void
va_start(
va_list ap
)
type
va_arg(
va_list ap
, type
)
void
va_end(
va_list ap
)
A function may be called with a varying number of arguments of varying
types.
The include file
<varargs.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 name an argument
va_alist
,
which marks the start of the variable argument list,
and which is naturally the last argument named.
It is declared by
va_dcl
,
which should not be followed by a semicolon.
The called function also 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.
It is possible for
va_alist
to be the only parameter to a function, resulting in it being possible
for a function to have no fixed arguments preceding the variable
argument list.
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.
void foo(fmt, va_alist)
char *fmt;
va_dcl
{
va_list ap;
int d;
char c, *p, *s;
va_start(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 */
c = va_arg(ap, char);
printf("char %c\n", c);
break;
}
}
va_end(ap);
}
stdarg.h
>;
see
stdarg(3)
for its description.