man3/stdarg.3

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.\"	@(#)stdarg.3	6.8 (Berkeley) 6/29/91
.\"
.\" Converted for Linux, Mon Nov 29 15:11:11 1993, faith@cs.unc.edu
.\" Additions, 2001-10-14, aeb
.\"
.TH STDARG 3  2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
stdarg, va_start, va_arg, va_end, va_copy \- variable argument lists
.SH SYNOPSIS
.nf
.B #include <stdarg.h>
.PP
.BI "void va_start(va_list " ap ", " last );
.IB type " va_arg(va_list " ap ", " type );
.BI "void va_end(va_list " ap );
.BI "void va_copy(va_list " dest ", va_list " src );
.fi
.SH DESCRIPTION
A function may be called with a varying number of arguments of varying
types.
The include file
.I <stdarg.h>
declares a type
.I 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
.I va_list
which is used by the macros
.BR va_start (),
.BR va_arg (),
and
.BR va_end ().
.SS va_start()
The
.BR va_start ()
macro initializes
.I ap
for subsequent use by
.BR va_arg ()
and
.BR va_end (),
and must be called first.
.PP
The argument
.I last
is the name of the last argument before the variable argument list, that is,
the last argument of which the calling function knows the type.
.PP
Because the address of this argument may be used in the
.BR va_start ()
macro, it should not be declared as a register variable,
or as a function or an array type.
.SS va_arg()
The
.BR va_arg ()
macro expands to an expression that has the type and value of the next
argument in the call.
The argument
.I ap
is the
.I va_list
.I ap
initialized by
.BR va_start ().
Each call to
.BR va_arg ()
modifies
.I ap
so that the next call returns the next argument.
The argument
.I 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
.IR type .
.PP
The first use of the
.BR va_arg ()
macro after that of the
.BR va_start ()
macro returns the argument after
.IR last .
Successive invocations return the values of the remaining arguments.
.PP
If there is no next argument, or if
.I 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
.I ap
is passed to a function that uses
.BI va_arg( ap , type ),
then the value of
.I ap
is undefined after the return of that function.
.SS va_end()
Each invocation of
.BR va_start ()
must be matched by a corresponding invocation of
.BR va_end ()
in the same function.
After the call
.BI va_end( ap )
the variable
.I ap
is undefined.
Multiple traversals of the list, each
bracketed by
.BR va_start ()
and
.BR va_end ()
are possible.
.BR va_end ()
may be a macro or a function.
.SS va_copy()
The
.BR va_copy ()
macro copies the (previously initialized) variable argument list
.I src
to
.IR dest .
The behavior is as if
.BR va_start ()
were applied to
.IR dest
with the same
.I last
argument, followed by the same number of
.BR va_arg ()
invocations that was used to reach the current state of
.IR src .
.PP
.\" Proposal from clive@demon.net, 1997-02-28
An obvious implementation would have a
.I va_list
be a pointer to the stack frame of the variadic function.
In such a setup (by far the most common) there seems
nothing against an assignment
.PP
.in +4n
.EX
va_list aq = ap;
.EE
.in
.PP
Unfortunately, there are also systems that make it an
array of pointers (of length 1), and there one needs
.PP
.in +4n
.EX
va_list aq;
*aq = *ap;
.EE
.in
.PP
Finally, on systems where arguments are passed in registers,
it may be necessary for
.BR va_start ()
to allocate memory, store the arguments there, and also
an indication of which argument is next, so that
.BR va_arg ()
can step through the list.
Now
.BR va_end ()
can free the allocated memory again.
To accommodate this situation, C99 adds a macro
.BR va_copy (),
so that the above assignment can be replaced by
.PP
.in +4n
.EX
va_list aq;
va_copy(aq, ap);
\&...
va_end(aq);
.EE
.in
.PP
Each invocation of
.BR va_copy ()
must be matched by a corresponding invocation of
.BR va_end ()
in the same function.
Some systems that do not supply
.BR va_copy ()
have
.B __va_copy
instead, since that was the name used in the draft proposal.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR va_start (),
.BR va_end (),
.BR va_copy ()
T}	Thread safety	MT-Safe
T{
.BR va_arg ()
T}	Thread safety	MT-Safe race:ap
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
The
.BR va_start (),
.BR va_arg (),
and
.BR va_end ()
macros conform to C89.
C99 defines the
.BR va_copy ()
macro.
.SH BUGS
Unlike the historical
.B varargs
macros, the
.B stdarg
macros do not permit programmers to code a function with no fixed
arguments.
This problem generates work mainly when converting
.B varargs
code to
.B 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
.I va_list
argument, such as
.BR vfprintf (3).
.SH EXAMPLES
The function
.I foo
takes a string of format characters and prints out the argument associated
with each format character based on the type.
.PP
.EX
#include <stdio.h>
#include <stdarg.h>

void
foo(char *fmt, ...)   /* \(aq...\(aq is C syntax for a variadic function */

{
    va_list ap;
    int d;
    char c;
    char *s;

    va_start(ap, fmt);
    while (*fmt)
        switch (*fmt++) {
        case \(aqs\(aq:              /* string */
            s = va_arg(ap, char *);
            printf("string %s\en", s);
            break;
        case \(aqd\(aq:              /* int */
            d = va_arg(ap, int);
            printf("int %d\en", d);
            break;
        case \(aqc\(aq:              /* char */
            /* need a cast here since va_arg only
               takes fully promoted types */
            c = (char) va_arg(ap, int);
            printf("char %c\en", c);
            break;
        }
    va_end(ap);
}
.EE
.SH SEE ALSO
.BR vprintf (3),
.BR vscanf (3),
.BR vsyslog (3)