Variadic functions are functions that may be called with different number of arguments.
Only prototypedfunction 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(until C23). The ellipsis parameter and the proceeding parameter must be delimited by,.
// Prototyped declarationint printx(constchar* fmt, ...);// function declared this wayprintx("hello world");// may be called with oneprintx("a=%d b=%d", a, b);// or more arguments int printz(...);// OK since C23 and in C++// Error until C23: ... must follow at least one named parameter // int printy(..., const char* fmt); // Error: ... must be the last// int printa(const char* fmt...); // Error in C: ',' is required; OK in C++
At thefunction call, each argument that is a part of the variable argument list undergoes special implicit conversions known asdefault 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:
Defined in header <stdarg.h> | |
| enables access to variadic function arguments (function macro)[edit] | |
| accesses the next variadic function argument (function macro)[edit] | |
(C99) | makes a copy of the variadic function arguments (function macro)[edit] |
| ends traversal of the variadic function arguments (function macro)[edit] | |
| holds the information needed byva_start,va_arg,va_end, andva_copy (typedef)[edit] | |
Contents |
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 thestdarg.h macros.
// old-style declaration, removed in C23int printx();// function declared this wayprintx("hello world");// may be called with oneprintx("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
#include <stdio.h>#include <time.h>#include <stdarg.h> void tlog(constchar* 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);}
Output:
[10:21:38] logging 1 2 3...
C++ documentation forVariadic arguments |
