Variadic arguments

Variadic functions are functions that may be called with different number of arguments.

Only new-style (prototyped) function declarations may be variadic. This is indicated by the parameter of the form ... which must appear last in the parameter list and must follow at least one named parameter.

//New-style declaration
int printx(const char* fmt, ...); // function declared this way
printx("hello world"); // may be called with one
printx("a=%d b=%d", a, b); // or more arguments
 
// int printy(..., const char* fmt); // Error: ... must be the last
// int printz(...); // Error: ... must follow at least one named parameter

At the function call, each argument that is a part of the variable argument list undergoes special implicit conversions known as default argument promotions.

Within the body of a function that uses variadic arguments, the values of these arguments may be accessed using the <stdarg.h> library facilities:

Notes

Although old-style (prototype-less) function declarations allow the subsequent function calls to use any number of arguments, they are not allowed to be variadic (as of C89). The definition of such function must specify a fixed number of parameters and cannot use the stdarg.h macros.

//old-style declaration
int printx(); // function declared this way
printx("hello world"); // may be called with one
printx("a=%d b=%d", a, b); // or more arguments
// the behavior of at least one of these calls is undefined, depending on
// the number of parameters the function is defined to take

Example

#include <stdio.h>
#include <time.h>
#include <stdarg.h>
 
void tlog(const char* fmt,...)
{
    char msg[50];
    strftime(msg, sizeof msg, "%T", localtime(&(time_t){time(NULL)}));
    printf("[%s] ", msg);
    va_list args;
    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);
}
 
int main(void)
{
   tlog("logging %d %d %d...\n", 1, 2, 3);
}

[10:21:38] logging 1 2 3...

References