[SOLVED] how to access linker symbol in c code with MinGW?

Issue

I am using Mingw gcc and ld to generate an executable for windows. I would like to access the start address of a section in c code, but I have tried a lot of methods and no one works.

My linker script file has this:

  .data BLOCK(__section_alignment__) :
  {
    __data_start__ = . ;
    *(.data)
    *(.data2)
    *(SORT(.data$*))
    KEEP(*(.jcr))
    __data_end__ = . ;
    *(.data_cygwin_nocopy)
  }

In C code I do this:

extern char  __data_start__;
uint32_t  test = &__data_start__;

And get this error: undefined reference to __data_start__

Could anyone help me with this? Thanks

Solution

mingw32 is using leading underscores, i.e. the compiler will add a _ (underscore) to each symbol. The part of the linker description file you are showing is part of the default linker script (at least in my installation), and for your code I am getting the mentioned error for

int main()
{
    extern char __data_start__;
    return (int) &__data_start__;
}

with -save-temps, the assembly code has

    movl    $___data_start__, %eax

which has 3 leading _'s not just 2.

Thus, there are 2 solutions: You can advise the compiler to emit __data_start__ as assembly name, which is a GCC extension:

int main()
{
    extern char some_identifier __asm("__data_start__");
    return (int) &some_identifier;
}

Or you can use _data_start:

int main()
{
    extern char _data_start__;
    return (int) &_data_start__;
}

Both use __data_start__ in assembly / object file:

    movl    $__data_start__, %eax

i686-w64-mingw32-nm module.o

         U __data_start__

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