обрабатывать список переменных аргументов (handle variable argument list)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
stdarg.h — handle variable argument list
Синопсис (Synopsis)
#include <stdarg.h>
void va_start(va_list ap, argN);
void va_copy(va_list dest, va_list src);
type va_arg(va_list ap, type);
void va_end(va_list ap);
Описание (Description)
The functionality described on this reference page is aligned
with the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The <stdarg.h> header shall contain a set of macros which allows
portable functions that accept variable argument lists to be
written. Functions that have variable argument lists (such as
printf()) but do not use these macros are inherently non-
portable, as different systems use different argument-passing
conventions.
The <stdarg.h> header shall define the va_list type for variables
used to traverse the list.
The va_start() macro is invoked to initialize ap to the beginning
of the list before any calls to va_arg().
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.
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 unspecified and shall
be passed to the va_end() macro prior to any further reference to
ap. The parameter argN is the identifier of the rightmost
parameter in the variable parameter list in the function
definition (the one just before the ...). If the parameter argN
is declared with the register storage class, with a function type
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 va_arg() macro shall return the next argument in the list
pointed to by ap. Each invocation of va_arg() modifies ap so
that the values of successive arguments are returned in turn. The
type parameter 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 a pointer to void and the other is a pointer to a
character type.
* Both types are pointers.
Different types can be mixed, but it is up to the routine to know
what type of argument is expected.
The va_end() macro is used to clean up; it invalidates ap for use
(unless va_start() or va_copy() is invoked again).
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.
Multiple traversals, each bracketed by va_start() ... va_end(),
are possible.
The following sections are informative.
Примеры (Examples)
This example is a possible implementation of execl():
#include <stdarg.h>
#define MAXARGS 31
/*
* execl is called by
* execl(file, arg1, arg2, ..., (char *)(0));
*/
int execl(const char *file, const char *args, ...)
{
va_list ap;
char *array[MAXARGS +1];
int argno = 0;
va_start(ap, args);
while (args != 0 && argno < MAXARGS)
{
array[argno++] = args;
args = va_arg(ap, const char *);
}
array[argno] = (char *) 0;
va_end(ap);
return execv(file, array);
}
Использование в приложениях (Application usage)
It is up to the calling routine to communicate to the called
routine how many arguments there are, since it is not always
possible for the called routine to determine this in any other
way. For example, execl() is passed a null pointer to signal the
end of the list. The printf() function can tell how many
arguments are there by the format argument.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
The System Interfaces volume of POSIX.1‐2017, exec(1p),
fprintf(3p)
