The macros are invoked in the program prior to invoking the vaarg and vaend within the function.
Syntax for using va_start function
The definition of the va-start macro in your code:
Example
void va_start( va_list ap, last_arg);
Return Type in va_start function
void: Because the function does not provide any output, the return type specified when defining the va_start function is void.
Parameters Passed in va_start function
- ap - This parameter refers to the variable list. It is an object that is specifically created for the valist. This object is used for storing the information that the user requires to find the additional arguments using the vaarg function.
- lastarg - This is the last argument entered in the vastart function. It is a fixed-length argument to be passed in it just before the ellipsis.
Header file:
Given that this macro is specified within the stdarg library, it is imperative to incorporate the corresponding header file into your program.
Example
#include<stdarg.h>
Implementation of va_start function in C Program:
Example
#include <stdio.h>
#include <stdarg.h>
//let us define a function that will be used to perform addition
//here we have not defined the total parameters that can be passed in the function
int addition(int n, ...)
{
// initialize a temporary variable that can be used
// to store the parameters passed in the functions
// so that we can add them
va_list numtoadd;
int totalsum;
// Initialize a variable to store
// the sum of all the parameters in the function.
totalsum = 0;
// Use the va_start() function
// define the variable part of the function and
// then define the end of the argument lists
va_start(numtoadd, n);
// use the loop to add each argument
// and add all the arguments in the totalsum variable
for (int i=0; i < n; i++)
totalsum = totalsum + va_arg(numtoadd, int);
// now once you have used the list
// clear the memory
va_end(numtoadd);
// now the addition is performed and stored in totalsum
// return the totalsum
return totalsum;
}
int main(int argc, const char * argv[])
{
// Define variables to store the value required for addition
int val1, val2, val3;
int res;
val1 = 3;
val2 = 4;
val3 = 5;
/* Calling the addition function */
res = addition(3, val1, val2, val3);
// Print the answers after performing the addition
printf("The sum of %d, %d and %d is %d\n", val1, val2, val3, res);
return 0;
}
Output:
Output
[Program Output]
Let's explore another instance of displaying the result of multiplying a series of numbers.
Example
#include <stdio.h>
#include <stdarg.h>
//let us define a function that will be used to perform multiplication
//here we have not defined the total parameters that can be passed in the function
int product(int n, ...)
{
// initialize a temporary variable that can be used
// to store the parameters passed in the functions
// that we have to multiply
va_list numtomul;
int totalproduct;
// Initialize a variable to store
// the product of all the parameters in the function.
totalproduct = 1;
// Use the va_start() function
// define the variable part of the function and
// then define the end of the argument lists
va_start(numtomul, n);
// use the loop to add each argument
// and add all the arguments in the totalsum variable
for (int i=0; i < n; i++)
totalproduct = totalproduct * va_arg(numtomul, int);
// now once you have used the list
// clear the memory
va_end(numtomul);
// now the multiplication is performed and stored in totalproduct
// return the totalsum
return totalproduct;
}
int main(int argc, const char * argv[])
{
// Define variables to store the value required for performing multiplication
int val1, val2, val3;
int res;
val1 = 3;
val2 = 4;
val3 = 5;
/* Calling the product function */
res = product(3, val1, val2, val3);
// Print the answers after computing product
printf("The product of %d, %d and %d is %d\n", val1, val2, val3, res);
return 0;
}
Output:
Output
[Program Output]
Conclusion: Prior to incorporating the vaarg function into your program, it is crucial to first integrate the vastart function within your code and finalize it with the va_end function.