STDARG (3)
variable argument lists
SYNOPSIS
#include <stdarg.h>
void va_start( va_list ap , last );
type va_arg( va_list ap , type );
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 ,
and
va_end .
The
va_start
macro initializes
ap
for subsequent use by
va_arg
and
va_end ,
and must be called first.
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_start
macro returns no value.
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_end
macro handles a normal return from the function whose variable argument
list was initialized by
va_start .
The
va_end
macro returns no value.
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;
int d;
char c, *p, *s;
va_start(ap, fmt);
while (*fmt)
switch(*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\en", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\en", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c\en", c);
break;
}
va_end(ap);
}
STANDARDS
The
va_start ,
va_arg ,
and
va_end
macros conform to ANSI C3.159-1989 (``ANSI C'').
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 .
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).
man3/stdin.3
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