
Welcome to the world of C
2. Explain command line arguments of main function?
In C, we can supply arguments to 'main' function. The arguments that we pass to main ( ) at command prompt are called command line arguments. These arguments are supplied at the time of invoking the program.
The main( )function can take arguments as:
main(int argc, char *argv[]) { }
The first argument argc is known as 'argument counter'. It represents the number of arguments in the command line. The second argument argv is known as 'argument vector'. It is an array of char type pointers that points to the command line arguments. Size of this array will be equal to the value of argc.
Example:
at the command prompt if we give:
C:\> fruit.exe apple mango
Then
argc would contain value 3
argv [0]
would contain base address of string " fruit.exe" which is the command name that invokes the program.
argv [1] would contain base address of string "apple"
argv [2] would contain base address of string "mango"
here apple and mango are the arguments passed to the program fruit.exe
Program:
#include <stdio.h>
int main(int argc, char *argv[]) {
int n;
printf("Following are the arguments entered in the command line");
for (n = 0; n < argc; n++) {
printf("\n %s", argv[n]);
}
printf("\n Number of arguments entered are\n %d\n", argc);
return 0;
}
Output:
Following are the arguments entered in the command line
C:\testproject.exe
apple
mango
Number of arguments entered are 3
3. What are header files? Are functions declared or defined in header files ?
Functions and macros are declared in header files. Header files would be included in source files by the compiler at the time of compilation.
Header files are included in source code using #include directive. #include<some.h> includes all the declarations present in the header file 'some.h'.
A header file may contain declarations of sub-routines, functions, macros and also variables which we may want to use in our program. Header files help in reduction of repetitive code.
Syntax of include directive:
#include<stdio.h> //includes the header file stdio.h, standard input output header into the source code
Functions can be declared as well as defined in header files. But it is recommended only to declare functions and not to define in the header files. When we include a header file in our program we actually are including all the functions, macros and variables declared in it.
In case of pre-defined C standard library header files ex(stdio.h), the functions calls are replaced by equivalent binary code present in the pre-compiled libraries. Code for C standard functions are linked and then the program is executed. Header files with custom names can also be created.
Program: Custom header files example
/****************
Index: restaurant.h
****************/
int billAll(int food_cost, int tax, int tip);
/****************
Index: restaurant.c
****************/
#include<stdio.h>
int billAll(int food_cost, int tax, int tip) {
int result;
result = food_cost + tax + tip;
printf("Total bill is %d\n",result);
return result;
}
/****************
Index: main.c
****************/
#include<stdio.h>
#include"restaurant.h"
int main() {
int food_cost, tax, tip;
food_cost = 50;
tax = 10;
tip = 5;
billAll(food_cost,tax,tip);
return 0;
}
4. What are the differences between formal arguments and actual arguments of a function?
Argument: An argument is an expression which is passed to a function by its caller (or macro by its invoker) in order for the function(or macro) to perform its task. It is an expression in the comma-separated list bound by the parentheses in a function call expression.
Actual arguments:
The arguments that are passed in a function call are called actual arguments. These arguments are defined in the calling function.
Formal arguments:
The formal arguments are the parameters/arguments in a function declaration. The scope of formal arguments is local to the function definition in which they are used. Formal arguments belong to the called function. Formal arguments are a copy of the actual arguments. A change in formal arguments would not be reflected in the actual arguments.
Example:
#include <stdio.h>
void sum(int i, int j, int k);
/* calling function */
int main() {
int a = 5;
// actual arguments
sum(3, 2 * a, a);
return 0;
}
/* called function */
/* formal arguments*/
void sum(int i, int j, int k) {
int s;
s = i + j + k;
printf("sum is %d", s);
}
Here 3,2*a,a are actual arguments and i,j,k are formal arguments.
5. What is pass by value in functions?
Pass by Value: In this method, the value of each of the actual arguments in the calling function is copied into corresponding formal arguments of the called function. In pass by value, the changes made to formal arguments in the called function have no effect on the values of actual arguments in the calling function.
Example:
#include <stdio.h>
void swap(int x, int y) {
int t;
t = x;
x = y;
y = t;
} int main()
{
int m = 10, n = 20;
printf("Before executing swap m=%d n=%d\n", m, n);
swap(m, n);
printf("After executing swap m=%d n=%d\n", m, n);
return 0;
}
Output:
Before executing swap m=10 n=20
After executing swap m=10 n=20
Explanation:
In the main function, value of variables m, n are not changed though they are passed to function 'swap'. Swap function has a copy of m, n and hence it can not manipulate the actual value of arguments passed to it.
