mirror of https://github.com/mkerrisk/man-pages
291 lines
7.4 KiB
Groff
291 lines
7.4 KiB
Groff
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the American National Standards Committee X3, on Information
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.\" Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)stdarg.3 6.8 (Berkeley) 6/29/91
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.\"
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.\" Converted for Linux, Mon Nov 29 15:11:11 1993, faith@cs.unc.edu
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.\" Additions, 2001-10-14, aeb
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.\"
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.TH STDARG 3 2001-10-14 "" "Linux Programmer's Manual"
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.SH NAME
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stdarg \- variable argument lists
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.SH SYNOPSIS
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.B #include <stdarg.h>
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.sp
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.BI "void va_start(va_list " ap ", " last );
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.br
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.BI "" type " va_arg(va_list " ap ", " type );
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.br
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.BI "void va_end(va_list " ap );
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.br
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.BI "void va_copy(va_list " dest ", va_list " src );
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.SH DESCRIPTION
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A function may be called with a varying number of arguments of varying
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types. The include file
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.I stdarg.h
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declares a type
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.B va_list
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and defines three macros for stepping through a list of arguments whose
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number and types are not known to the called function.
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.PP
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The called function must declare an object of type
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.B va_list
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which is used by the macros
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.BR va_start ,
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.BR va_arg ,
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and
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.BR va_end .
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.SS va_start
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The
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.B va_start
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macro initializes
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.I ap
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for subsequent use by
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.B va_arg
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and
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.BR va_end ,
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and must be called first.
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.PP
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The parameter
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.I last
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is the name of the last parameter before the variable argument list, i.e.,
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the last parameter of which the calling function knows the type.
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.PP
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Because the address of this parameter may be used in the
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.B va_start
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macro, it should not be declared as a register variable,
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or as a function or an array type.
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.SS va_arg
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The
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.B va_arg
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macro expands to an expression that has the type and value of the next
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argument in the call. The parameter
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.I ap
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is the
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.B va_list
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.I ap
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initialized by
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.BR va_start .
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Each call to
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.B va_arg
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modifies
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.I ap
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so that the next call returns the next argument. The parameter
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.I type
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is a type name specified so that the type of a pointer to an object that
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has the specified type can be obtained simply by adding a * to
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.IR type .
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.PP
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The first use of the
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.B va_arg
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macro after that of the
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.B va_start
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macro returns the argument after
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.IR last .
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Successive invocations return the values of the remaining arguments.
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.PP
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If there is no next argument, or if
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.I type
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is not compatible with the type of the actual next argument (as promoted
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according to the default argument promotions), random errors will occur.
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.PP
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If
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.I ap
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is passed to a function that uses
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.BI va_arg( ap , type )
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then the value of
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.I ap
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is undefined after the return of that function.
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.SS va_end
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Each invocation of
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.B va_start
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must be matched by a corresponding invocation of
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.B va_end
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in the same function. After the call
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.BI va_end( ap )
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the variable
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.I ap
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is undefined. Multiple transversals of the list, each
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bracketed by
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.B va_start
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and
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.B va_end
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are possible.
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.B va_end
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may be a macro or a function.
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.SS va_copy
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.\" Proposal from clive@demon.net, 1997-02-28
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An obvious implementation would have a
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.B va_list
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a pointer to the stack frame of the variadic function.
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In such a setup (by far the most common) there seems
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nothing against an assignment
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.RS
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.nf
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va_list aq = ap;
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.fi
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.RE
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Unfortunately, there are also systems that make it an
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array of pointers (of length 1), and there one needs
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.RS
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.nf
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va_list aq;
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*aq = *ap;
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.fi
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.RE
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Finally, on systems where parameters are passed in registers,
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it may be necessary for
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.B va_start
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to allocate memory, store the parameters there, and also
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an indication of which parameter is next, so that
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.B va_arg
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can step through the list. Now
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.B va_end
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can free the allocated memory again.
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To accommodate this situation, C99 adds a macro
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.BR va_copy ,
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so that the above assignment can be replaced by
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.RS
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.nf
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va_list aq;
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va_copy(aq, ap);
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...
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va_end(aq);
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.fi
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.RE
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Each invocation of
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.B va_copy
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must be matched by a corresponding invocation of
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.B va_end
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in the same function.
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Some systems that do not supply
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.B va_copy
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have
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.B __va_copy
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instead, since that was the name used in the draft proposal.
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.SH EXAMPLES
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The function
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.I foo
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takes a string of format characters and prints out the argument associated
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with each format character based on the type.
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.RS
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.nf
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#include <stdio.h>
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#include <stdarg.h>
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void foo(char *fmt, ...) {
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va_list ap;
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int d;
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char c, *s;
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va_start(ap, fmt);
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while (*fmt)
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switch(*fmt++) {
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case 's': /* string */
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s = va_arg(ap, char *);
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printf("string %s\en", s);
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break;
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case 'd': /* int */
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d = va_arg(ap, int);
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printf("int %d\en", d);
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break;
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case 'c': /* char */
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/* need a cast here since va_arg only
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takes fully promoted types */
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c = (char) va_arg(ap, int);
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printf("char %c\en", c);
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break;
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}
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va_end(ap);
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}
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.fi
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.RE
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.SH "CONFORMING TO"
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The
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.BR va_start ,
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.BR va_arg ,
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and
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.B va_end
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macros conform to ANSI X3.159-1989 (``C89'').
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C99 defines the
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.B va_copy
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macro.
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.SH COMPATIBILITY
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These macros are
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.I not
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compatible with the historic macros they replace. A backward compatible
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version can be found in the include file
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.IR varargs.h .
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.SH COMPARISON
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The historic setup is:
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.RS
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.nf
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#include <varargs.h>
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void foo(va_alist) va_dcl {
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va_list ap;
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va_start(ap);
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while(...) {
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...
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x = va_arg(ap, type);
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...
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}
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va_end(ap);
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}
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.fi
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.RE
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On some systems,
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.I va_end
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contains a closing '}' matching a '{' in
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.IR va_start ,
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so that both macros must occur in the same function, and in a way
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that allows this.
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.SH BUGS
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Unlike the
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.B varargs
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macros, the
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.B stdarg
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macros do not permit programmers to code a function with no fixed
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arguments. This problem generates work mainly when converting
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.B varargs
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code to
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.B stdarg
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code, but it also creates difficulties for variadic functions that wish to
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pass all of their arguments on to a function that takes a
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.B va_list
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argument, such as
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.BR vfprintf (3).
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