NAME
stdarg —
variable argument lists
SYNOPSIS
#include
<stdarg.h>
void
va_start(va_list
ap, last);
type
va_arg(va_list
ap, type);
void
va_copy(va_list
dest, va_list
src);
void
va_end(va_list
ap);
DESCRIPTION
A function may be called with a varying number of arguments of
varying types. The include file
<stdarg.h> declares a type
(va_list) and defines three macros for stepping through a
list of arguments whose number and types are not known to the called
function.
The called function must declare an object of type
va_list which is used by the macros
va_start(),
va_arg(), va_copy(), and
va_end().
The
va_start()
macro must be called first, and it initializes ap,
which can be passed to va_arg() for each argument to
be processed. Calling va_end() signals that there
are no further arguments, and causes ap to be
invalidated. Note that each call to va_start() must
be matched by a call to va_end(), from within the
same function.
The parameter last is the name of the last parameter before the variable argument list, i.e., the last parameter of which the calling function knows the type.
Because the address of this parameter is used in
the
va_start()
macro, it should not be declared as a register variable, or as a function or
an array type.
The
va_arg()
macro expands to an expression that has the type and value of the next
argument in the call. The parameter ap is the
va_list ap initialized by
va_start(). Each call to
va_arg() modifies ap so that
the next call returns the next argument. The parameter
type is a type name specified so that the type of a
pointer to an object that has the specified type can be obtained simply by
adding a * to type.
If there is no next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur.
The first use of the
va_arg()
macro after that of the va_start() macro returns the
argument after last. Successive invocations return the
values of the remaining arguments.
The
va_copy()
macro copies the state of the variable argument list,
src, previously initialized by
va_start(), to the variable argument list,
dest, which must not have been previously initialized
by va_start(), without an intervening call to
va_end(). The state preserved in
dest is equivalent to calling
va_start() and va_arg() on
dest in the same way as was used on
src. The copied variable argument list can
subsequently be passed to va_arg(), and must finally
be passed to va_end() when through with it.
After a variable argument list is invalidated by
va_end(), it
can be reinitialized with va_start() or made a copy
of another variable argument list with
va_copy().
EXAMPLES
The function foo takes a string of format characters and prints out the argument associated with each format character based on the type.
void foo(char *fmt, ...)
{
va_list ap, ap2;
int d;
char c, *s;
va_start(ap, fmt);
va_copy(ap2, ap);
while (*fmt)
switch(*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\n", d);
break;
case 'c': /* char */
/* Note: char is promoted to int. */
c = va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
...
/* use ap2 to iterate over the arguments again */
...
va_end(ap2);
}
COMPATIBILITY
These macros are
not
compatible with the historic macros they replace. A backward compatible
version can be found in the include file
<varargs.h>.
STANDARDS
The va_start(),
va_arg(), va_copy(), and
va_end() macros conform to ISO/IEC
9899:1999 (“ISO C99”).
BUGS
Unlike the varargs macros, the
stdarg macros do not permit programmers to code a
function with no fixed arguments. This problem generates work mainly when
converting varargs code to stdarg
code, but it also creates difficulties for variadic functions that wish to
pass all of their arguments on to a function that takes a
va_list argument, such as
vfprintf(3).