NAME
va_start, va_arg,
va_copy, va_end —
variable argument lists
SYNOPSIS
#include
<stdarg.h>
void
va_start(va_list
ap, last);
type
va_arg(va_list
ap, type);
void
va_copy(va_list
dst, va_list
src);
void
va_end(va_list
ap);
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(), 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, nor as a function,
nor 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 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, see below), random errors will occur.
If the type in question is one that would normally
be promoted, the promoted type should be used as the argument to
va_arg().
The following describes which types should be promoted (and to what):
- short is promoted to int
- float is promoted to double
- char is promoted to int
The same rules apply to unsigned versions of the above types, as well as their bit-type equivalents (e.g. int8_t and int16_t).
The
va_copy()
macro makes dst 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_end()
macro handles a normal return from the function whose variable argument list
was initialized by va_start() or
va_copy().
RETURN VALUES
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_start(),
va_copy() and va_end()
macros return no value.
EXAMPLES
The function foo() 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);
}
STANDARDS
These macros are
not
compatible with the historic macros they replace. A backward compatible
version can be found in the include file
<varargs.h>.
The va_start(),
va_arg() and va_end() macros
conform to ISO/IEC 9899:1999
(“ISO C99”).
HISTORY
The va_start(),
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”).
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 an argument of type
va_list, such as
vfprintf(3).