man/man3/stdarg.3

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.Dd February 25, 2020
.Dt STDARG 3
.Os
.Sh NAME
.Nm stdarg
.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 dest" "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
.Pq Em va_list
and defines four 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
.Em va_list
which is used by the macros
.Fn va_start ,
.Fn va_arg ,
.Fn va_copy ,
and
.Fn va_end .
.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, or as a
function or 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
.Em va_list Fa ap
initialized by
.Fn va_start
or
.Fn va_copy .
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 *
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),
random errors will occur.
.Pp
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_copy
macro copies a variable argument list, previously initialized by
.Fn va_start ,
from
.Fa src
to
.Fa dest .
The state is preserved such that it is equivalent to calling
.Fn va_start
with the same second argument used with
.Fa src ,
and calling
.Fn va_arg
the same number of times as called with
.Fa src .
.Pp
The
.Fn va_end
macro cleans up any state associated with the variable argument list
.Fa ap .
.Pp
Each invocation of
.Fn va_start
or
.Fn va_copy
must be paired with a corresponding invocation of
.Fn va_end
in the same function.
.Sh RETURN VALUES
The
.Fn va_arg
macro returns the value of the next argument.
.Pp
The
.Fn va_start ,
.Fn va_copy ,
and
.Fn va_end
macros return no value.
.Sh EXAMPLES
The function
.Em 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;
	char c, *s;

	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 */
			/* Note: char is promoted to int. */
			c = va_arg(ap, int);
			printf("char %c\en", c);
			break;
		}
	va_end(ap);
}
.Ed
.Sh COMPATIBILITY
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 .
.Sh STANDARDS
The
.Fn va_start ,
.Fn va_arg ,
.Fn va_copy ,
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
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
code,
but it also creates difficulties for variadic functions that
wish to pass all of their arguments on to a function
that takes a
.Em va_list
argument, such as
.Xr vfprintf 3 .