GCC: how to tell GCC to put the 'main' function at the start of the .text section?

Question

I've just started learning some ARM programming and I've got stuck in a slightly annoying problem. The toolchain I'm using to compile my sources is Sourcery CodeBench Lite 2013.05-23 (can be found here: https://sourcery.mentor.com/GNUToolchain/release2449)

What I would need is to tell GCC or LD or OBJCOPY to put the compiled bytecode of the 'main' function at the beginning of the .text section.

Is there any way to achieve this? (maybe through a linker script?)

Thank you

Answer 1

Solved the problem. For whoever faces it:

• When compiling with GCC, add the -ffunction-sections option in the command-line. This will tell GCC to put each function in a separate section. The format of the section name will be .text.#function name#, without the # (that is, if the function belongs to the .text section [ which by default is true ]).

• Secondly, use a linker script to order these "function-sections" into the final big .text section. As an example, putting the main function at the beginning of the .text section would result in an LD script that looks approximately like this:

  ENTRY(main)
  SECTIONS
  {
      .text :
      {
          *(.text.main);
          *(.text*);
      }
  }
  

Answer 2

First, see how is the .text section defined in your gcc's default linker script (so you don't have to make your own), by calling it as:

gcc -Wl,-verbose

that will print out the default linker script. Mine shows this for the .text section:

/* text: Program code section */
  .text : 
  {
    *(.text)
    *(.text.*)
    *(.gnu.linkonce.t.*)
  }

So in order to have the "main" function be the first in the .text section (and the rest be contiguous), you have to set the "section" attribute for all other functions. For example:

void main(void);
void funct1(....) __attribute__ ((section (".text.A")));
void funct2(....) __attribute__ ((section (".text.A")));
void funct3(....) __attribute__ ((section (".text.A")));

It's enough with "attributing" the function prototypes. That way, when you compile now, the "main" function will be the first one in the ".text" section and all others will follow on the immediately consecutive addresses.

If you want to place the ".text" section (i.e. "main" function) at a specific address (for example 0x1000), remember to link with:

gcc .... -Wl,-Ttext=0x1000

Answer 3

You can also just put 'main' in its own section using an __attribute__:

int main (void) __attribute__ ((section ("entry")));

and then in the ld file:

ENTRY(main)
SECTIONS
{
    .text :
    {
        *(main)
        *(.text)
    }
}

There are plenty of other interesting __attributes__, read more about them here: http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html