Variable number of arguments to a function

Variable number of arguments:

     Till now we have seen various types of functions in C. Some them were built-in and some user defined. There are a lot of noticeable point about them. Intuitively We can say that all the functions should possess same characteristics  whether it is user defined or built-in. We have also seen function returning some value, not returning any value, taking no argument and taking certain fixed number of arguments with certain data-types.
    Some-times a question can come in your mind if it is possible for a function to accept any number of arguments in C language. Off course it is possible !!! In fact we have used such function many times. Don't remember??? Recall our most commonly used functions printf() and scanf(). How many arguments does printf() function take??
     Let me remind you when we pass arguments to a function while calling it, the arguments are separated by a comma(,). So printf() function takes variable number of arguments. First argument to printf() is a character string and after that some other arguments of various types may or may not come. This fact encourages us to create our own user defined function which would take variable number of arguments.
Problem and solution:
    There may be some problems while implementing variable arguments. One such problem is to determine how many arguments have been passed during a particular call. To overcome that problem there is a rule that at least one argument must be specified in the declaration of function. For more ease I will be giving that argument as a number equal to the count of following arguments so that I can easily know that how many arguments are being passed.
Sample program:
     This program is one of the simplest programs for illustrating the use of variable number of arguments. There is a function sum which can take may arguments and print the sum of those.
Have a look.
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#include <stdio.h>
#include <stdarg.h>
int sum(int num, ... );
int main( void )
{
    printf("This is program to show the use of variable numbers of arguments to a function\n");
    printf( "Sum of 1,2,3,4 is %d\n", sum(4,1,2,3,4) );
    printf( "Sum of 4,5,3 is %d\n", sum(3,4,5,3) );
    printf( "Sum of 7,3,4,9,1,5 is %d\n", sum(6,7,3,4,9,1,5) );
    return( 0 );
 }
int sum(int num, ... )
{
    int answer = 0;
    va_list argptr;
    va_start( argptr, num );
    while(num-- > 0)
    answer += va_arg( argptr, int );
    va_end( argptr );
    return( answer );
} 

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Explanation:
    Declaration of the function is clear as shown. Here argptr is a variable of type va_list. After va_start function is executed here, argptr will contain list of rest of the variables. After that each value of each argument can be accessed with the help of va_arg() function. When all the arguments are finished va_end() function should be put at the end. This va_end() is analogous to fclose() in file handling.
 

Remarks: 
1. variable number of arguments means the function can take some random(but finite) number of arguments. This does not mean that the function can take infinite number of arguments. Maximum number of arguments to a function are limited and depend on the compiler.