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Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!cwruecmp!hal!ncoast!allbery
From: allbery@ncoast.UUCP
Newsgroups: net.sources
Subject: Re: Public Domain _doprnt in C
Message-ID: <2285@ncoast.UUCP>
Date: Sat, 4-Apr-87 11:01:34 EST
Article-I.D.: ncoast.2285
Posted: Sat Apr  4 11:01:34 1987
Date-Received: Sun, 5-Apr-87 13:01:33 EST
References: <938@copper.TEK.COM> <100@asi.UUCP>
Reply-To: allbery@ncoast.UUCP (Brandon Allbery)
Followup-To: net.sources
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 70

As quoted from <100@asi.UUCP> by metro@asi.UUCP (Metro T. Sauper):
+---------------
| I am not familiar with the _doprnt() routine, however I had a need to perform
| a similar function.  I wanted to write a function of the form:
>...
| fperror(fmt,args) char *fmt; int args;
| {
| 	char msgbuff[128];
| 
| 	vsprintf(msgbuff,fmt,&args);	/* format args into msgbuff based on fmt */
+---------------

This works on some systems, but if you have vsprintf() you canb also make 
these functions fully portable.  Use varargs.

Skeleton which appears in many programs I write (see io.c in UNaXcess
Conferencing as an example):

#include <varargs.h>

void writef(va_alist)
va_dcl {
	register char *fmt;
	static char _buf[5120];
	va_list args;
	
	va_start(args);
	fmt = va_arg(args, char *);	/* NO PARENTHESES AROUND THE TYPE! */
					/* They will cause a syntax error */
	(void) vsprintf(_buf, fmt, args);
	va_end(args);
	for (fmt = _buf; *fmt != '\0'; fmt++)
		writec(*fmt);
}

Note that varargs is defined such that it will work even on machines such as
the Pyramid which pass arguments in registers (common on RISC machines).
You MUST declare only one argument -- va_alist -- and include the type of it
as "va_dcl" without a semicolon.  (C compilers can then recognize this
directly to do any special processing needed, as for the Pyramid.)  Variables
declared as "va_list" can then be passed to other functions as arguments,
which is what makes the v*printf() functions work.

I have seen varargs implemented under:

AT&T System V
Plexus Sys3 -- and possibly vanilla AT&T System III
Xenix 2.x (Tandy 16a)
Xenix 3.x (Tandy 6000, Altos x86)
Pyramid OS/x
Ultrix 1.1 (MicroVax)

Moreover, UNaXcess Conferencing seems to be running on quite a few systems
by means of it.  (Actually, UC doesn't call vsprintf, since that function
doesn't exist on all systems; it has its own format code.  You're free to
lift it out and use it, provided you attribute it to me and clearly state
any changes you make to it so I don't get bug reports on your changes.)

++Brandon
-- 
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