varargs - variable argument lists
#include <varargs.h>
void
va_start(va_list ap);
type
va_arg(va_list ap, type);
void
va_end(va_list ap);
These historic interfaces are provided to support compilation of existing
programs only. New code should use the stdarg(3) interfaces.
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
preceeding 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.
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(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(3)
These historic macros were replaced in ANSI X3.159-1989 (``ANSI C'') by
the include file <stdarg.h>; see stdarg(3) for its description.
These macros are not compatible with the new macros they were replaced
by. In particular, it is not possible for a stdarg function to have no
fixed arguments.
BSD February 4, 2002 BSD
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