[lug] Linux syscall 19 lseek() produces unexpected output

[Lori Reed]

Why assembly rather than C?

L.


Carl Hamlin wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Folks, I am *really* stumped. Here's the code:
> 
> section .data
> 
> 	fd1	dd	0
> 	testd	db	'Generic Text'
> 	testsz	equ	$ - testd
> 
> section .text
> global	_start
> 
> _start:
> 
> 	pop	ebx		;	ebx - argc
> 	pop	ebx		;	ebx - argv
> 	pop	ebx		;	ebx - filename to write to
> 	
> 	mov	eax, 8		;	eax - syscall_creat
> 	mov	ecx, 00664Q	;	ecx - read/write
> 	int	80h		;	syscall_creat
> 	mov	[fd1], eax	;	eax - file descriptor
> 	
> 	mov	eax, 19		;	eax - syscall_lseek
> 	mov	ebx, [fd1]	;	ebx - file descriptor
> 	sub	ecx, ecx	;	ecx - offset 0
> 	sub	edx, 2		;	edx - SEEK_END
> 	int	80h		;	syscall_lseek
> 	
> 	mov	eax, 4		;	eax - syscall_write
> 	mov	ebx, [fd1]	;	ebx - file descriptor
> 	mov	ecx, testd	;	ecx - pointer to data to be
> 				;	      written
> 	mov	edx, testsz	;	edx - number of bytes to be
> 				;	      written
> 	int	80h		;	syscall_write
> 	
> 	mov	eax, 1		;	eax - syscall_exit
> 	sub	ebx, ebx	;	ebx - errorcode 0
> 	int	80h		;	syscall_exit
> 
> This code *should* write 'Generic Text' at the end of a file specified
> on the command line.
> 
> What it does instead is to overwrite the file, if it exists. I've tried
> using open() instead of creat() with no luck. There is *very* little
> documentation on this on the internet, and believe me, if there is
> something that specifically referenced lseek(), I've seen it. This is my
> third day trying to figure this out.
> 
> Can anyone tell me what gives here?
> 
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.6 (GNU/Linux)
> 
> iD8DBQFHJfbahnZ133XoNC8RAkonAJ967lou8BRgp8w5j4UGayHWF2//XgCaAggM
> ec34snK9u4YsMZ3e/Uxoto0=
> =K0Ja
> -----END PGP SIGNATURE-----
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
> Join us on IRC: lug.boulder.co.us port=6667 channel=#colug
> 
> 
>