STDARG(3) OpenBSD Programmer's Manual STDARG(3)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_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, 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 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_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_copy() macro returns no value.
The va_end() macro handles a normal return from the function whose
variable argument list was initialized by va_start() or va_copy().
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, 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 ANSI/ISO/IEC
9899-1999 (``ANSI C99'').
HISTORY
The va_start(), va_arg() and va_end() macros were introduced in ANSI
X3.159-1989 (``ANSI C''). The va_copy() macro was introduced in
ANSI/ISO/IEC 9899-1999 (``ANSI 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 a va_list argument, such as
vfprintf(3).
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9
