Duplicately defined symbols -- why?

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kc5tja

Joined: 17 Mar 2005
Posts: 3

kc5tja 17 Mar 2005, 20:50

I have a problem where I use a symbol in a .text section, but the symbol is defined in a .data section.

Code:

  format elf
  use32

  section ".text"
  extrn banner

myFunction:
  ...
  mov eax,banner
  mov ebx,bannerLength
  call textOut
  ...

  section ".data"
  public banner

banner:
  db "Hello world",13,10,0
bannerLength equ $-banner

The problem is, it won't assemble regardless of the combination of public and extrn. Here's a table that gives the output I'm seeing:

Code:

EXTRN   PUBLIC   ERROR (where error is flagged)
----------------------------------------------------------
  -       -      undefined symbol (in .text section)
  -       +      undefined symbol (in .text section)
  +       -      symbol already defined (in .data section)
  +       +      symbol already defined (in .data section)
----------------------------------------------------------

I cannot find any example or FAQ or other documentation item that would explain this behavior. This, by all appearances, is a bug. Is there some kind of convenient work-around to this? I'm using FASM 1.60 under Linux.

Thanks for your help.

17 Mar 2005, 20:50

kc5tja

Joined: 17 Mar 2005
Posts: 3

kc5tja 17 Mar 2005, 21:28

And, actually, it appears that EQU symbols do not span section boundaries. A big pity. Sad

17 Mar 2005, 21:28

mike.dld

Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk

mike.dld 17 Mar 2005, 22:20

It compiles OK with '=' and without 'extrn'... And, actually 'extrn' means that 'banner' isn't defined in your source elsewhere.

Code:

  format elf
  use32

  section ".text"

myFunction:
 ...
  mov eax,banner
  mov ebx,bannerLength
  call textOut
 ...

  section ".data"
  public banner

banner:
  db "Hello world",13,10,0
bannerLength = $-banner

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Last edited by mike.dld on 17 Mar 2005, 23:08; edited 1 time in total

17 Mar 2005, 22:20

kc5tja

Joined: 17 Mar 2005
Posts: 3

kc5tja 17 Mar 2005, 23:00

mike.dld wrote:

It compiles OK with '=' and without 'extrn'... And, actually 'extrn' means that 'banner' isn't defined is your source elsewhere.

Thanks for the help and clarification. I think I understand the intended use of extrn (err...any reason why we're missing the 'e' in extern? Is this anything like creat? ;D Although, being that I come from an Amiga/Commodore background, I much prefer XREF and XDEF myself).

I am still confused by the difference in semantics between = and EQU though. To me, they are identical. I'll re-read the documentation again to make sure I didn't miss anything important.

Thanks again!

17 Mar 2005, 23:00

mike.dld

Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk

mike.dld 17 Mar 2005, 23:06

The first striking thing for me is that you cannot write 'abc = esp' i.e. '=' is only (and preffered) for numeric expressions AFAIK.

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17 Mar 2005, 23:06

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 18 Mar 2005, 00:05

kc5tja wrote:

I am still confused by the difference in semantics between = and EQU though. To me, they are identical. I'll re-read the documentation again to make sure I didn't miss anything important.

"equ" is not equal to "=". Actually they are very different and intended for different things.
Just stop using "equ" until you really need it. Actually its use is needed only in very few cases, mainly in macro declaration and other preprocessor tricks.

Regards.

18 Mar 2005, 00:05

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