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To: xavicardona AT rocketmail DOT com, djgpp AT delorie DOT com
From: Nate Eldredge <nate AT cartsys DOT com>
Subject: Re: assembler & C variables
Date: Mon, 4 May 1998 17:06:56 -0700
Message-ID: <19980505000653.AAA24646@ppp117.cartsys.com>
Precedence: bulk

At 09:24  5/4/1998 -0700, Xavi Cardona wrote:
>!
>I'm starting assembler programming. I have read all FAQs,docs & txt
>about this, but I don't know why this program not works:
>
>/* Program asmvar.cc: this program puts 10 in myvar */
>#include <stdio.h>
>main()
>{char myvar =0;
>__asm__ __volatile__ ("movb $10,_myvar"
>        	      :
>      	              :
>      	              :"memory");
>printf("i=%d\n",myvar);
>}
>
>I compile it with:
>	gcc -O2 -o asmvar.exe asmvar.cc
>
>But, the result is: 
>	In function main
>		undefined reference to "_myvar"

Because in C++ variable names are mangled, and local variables lose their
names before assembly time anyway. This is why GCC has its extended
assembly. Try:

char myvar = 0;
asm("movb $10, %0"
    : "=g" (myvar)
    : ); /* no inputs, nothing unusual clobbered */

Nate Eldredge
nate AT cartsys DOT com