NAME¶
stdarg - variable argument lists
SYNOPSIS¶
#include <stdarg.h>
void va_start( va_list ap, last);
type va_arg( va_list ap, type);
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, and
va_end.
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 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.
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.
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;
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);
}
The
va_start,
va_arg, and
va_end macros conform to ANSI
X3.159-1989 (``ANSI C'').
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.
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).