Variadic functions

 

C

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Technical Specifications

Symbol index

 

Variadic functions

va_start

va_arg

va_copy (C99)

va_end

va_list

 

Variadic functions are functions (e.g. printf) which take a variable number of arguments.

The declaration of a variadic function uses an ellipsis as the last parameter, e.g. int printf(const char* format, ...);. See variadic arguments for additional detail on the syntax and automatic argument conversions.

Accessing the variadic arguments from the function body uses the following library facilities:

Types
va_list	holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)
Macros
Defined in header <stdarg.h>
va_start	enables access to variadic function arguments (function macro)
va_arg	accesses the next variadic function argument (function macro)
va_copy (C99)	makes a copy of the variadic function arguments (function macro)
va_end	ends traversal of the variadic function arguments (function macro)

Example

Print values of different types.

Run this code

#include <stdarg.h>
#include <stdio.h>
 
void simple_printf(const char* fmt, ...)
{
    va_list args;
 
    for (va_start(args, fmt); *fmt != '\0'; ++fmt)
    {
        switch(*fmt)
        {
            case 'd':
            {
                int i = va_arg(args, int);
                printf("%d\n", i);
                break;
            }
            case 'c':
            {
                // A 'char' variable will be promoted to 'int'
                // A character literal in C is already 'int' by itself
                int c = va_arg(args, int);
                printf("%c\n", c);
                break;
            }
            case 'f':
            {
                double d = va_arg(args, double);
                printf("%f\n", d);
                break;
            }
            default:
                puts("Unknown formatter!");
                goto END;
        }
    }
END:
    va_end(args);
}
 
int main(void)
{
    simple_printf("dcff", 3, 'a', 1.969, 42.5);
}

Output:

3
a
1.969000
42.50000

References

• C23 standard (ISO/IEC 9899:2024):

• 7.16 Variable arguments <stdarg.h> (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.16 Variable arguments <stdarg.h> (p: TBD)

• C11 standard (ISO/IEC 9899:2011):

• 7.16 Variable arguments <stdarg.h> (p: 269-272)

• C99 standard (ISO/IEC 9899:1999):

• 7.15 Variable arguments <stdarg.h> (p: 249-252)

• C89/C90 standard (ISO/IEC 9899:1990):

• 4.8 VARIABLE ARGUMENTS <stdarg.h>

See also

C++ documentation for Variadic functions