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Date: Wed, 18 Jan 95 17:50:38 IST
From: "Ugen J.S.Antsilevich" <ugen@netvision.net.il>
Subject: bug in GCC or in lseek/read???
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Here it comes.
I am not sure but it seems like GCC bug in handling quad (long long)
types....
Here is the programm.What it tries to do is - to read some buffer from arbitrary
file,then lseek back and re-read same place again.Actually i ran into the
problemm when really working on very same code.
Now look into text and try compiling it,you will see very interesting behaviour.

If you'll take a look at assembly code you'll see that -sizeof(<anything>)
replaced in quad_t as 0 and then -size(-5 in this example).
on the other hand -<number> is turned into -1 -<number> (-1 -5 here).
The cast to (int) makes same result and works.The cast to off_t type which
is right does not works.

Now: lseek returns same value in all cases-both in those where read fails
and works.

So probably two things here: bug or at least inconsistency in GCC which
makes worng conversions + some strange bug in lseek/read calls 
which i personally can't still figure out..
or may be i am too dumb?????
-------------------------CUT HERE(and look in code)-----------------------

#include <fcntl.h>
#include <unistd.h>

main()
{
int f;
char buf[5],buf1[5];

f=open("/bin/ls",O_RDONLY); /* any file actually we don't care */
printf("seek %u\n",lseek(f,100,SEEK_SET)); /* move into the middle 
					      somewhere.Actually bug
					      shows without this string */
if (read(f,buf,5)<=0) {
	printf("First read failed\n");
	exit(1);
}

printf("%d\n",-sizeof(buf)); /* just to show you size of this buffer */
#error ------------ Here choose *ONE* from following 4 strings-------
printf("seek %u\n",lseek(f,-sizeof(buf),SEEK_CUR)); /* No cast - does not works */
printf("seek %u\n",lseek(f,(off_t)-sizeof(buf),SEEK_CUR)); /*right cast-does not works 
*/ 
printf("seek %u\n",lseek(f,(int)-sizeof(buf),SEEK_CUR)); /*wrong cast-works OK*/ 
printf("seek %u\n",lseek(f,-5,SEEK_CUR)); /* number only-works OK*/

if (read(f,buf1,5)<=0) {
	printf("Second read failed\n"); /* This one will fail BUT!!!
					   'errno' is set to 0!!! */
	exit(1);
}
if (!strncmp(buf,buf1,5))   /* just to show thet if everything OK, we read same string 
*/
	printf("match\n");
}


-------------------------------END-----------------------------------

-- 
-=Ugen J.S.Antsilevich=-
NetVision - Israeli Commercial Internet          |  Learning 
E-mail: ugen@NetVision.net.il                    | To Fly. [c]
Phone : +972-4-550330                            |