Variadic functions

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

 

Variadic functions

va_start

va_arg

va_copy

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:

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)
Type
va_list	holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)

Example

Print values of different types.

Run this code

#include <stdio.h>
#include <stdarg.h>
 
void simple_printf(const char* fmt, ...)
{
    va_list args;
    va_start(args, fmt);
 
    while (*fmt != '\0') {
        if (*fmt == 'd') {
            int i = va_arg(args, int);
            printf("%d\n", i);
        } else if (*fmt == '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);
        } else if (*fmt == 'f') {
            double d = va_arg(args, double);
            printf("%f\n", d);
        }
        ++fmt;
    }
 
    va_end(args);
}
 
int main(void)
{
    simple_printf("dcff", 3, 'a', 1.999, 42.5); 
}

Output:

3
a
1.999000
42.50000

References

• 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