C library macro - va_start()

Description

The C library macro void va_start(va_list ap, last_arg) initializes ap variable to be used with the va_arg and va_end macros. The last_arg is the last known fixed argument being passed to the function ie. the argument before the ellipsis.

This macro must be called before using va_arg and va_end.

Declaration

Following is the declaration for va_start() macro.

void va_start(va_list ap, last_arg);

Parameters

ap -- This is the object of va_list and it will hold the information needed to retrieve the additional arguments with va_arg.
last_arg -- This is the last known fixed argument being passed to the function.

Return Value

Example

The following example shows the usage of va_start() macro.

#include<stdarg.h>
#include<stdio.h>

int sum(int, ...);

int main(void)
{
   printf("Sum of 10, 20 and 30 = %d\n",  sum(3, 10, 20, 30) );
   printf("Sum of 4, 20, 25 and 30 = %d\n",  sum(4, 4, 20, 25, 30) );

   return 0;
}

int sum(int num_args, ...)
{
   int val = 0;
   va_list ap;
   int i;

   va_start(ap, num_args);
   for(i = 0; i < num_args; i++)
   {
      val += va_arg(ap, int);
   }
   va_end(ap);
 
   return val;
}

Let us compile and run the above program, this will produce the following result:

Sum of 10, 20 and 30 = 60
Sum of 4, 20, 25 and 30 = 79

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