How to pick up a symbol in C code which was defined in a .ld linkfile?

[peter-h]

This is weird.

In the linkfile I have

 _build_type = 1;

and sure enough in the .map file I see

0x0000000000000001                _build_type = 0x1

but this code

Code: [Select]

extern char _build_type;
strcat((char *)PAGE_BODY, "<br>Build: ");
if ( _build_type == 0 )
{
strcat((char *)PAGE_BODY, "v0");
}
else if ( _build_type == 1 )
{
strcat((char *)PAGE_BODY, "v1");
}
always sees it as zero. Confirmed by stepping through it.

I have also tried int instead of char - doesn't pick up a value at all. I am wondering whether every value in a linkfile is actually implicitly an address, so one can't do this?

But then this type of construct works

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extern char _s_nonboot_data;
extern char _e_nonboot_data;
extern char _si_nonboot_data;
memcpy(&_s_nonboot_data, &_si_nonboot_data, &_e_nonboot_data - &_s_nonboot_data);

and "obviously" 0 and 1 are also valid addresses.

Obviously I could have a .h file with a #define in it, etc, but for other reasons I am trying to select a text string based on a symbol defined 0/1 in the linkfile. I can build two different versions of the project which are very similar but have different linkfiles.

[pcprogrammer]

I have used this in projects of mine and it looks like this:

Link file

Code: [Select]

MY_LINK_VARIABLE = 10;

Code file

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if(&MY_LINK_VARIABLE == 10)
{
  ...
}
else
{
  ...
}

This is with the GNU compiler for ARM.

[peter-h]

I tried that (my memcpy example above uses it) but



Maybe it works with values other than 0 but that produces "warning: comparison between pointer and integer".

So maybe (char*) 0 or (char*) 1.

[pcprogrammer]

Ah yes, that is how it is used in my code, for addresses. Maybe there is another way then the & sign to have it be the actual variable instead of the address.

Could it be that the variable needs to be in a section part of the link file?

[peter-h]

I am defining other stuff in the linkfile, and not inside any special section, but they are all memory addresses.

Such a simple thing! But this works:

Code: [Select]

extern char _build_type;
strcat((char *)PAGE_BODY, "<br>Build: ");
if ( &_build_type == (char*) 1 )
{
strcat((char *)PAGE_BODY, "v1");
}
else if ( &_build_type == (char*) 2 )
{
strcat((char *)PAGE_BODY, "v2");
}

so using 1 or 2, not 0. Odd since addresses can be 0 too, although usually this indicates a cockup (null pointer, etc). Maybe to test for ==0 one should use ==NULL.

[Nominal Animal]

In the linkfile I have

 _build_type = 1;

As you already realized, in the linker script you can define symbol addresses, not symbol values.

In C code, you could use
    extern const char _build_type[];
    #define  BUILD_TYPE  ((uintptr_t)(_build_type))
since the name of an array decays to a pointer.  With this, _build_type == (void *)1 and BUILD_TYPE == 1.