stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
#include <stdarg.h>
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);
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 argument
last is the name of the last argument before the variable
argument list, that is, the last argument of which the calling function knows
the type.
Because the address of this argument 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.
The
va_arg() macro expands to an expression that has the type and value
of the next argument in the call. The argument
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
argument
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.
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
traversals of the list, each bracketed by
va_start() and
va_end() are possible.
va_end() may be a macro or a function.
The
va_copy() macro copies the (previously initialized) variable argument
list
src to
dest. The behavior is as if
va_start() were
applied to
dest with the same
last argument, followed by the
same number of
va_arg() invocations that was used to reach the current
state of
src.
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 arguments are passed in registers, it may be necessary
for
va_start() to allocate memory, store the arguments there, and also
an indication of which argument 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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| va_start (), va_end (), va_copy () |
Thread safety |
MT-Safe |
| va_arg () |
Thread safety |
MT-Safe race:ap |
The
va_start(),
va_arg(), and
va_end() macros conform to
C89. C99 defines the
va_copy() macro.
Unlike the historical
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).
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, ...) /* '...' is C syntax for a variadic function */
{
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);
}