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
)
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_end(
),
and, optionally,
va_copy(
).
The
va_start()
macro initializes
ap
for subsequent use by
va_arg(),
va_copy(
)
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.
If the type in question is one that gets promoted, the promoted type
should be used as the argument to
va_arg().
The following describes which types are promoted (and to what):
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_copy()
macro makes
dest
a copy of
src
as if the
va_start()
macro had been applied to it followed by the same sequence of uses of the
va_arg(
)
macro as had previously been used to reach the present state of
src
.
The
va_copy()
macro returns no value.
The
va_end()
macro handles a normal return from the function whose variable argument
list was initialized by
va_start(
)
or
va_copy(
).
The
va_end()
macro returns no value.
)
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, c;
char *s;
double f;
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 */
c = va_arg(ap, int); /* promoted */
printf("char %c\n", c);
break;
case 'f': /* float */
f = va_arg(ap, double); /* promoted */
printf("float %f\n", f);
}
va_end(ap);
}
),
va_arg(
),
va_copy(
),
and
va_end(
)
macros conform to
ISO/IEC 9899:1999 (``ISO C99'') .
),
va_arg(
)
and
va_end(
)
macros were introduced in
ANSI X3.159-1989 (``ANSI C89'') .
The
va_copy(
)
macro was introduced in
ISO/IEC 9899:1999 (``ISO C99'') .
varargs.h
>.