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Re: GOT error in gas

From: Nick Clifton
Subject: Re: GOT error in gas
Date: Thu, 15 Feb 2007 17:33:18 +0000
User-agent: Thunderbird (X11/20061215)

Hi Mikulas,

__asm__ (".global number; number = 0x12345678");
extern void number;

These two declarations are not compatible.  The latter declares number as
a data symbol, but the former defines it is an absolute symbol.

I thought that .types do not care for linking,

Andreas is not talking about .types. He is talking about the sections to which the symbol belongs. Writing "extern void number" declares "number" as a symbol that will live in the .data section(1). The address of "number" is not known until the linker has performed a final link (for static code) or the loader has initialised the executable (for PIC code).

Writing "__asm__(".global number; number=0x12345678")" however declares "number" as a symbol with an absolute *address*. The symbol does not have a value, or rather its value is whatever happens to be in the memory location 0x12345678. This symbol does not live in a section, and its address does not change during linking or loading.

Hence the two declarations are inconsistent and you get undefined behaviour.

How otherwise should external C variables be placed at absolute locations?

You could adapt the mechanism that you already have. You say that everything works if the __asm__ statement is in a separate compilation unit, so just split out all of your absolute C variables into one (or more) separate files and have a header file containing "extern void..." declarations for them.

Alternatively you could provide the addresses for these symbols via a linker script, rather than trying to define them in C. For example:

  % cat addr.t
  number = ABSOLUTE (0x12345678);

  % cat test.c
  #include <stdio.h>
  extern void number;
  int main (void) { return printf ("%p\n", & number); }

  % gcc test.c -Wl,addr.t -fPIC

  % ./a.out


(1) Or some similar section such as .common or .sdata.

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