va_start() in C programming

The va_start is a macro defined in the stdarg header file in the C Programming languages. The va_start() function is used to initialize the argument list.

The argument list is referred to by ap. Initializing the argument list is necessary before using the va_arg() and va_end() functions in the program. The second argument passed in the va_start() function is the last_arg. This argument is also called the fixed argument. This argument is passed in the function that is the argument before the ellipsis.

The macros are called in the program before calling the va_arg and va_end in the function.

Syntax for using va_start() function

The declaration for va-start() macro in your program:

Return Type in va_start() function

void: Since the function does not return any value, the return type used when declaring 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 va_list. This object is used for storing the information that the user requires to find the additional arguments using the va_arg() function.
last_arg - This is the last argument entered in the va_start() function. It is a fixed-length argument to be passed in it just before the ellipsis.

Header file:

Since this macro is defined in the stdarg library, thus, it is necessary to include the header file in your program

Implementation of va_start() function in C Program:

#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:

Let us discuss another example of printing the product of a list of numbers.

#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:

Conclusion: Before implementing the va_arg() function in the your program, it is necessary to implement the va_start() function in your code and end it with va_end() function.

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