STDARG(3) BSD Programmer's Manual STDARG(3)
NAME
stdarg - variable argument lists
SYNOPSIS
#include <stdarg.h>
void
vastart(valist ap, last)
type
vaarg(valist ap, type)
void
vaend(valist ap)
DESCRIPTION
A function may be called with a varying number of arguments of varying
types. The include file <stdarg.h> declares a type (valist) 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 valist which is used
by the macros vastart(), vaarg(), and vaend().
The vastart() macro initializes ap for subsequent use by vaarg() and
vaend(), 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 vastart() macro, it
should not be declared as a register variable, or as a function or an ar-
ray type.
The vastart() macro returns no value.
The vaarg() macro expands to an expression that has the type and value
of the next argument in the call. The parameter ap is the valist ap
initialized by vastart(). Each call to vaarg() 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 speci-
fied 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 argu-
ment promotions), random errors will occur.
The first use of the vaarg() macro after that of the vastart() macro
returns the argument after last. Successive invocations return the values
of the remaining arguments.
The vaend() macro handles a normal return from the function whose vari-
able argument list was initialized by vastart().
The vaend() 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 */
c = va_arg(ap, char);
printf("char %c\n", c);
break;
}
va_end(ap);
}
STANDARDS
The vastart(), vaarg(), and vaend() macros conform to ANSI C3.159-1989
(``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 argu-
ments on to a function that takes a valist argument, such as
vfprintf(3).
BSDI BSD/386 March 27, 1993 2