stdarg(3) BSD Library Functions Manual stdarg(3)
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 speci-
fied 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 argu-
ment 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 val-
ues 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 argu-
ments on to a function that takes a va_list argument, such as
vfprintf(3).
BSD October 25, 2002 BSD
Mac OS X 10.8 - Generated Fri Aug 31 05:33:11 CDT 2012