void va_start(va_list ap , last ); type va_arg(va_list ap , type ); void va_end(va_list ap ); 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
va_start , va_arg , and
va_end . va_start 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 may be used in the
va_start macro, it should not be declared as a register variable,
or as a function or an array type.
va_arg 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 .
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.
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
va_arg( ap , type ) then the value of
ap is undefined after the return of that function.
va_end Each invocation of
va_start must be matched by a corresponding invocation of
va_end in the same function. After the call
va_end( ap ) the variable
ap is undefined. Multiple transversals of the list, each
bracketed by
va_start and
va_end are possible.
va_end may be a macro or a function.
va_copy
An obvious implementation would have a
va_list 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
va_start to allocate memory, store the parameters there, and also
an indication of which parameter is next, so that
va_arg can step through the list. Now
va_end can free the allocated memory again.
To accommodate this situation, C99 adds a macro
va_copy , so that the above assignment can be replaced by
va_list aq;
va_copy(aq, ap);
...
va_end(aq);
Each invocation of
va_copy must be matched by a corresponding invocation of
va_end in the same function.
Some systems that do not supply
va_copy have
__va_copy instead, since that was the name used in the draft proposal.
EXAMPLES
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, *p, *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);
}
CONFORMING TO
The
va_start , va_arg , and
va_end macros conform to ANSI X3.159-1989 (``C89'').
C99 defines the
va_copy macro.
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 .
On some systems,
va_end contains a closing '}' matching a '{' in
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) .
