STDARG
NAME
stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
SYNOPSIS
#include <stdarg.h>
I void va_start(va_list ap , last );
I type va_arg(va_list ap , type );
I void va_end(va_list ap );
I void va_copy(va_list dest , va_list src );
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
R va_start (),
R va_arg (),
and
R va_end ().
va_start()
The
R va_start ()
macro initializes
ap
for subsequent use by
R va_arg ()
and
R va_end (),
and must be called first.
The parameter
last
is the name of the last parameter before the variable argument list, that is,
the last parameter of which the calling function knows the type.
Because the address of this parameter may be used in the
R va_start ()
macro, it should not be declared as a register variable,
or as a function or an array type.
va_arg()
The
R 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
R va_start ().
Each call to
R 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
R type .
The first use of the
R va_arg ()
macro after that of the
R va_start ()
macro returns the argument after
R last .
Successive invocations return the values of the remaining arguments.
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.
If
ap
is passed to a function that uses
I va_arg( ap , type )
then the value of
ap
is undefined after the return of that function.
va_end()
Each invocation of
R va_start ()
must be matched by a corresponding invocation of
R va_end ()
in the same function.
After the call
I va_end( ap )
the variable
ap
is undefined.
Multiple traversals of the list, each
bracketed by
R va_start ()
and
R va_end ()
are possible.
R va_end ()
may be a macro or a function.
va_copy()
An obvious implementation would have a
va_list
be a pointer to the stack frame of the variadic function.
In such a setup (by far the most common) there seems
nothing against an assignment
va_list aq = ap;
Unfortunately, there are also systems that make it an
array of pointers (of length 1), and there one needs
va_list aq;
*aq = *ap;
Finally, on systems where parameters are passed in registers,
it may be necessary for
R va_start ()
to allocate memory, store the parameters there, and also
an indication of which parameter is next, so that
R va_arg ()
can step through the list.
Now
R va_end ()
can free the allocated memory again.
To accommodate this situation, C99 adds a macro
R va_copy (),
so that the above assignment can be replaced by
va_list aq;
va_copy(aq, ap);
...
va_end(aq);
Each invocation of
R va_copy ()
must be matched by a corresponding invocation of
R va_end ()
in the same function.
Some systems that do not supply
R va_copy ()
have
__va_copy
instead, since that was the name used in the draft proposal.
CONFORMING TO
The
R va_start (),
R va_arg (),
and
R va_end ()
macros conform to C89.
C99 defines the
R va_copy ()
macro.
NOTES
These macros are
not
compatible with the historic macros they replace.
A backward compatible version can be found in the include file
R varargs.h .
The historic setup is:
#include <varargs.h>
void
foo(va_alist)
va_dcl
{
va_list ap;
va_start(ap);
while (...) {
...
x = va_arg(ap, type);
...
}
va_end(ap);
}
On some systems,
va_end
contains a closing '}' matching a '{' in
R va_start ,
so that both macros must occur in the same function, and in a way
that allows this.
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).
EXAMPLE
The function
foo
takes a string of format characters and prints out the argument associated
with each format character based on the type.
#include <stdio.h>
#include <stdarg.h>
void
foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
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 */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
}
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