STDARG(3EXT) Extended Library Functions STDARG(3EXT)

NAME

stdarg - handle variable argument list

SYNOPSIS

#include <stdarg.h>
va_list pvar;

void va_start(va_list pvar, void name);


(type *) va_arg(va_list pvar, type);


void va_copy(va_list dest, va_list src);


void va_end(va_list pvar);

DESCRIPTION

This set of macros allows portable procedures that accept variable
numbers of arguments of variable types to be written. Routines that have
variable argument lists (such as printf) but do not use stdarg are
inherently non-portable, as different machines use different argument-
passing conventions.


va_list is a type defined for the variable used to traverse the list.


The va_start macro is invoked before any access to the unnamed arguments
and initializes pvar for subsequent use by va_arg() and va_end(). The
parameter name is the identifier of the rightmost parameter in the
variable parameter list in the function definition (the one just before
the , ...). If this parameter is declared with the register storage class
or 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.


The parameter name is required under strict ANSI C compilation. In other
compilation modes, name need not be supplied and the second parameter to
the va_start() macro can be left empty (for example, va_start(pvar, );).
This allows for routines that contain no parameters before the ... in the
variable parameter list.


The va_arg() macro expands to an expression that has the type and value
of the next argument in the call. The parameter pvar should have been
previously initialized by va_start(). Each invocation of va_arg()
modifies pvar so that the values of successive arguments are returned in
turn. The parameter type is the type name of the next argument to be
returned. The type name must be specified in such a way so 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.


The va_copy() macro saves the state represented by the va_listsrc in the
va_list dest. The va_list passed as dest should not be initialized by a
previous call to va_start(), and must be passed to va_end() before being
reused as a parameter to va_start() or as the dest parameter of a
subsequent call to va_copy(). The behavior is undefined should any of
these restrictions not be met.


The va_end() macro is used to clean up.


Multiple traversals, each bracketed by va_start() and va_end(), are
possible.

EXAMPLES

Example 1: A sample program.

This example gathers into an array a list of arguments that are pointers
to strings (but not more than MAXARGS arguments) with function f1, 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 shall have visible the definition of the function or a
declaration such as


void f1(int, ...)

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+

SEE ALSO

vprintf(3C), attributes(7), standards(7)

NOTES

It is the responsibility of the calling routine to specify in some manner
how many arguments there are, since it is not always possible to
determine the number of arguments from the stack frame. For example,
execl is passed a zero pointer to signal the end of the list. The printf
function can determine the number of arguments by the format. It is non-
portable to specify a second argument of char, short, or float to
va_arg(), because arguments seen by the called function are not char,
short, or float. C converts char and short arguments to int and converts
float arguments to double before passing them to a function.

March 22, 2006 STDARG(3EXT)