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\"      @(#)stdarg.3    8.1 (Berkeley) 6/5/93
\"
Dd June 14, 2015
Dt STDARG 3
Os
Sh NAME
Nm stdarg ,
Nm va_arg  ,
Nm va_copy  ,
Nm va_end ,
Nm va_start
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 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
Em 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, or as a
function or an array type.
Pp
The
Fn va_start
macro returns no value.
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 .
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
If the type in question is one that gets promoted, the promoted type
should be used as the argument to
Fn va_arg .
The following describes which types are promoted (and to what):
Bl -dash -compact
It
Va short
is promoted to
Va int
It
Va float
is promoted to
Va double
It
Va char
is promoted to
Va int
El
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 makes
Fa dest
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_copy
macro returns no value.
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 .
Pp
The
Fn va_end
macro returns 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 COMPATIBILITY
These macros are
Em not
compatible with the historic
In varargs.h
macros they replaced.
Any remaining code using the pre-C89
In varargs.h
interface should be updated.
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 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 a
Em va_list
argument, such as
Xr vfprintf 3 .

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