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

[Ted Logan]

Do you want:

       mov     edx, 2          ;       edx - SEEK_END

instead of:

       sub     edx, 2          ;       edx - SEEK_END

That is, assigning edx = SEEK_END, rather than subtracting 2 from
whatever value edx held before?

On 10/29/07, Carl Hamlin <carl at eceria.com> wrote:
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> 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?
>
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-- 
Ted Logan
Software Engineer
ted.logan at gmail.com
http://jaeger.festing.org/