6. What is pass by reference in functions?
Pass by Reference:
In this method, the addresses of actual arguments in the calling function are copied into formal arguments of the called function. This means that using these addresses, we would have an access to the actual arguments and hence we would be able to manipulate them. C does not support Call by reference. But it can be simulated using pointers.
Example:
#include <stdio.h>
/* function definition */
void swap(int *x, int *y) {
int t;
t = *x; /* assign the value at address x to t */
*x = *y; /* put the value at y into x */
*y = t; /* put the value at to y */
} int main() {
int m = 10, n = 20;
printf("Before executing swap m=%d n=%d\n", m, n);
swap(&m, &n);
printf("After executing swap m=%d n=%d\n", m, n);
return 0;
}
Output:
Before executing swap m=10 n=20
After executing swap m=20 n=10
Explanation:
In the main function, address of variables m, n are sent as arguments to the function 'swap'. As swap function has the access to address of the arguments, manipulation of passed arguments inside swap function would be directly reflected in the values of m, n.
7. What are the differences between getchar() and scanf() functions for reading strings?
7. What are the differences between getchar() and scanf() functions for reading strings?
8. Out of the functions fgets() and gets(), which one is safer to use and why?
Out of functions fgets( ) and gets( ), fgets( ) is safer to use. gets( ) receives a string from the keyboard and it is terminated only when the enter key is hit. There is no limit for the input string. The string can be too long and may lead to buffer overflow.
Example:
gets(s) /* s is the input string */
Whereas fgets( ) reads string with a specified limit, from a file and displays it on screen.The function fgets( ) takes three arguments.
First argument : address where the string is stored.
Second argument : maximum length of the string.
Third argument : pointer to a FILE.
Example:
fgets(s,20,fp); /* s: address of the string, 20: maximum length of string, fp: pointer to a file */
The second argument limits the length of string to be read. Thereby it avoids overflow of input buffer. Thus fgets( ) is preferable to gets( ).
9. What is the difference between the functions strdup() and strcpy()?
strcpy function:
copies a source string to a destination defined by user. In strcpy function both source and destination strings are passed as arguments. User should make sure that destination has enough space to accommodate the string to be copied. 'strcpy' sounds like short form of "string copy".
Syntax:
strcpy(char *destination, const char *source);
Source string is the string to be copied and destination string is string into which source string is copied. If successful, strcpy subroutine returns the address of the copied string. Otherwise, a null pointer is returned.
Example Program:
#include<stdio.h>
#include<string.h>
int main() {
char myname[10];
//copy contents to myname
strcpy(myname, "ARVIND");
//print the string
puts(myname);
return 0;
}
Output:
ARVIND
Explanation:
If the string to be copied has more than 10 letters, strcpy cannot copy this string into the string 'myname'. This is because string 'myname' is declared to be of size 10 characters only.
In the above program, string "nodalo" is copied in myname and is printed on output screen.
strdup function:
duplicates a string to a location that will be decided by the function itself. Function will copy the contents of string to certain memory location and returns the address to that location. 'strdup' sounds like short form of "string duplicate"
Syntax:
strdup (const char *s);
strdup returns a pointer to a character or base address of an array.
Function returns address of the memory location where the string has been copied. In case free space could not be created then it returns a null pointer.
Both strcpy and strdup functions are present in header file <string.h>
Program:
Program to illustrate strdup().
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main() {
char myname[] = "ARVIND";
//name is pointer variable which can store the address of memory location of string
char* name;
//contents of myname are copied in a memory address and are assigned to name
name = strdup(myname);
//prints the contents of 'name'
puts(name);
//prints the contents of 'myname'
puts(myname);
//memory allocated to 'name' is now freed
free(name);
return 0;
}
Output:
ARVIND
ARVIND
Explanation:
String myname consists of "ARVIND" stored in it. Contents of myname are copied in a memory address and memory is assigned to name. At the end of the program, memory can be freed using free(name);
To be contd...
A Special Thanks to www.interviewmantra.net. for providing content and to Mr. Sridhar Jammalamadaka for their valuable guidance.
