path: root/share/man/man3/va_start.3

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.\"	@(#)stdarg.3	8.1 (Berkeley) 6/5/93
.\"
.Dd $Mdocdate: August 30 2019 $
.Dt VA_START 3
.Os
.Sh NAME
.Nm va_start ,
.Nm va_arg ,
.Nm va_copy ,
.Nm va_end
.Nd variable argument lists
.Sh SYNOPSIS
.In stdarg.h
.Ft void
.Fn va_start "va_list ap" last
.Ft type
.Fn va_arg "va_list ap" type
.Ft void
.Fn va_copy "va_list dst" "va_list src"
.Ft void
.Fn va_end "va_list ap"
.Sh DESCRIPTION
A function may be called with a varying number of arguments of varying
types.
The include file
.In stdarg.h
declares a type
.Vt va_list
and defines three macros for stepping
through a list of arguments whose number and types are not known to
the called function.
.Pp
The called function must declare an object of type
.Vt va_list
which is used by the macros
.Fn va_start ,
.Fn va_arg ,
.Fn va_end ,
and, optionally,
.Fn va_copy .
.Pp
The
.Fn va_start
macro initializes
.Fa ap
for subsequent use by
.Fn va_arg ,
.Fn va_copy
and
.Fn va_end ,
and must be called first.
.Pp
The parameter
.Fa 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.
.Pp
Because the address of this parameter is used in the
.Fn va_start
macro, it should not be declared as a register variable, nor as a
function, nor an array type.
.Pp
The
.Fn va_arg
macro expands to an expression that has the type and value of the next
argument in the call.
The parameter
.Fa ap
is the
.Va va_list ap
initialized by
.Fn va_start .
Each call to
.Fn va_arg
modifies
.Fa ap
so that the next call returns the next argument.
The parameter
.Fa 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
.Ql *
to
.Fa type .
.Pp
If there is no next argument, or if
.Fa 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.
.Pp
If the type in question is one that would normally be promoted, the
promoted type should be used as the argument to
.Fn va_arg .
The following describes which types should be promoted (and to what):
.Bl -dash -compact
.It
.Vt short
is promoted to
.Vt int
.It
.Vt float
is promoted to
.Vt double
.It
.Vt char
is promoted to
.Vt int
.El
.Pp
The same rules apply to unsigned versions of the above types, as well
as their bit-type equivalents (e.g.\&
.Vt int8_t
and
.Vt int16_t ) .
.Pp
The
.Fn va_copy
macro makes
.Fa dst
a copy of
.Fa src
as if the
.Fn va_start
macro had been applied to it followed by the same sequence of uses of the
.Fn va_arg
macro as had previously been used to reach the present state of
.Fa src .
.Pp
The
.Fn va_end
macro handles a normal return from the function whose variable argument
list was initialized by
.Fn va_start
or
.Fn va_copy .
.Sh RETURN VALUES
The first use of the
.Fn va_arg
macro after that of the
.Fn va_start
macro returns the argument after
.Fa last .
Successive invocations return the values of the remaining
arguments.
.Pp
The
.Fn va_start ,
.Fn va_copy
and
.Fn va_end
macros return no value.
.Sh EXAMPLES
The function
.Fn foo
takes a string of format characters and prints out the argument
associated with each format character based on the type.
.Bd -literal -offset indent
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\en", s);
			break;
		case 'd':			/* int */
			d = va_arg(ap, int);
			printf("int %d\en", d);
			break;
		case 'c':			/* char */
			c = va_arg(ap, int);	/* promoted */
			printf("char %c\en", c);
			break;
		case 'f':			/* float */
			f = va_arg(ap, double); /* promoted */
			printf("float %f\en", f);
		}
	va_end(ap);
}
.Ed
.Sh STANDARDS
These macros are
.Em not
compatible with the historic macros they replace.
A backward compatible version can be found in the include
file
.In varargs.h .
.Pp
The
.Fn va_start ,
.Fn va_arg
and
.Fn va_end
macros conform to
.St -isoC-99 .
.Sh HISTORY
The
.Fn va_start ,
.Fn va_arg
and
.Fn va_end
macros were introduced in
.St -ansiC .
The
.Fn va_copy
macro was introduced in
.St -isoC-99 .
.Sh BUGS
Unlike the
.Em varargs
macros, the
.Nm stdarg
macros do not permit programmers to
code a function with no fixed arguments.
This problem generates work mainly when converting
.Em varargs
code to
.Nm 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
.Vt va_list ,
such as
.Xr vfprintf 3 .