stdarg.h

Variadic functions are functions which may take a variable number of arguments and are declared with an ellipsis in place of the last parameter. An example of such a function is printf. A typical declaration is

int check(int a, double b, ...);

Variadic functions must have at least one named parameter, so, for instance,

char *wrong(...);

is not allowed in C. (In C++ and C23,[2] such a declaration is permitted.) In C, a comma must precede the ellipsis if a named parameter is specified; in C++, it is optional.

The same syntax is used in a definition:

long func(char, double, int, ...);

long func(char a, double b, int c, ...)
{
    /* ... */
}

An ellipsis may not appear in old-style function definitions.

To access the unnamed arguments, one must declare a variable of type va_list in the variadic function. The macro va_start is then called with two arguments: the first is the variable declared of the type va_list, the second is the name of the last named parameter of the function. In C23 the second argument will be optional and will not be evaluated.[2] After this, each invocation of the va_arg macro yields the next argument. The first argument to va_arg is the va_list and the second is the type of the next argument passed to the function. Finally, the va_end macro must be called on the va_list before the function returns. (It is not required to read in all the arguments.)

C99 provides an additional macro, va_copy, which can duplicate the state of a va_list. The macro invocation va_copy(va2, va1) copies va1 into va2.

There is no defined method for counting or classifying the unnamed arguments passed to the function. The function is simply required to know or determine this somehow, the means of which vary. Common conventions include:

• Use of a printf or scanf-like format string with embedded specifiers that indicate argument types.
• A sentinel value at the end of the variadic arguments.
• A count argument indicating the number of variadic arguments.

Some C implementations provide C extensions that allow the compiler to check for the proper use of format strings and sentinels. Barring these extensions, the compiler usually cannot check whether the unnamed arguments passed are of the type the function expects, or convert them to the required type. Therefore, care should be taken to ensure correctness in this regard, since undefined behavior results if the types do not match. For example, if the expected type is int *, then a null pointer should be passed as (int *)NULL. Writing just NULL would result in an argument of type either int or void *, neither of which is correct. Another consideration is the default argument promotions applied to the unnamed arguments. A float will automatically be promoted to a double. Likewise, arguments of types narrower than an int will be promoted to int or unsigned int. The function receiving the unnamed arguments must expect the promoted type.

GCC has an extension that checks the passed arguments:

format(archetype, string-index, first-to-check)

The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments which should be type-checked against a format string. For example, the declaration:

extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format.

— "5.27 Extensions to the C Language Family - Declaring Attributes of Functions". Retrieved 2009-01-03.

#include <stdio.h>
#include <stdarg.h>

/* print all args one at a time until a negative argument is seen;
   all args are assumed to be of int type */
void printargs(int arg1, ...)
{
  va_list ap;
  int i;

  va_start(ap, arg1); 
  for (i = arg1; i >= 0; i = va_arg(ap, int))
    printf("%d ", i);
  va_end(ap);
  putchar('\n');
}

int main(void)
{
   printargs(5, 2, 14, 84, 97, 15, -1, 48, -1);
   printargs(84, 51, -1, 3);
   printargs(-1);
   printargs(1, -1);
   return 0;
}

This program yields the output:

5 2 14 84 97 15
84 51

1

To call other var args functions from within your function (such as sprintf) you need to use the var arg version of the function (vsprintf in this example):

void MyPrintf(const char *format, ...)
{
  va_list args;
  char buffer[BUFSIZ];

  va_start(args, format);
  vsnprintf(buffer, sizeof buffer, format, args);
  va_end(args);
  FlushFunnyStream(buffer);
}

Outdated versions of POSIX defined the legacy header varargs.h, which dates from before the standardization of C and provides functionality similar to stdarg.h. This header is part of neither ISO C nor POSIX. The file, as defined in the second version of the Single UNIX Specification, simply contains all of the functionality of C89 stdarg.h, with the exceptions that:

• it cannot be used in standard C new-style definitions
• the given argument may be omitted (standard C requires at least one argument)

The interface is also different. For printargs example, one would instead write:

#include <stdio.h>
#include <varargs.h>

/* There is no "void" type; use an implicit int return. */
printargs(arg1, va_alist)
  va_dcl /* no semicolon here! */
{
  va_list ap;
  int i;

  va_start(ap); /* only the va_list is given! */
  for (i = arg1; i >= 0; i = va_arg(ap, int))
    printf("%d ", i);
  va_end(ap);
  putchar('\n');
  return;
}

and is called the same way.

varargs.h requires old-style function definitions because of the way the implementation works.[3] Conversely, it is not possible to mix old-style function definitions with stdarg.h.