32. Variable arguments <stdarg.h>

The header <stdarg.h> declares a type and defines four macros, for advancing through a list of arguments whose number and types are not known to the called function when it is translated.

A function may be called with a variable number of arguments of varying types. As described in Function definitions, its parameter list contains one or more parameters. The rightmost parameter plays a special role in the access mechanism, and will be designated parmN in this description.

The type declared is

va_list

which is an object type suitable for holding information needed by the macros va_start, va_arg, va_end and va_copy. If access to the varying arguments is desired, the called function shall declare an object (generally referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap. [1]

[1]It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original function may make further use of the original list after the other function returns.

32.1. Variable argument list access macros

The va_start and va_arg macros described in this subclause shall be implemented as macros, not functions. It is unspecified whether va_copy and va_end are macros or identifiers declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the same name, the behavior is undefined. Each invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of the va_end macro in the same function.

32.1.1. The va_arg macro

Synopsis

#include <stdarg.h>
type va_arg(va_list ap, type);

Description

The va_arg macro expands to an expression that has the specified type and the value of the next argument in the call. The parameter ap shall have been initialized by the va_start or va_copy macro (without an intervening invocation of the va_end macro for the same ap). Each invocation of the va_arg macro modifies ap so that the values of successive arguments are returned in turn. The parameter type shall be a type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by postfixing a * to type. If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the behavior is undefined, except for the following cases:

  • one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types;
  • one type is pointer to void and the other is a pointer to a character type.

Returns

The first invocation of the va_arg macro after that of the va_start macro returns the value of the argument after that specified by parmN. Successive inv ocations return the values of the remaining arguments in succession.

32.1.2. The va_copy macro

Synopsis

#include <stdarg.h>
void va_copy(va_list dest, va_list src);

Description

The va_copy macro initializes dest as a copy of src, as if the va_start macro had been applied to dest followed by the same sequence of uses of the va_arg macro as had previously been used to reach the present state of src. Neither the va_copy nor va_start macro shall be invoked to reinitialize dest without an intervening invocation of the va_end macro for the same dest.

Returns

The va_copy macro returns no value.

32.1.3. The va_end macro

Synopsis

#include <stdarg.h>
void va_end(va_list ap);

Description

The va_end macro facilitates a normal return from the function whose variable argument list was referred to by the expansion of the va_start macro, or the function containing the expansion of the va_copy macro, that initialized the va_list ap. The va_end macro may modify ap so that it is no longer usable (without being reinitialized by the va_start or va_copy macro). If there is no corresponding invocation of the va_start or va_copy macro or if the va_end macro is not invoked before the return, the behavior is undefined.

Returns

The va_end macro returns no value.

32.1.4. The va_start macro

Synopsis

#include <stdarg.h>
void va_start(va_list ap, parmN);

Description

The va_start macro shall be invoked before any access to the unnamed arguments.

The va_start macro initializes ap for subsequent use by the va_arg and va_end macros. Neither the va_start nor va_copy macro shall be invoked to reinitialize ap without an intervening invocation of the va_end macro for the same ap.

The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ...). If the parameter parmN is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined.

Returns

The va_start macro returns no value.

EXAMPLE 1 The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS arguments), then passes the array as a single argument to function f2. The number of pointers is specified by the first argument to f1.

#include <stdarg.h>

#define MAXARGS 31

void f1(int n_ptrs, ...)
{
  va_list ap;
  char *array[MAXARGS];
  int ptr_no = 0;

    if (n_ptrs > MAXARGS)
      n_ptrs = MAXARGS;

    va_start(ap, n_ptrs);
    while (ptr_no < n_ptrs)
      array[ptr_no++] = va_arg(ap, char *);

    va_end(ap);
    f2(n_ptrs, array);
}

Each call to f1 is required to have visible the definition of the function or a declaration such as

void f1(int, ...);

EXAMPLE 2 The function f3 is similar, but saves the status of the variable argument list after the indicated number of arguments; after f2 has been called once with the whole list, the trailing part of the list is gathered again and passed to function f4.

#include <stdarg.h>

#define MAXARGS 31

void f3(int n_ptrs, int f4_after, ...)
{
  va_list ap, ap_save;
  char *array[MAXARGS];
  int ptr_no = 0;

  if (n_ptrs > MAXARGS)
    n_ptrs = MAXARGS;

  va_start(ap, f4_after);

  while (ptr_no < n_ptrs) {
    array[ptr_no++] = va_arg(ap, char *);
    if (ptr_no == f4_after)
      va_copy(ap_save, ap);
  }
  va_end(ap);
  f2(n_ptrs, array);
  // Now process the saved copy.
  n_ptrs -= f4_after;
  ptr_no = 0;
  while (ptr_no < n_ptrs)
    array[ptr_no++] = va_arg(ap_save, char *);
  va_end(ap_save);
  f4(n_ptrs, array);
}