208 lines
5.4 KiB
Groff
208 lines
5.4 KiB
Groff
.\" Copyright (c) 1990, 1991, 1993
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.\" The Regents of the University of California. 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 8.1 (Berkeley) 6/5/93
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.\" $FreeBSD$
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.\"
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.Dd June 5, 1993
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.Dt STDARG 3
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.Os
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.Sh NAME
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.Nm stdarg
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.Nd variable argument lists
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.Sh SYNOPSIS
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.Fd #include <stdarg.h>
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.Ft void
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.Fn va_start "va_list ap" last
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.Ft type
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.Fn va_arg "va_list ap" type
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.Ft void
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.Fn va_end "va_list ap"
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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.
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The include file
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.Aq Pa stdarg.h
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declares a type
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.Pq Em va_list
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and defines three macros for stepping
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through a list of arguments whose number and types are not known to
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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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.Em va_list
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which is used by the macros
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.Fn va_start ,
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.Fn va_arg ,
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and
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.Fn va_end .
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.Pp
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The
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.Fn va_start
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macro initializes
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.Fa ap
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for subsequent use by
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.Fn va_arg
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and
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.Fn 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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.Fa last
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is the name of the last parameter before the variable argument list,
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i.e. 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 is used in the
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.Fn va_start
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macro, it should not be declared as a register variable, or as a
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function or an array type.
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.Pp
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The
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.Fn va_start
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macro returns no value.
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.Pp
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The
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.Fn 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.
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The parameter
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.Fa ap
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is the
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.Em va_list Fa ap
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initialized by
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.Fn va_start .
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Each call to
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.Fn va_arg
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modifies
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.Fa ap
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so that the next call returns the next argument.
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The parameter
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.Fa type
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is a type name specified so that the type of a pointer to an
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object that has the specified type can be obtained simply by
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adding a *
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to
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.Fa type .
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.Pp
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If there is no next argument, or if
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.Fa type
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is not compatible with the type of the actual next argument
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(as promoted according to the default argument promotions),
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random errors will occur.
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.Pp
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The first use of the
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.Fn va_arg
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macro after that of the
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.Fn va_start
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macro returns the argument after
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.Fa last .
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Successive invocations return the values of the remaining
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arguments.
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.Pp
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The
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.Fn va_end
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macro handles a normal return from the function whose variable argument
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list was initialized by
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.Fn va_start .
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.Pp
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The
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.Fn va_end
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macro returns no value.
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.Sh EXAMPLES
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The function
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.Em foo
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takes a string of format characters and prints out the argument
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associated with each format character based on the type.
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.Bd -literal -offset indent
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void foo(char *fmt, ...)
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{
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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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/* Note: char is promoted to int. */
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c = 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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.Ed
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.Sh STANDARDS
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The
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.Fn va_start ,
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.Fn va_arg ,
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and
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.Fn va_end
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macros conform to
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.St -isoC .
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.Sh COMPATIBILITY
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These macros are
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.Em not
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compatible with the historic macros they replace.
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A backward compatible version can be found in the include
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file
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.Aq Pa varargs.h .
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.Sh BUGS
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Unlike the
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.Em varargs
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macros, the
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.Nm
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macros do not permit programmers to
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code a function with no fixed arguments.
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This problem generates work mainly when converting
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.Em varargs
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code to
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.Nm
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code,
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but it also creates difficulties for variadic functions that
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wish to pass all of their arguments on to a function
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that takes a
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.Em va_list
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argument, such as
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.Xr vfprintf 3 .
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