With MACH-O is there a way to register a function that will run before main?

Question

Under Linux, I can register a routine that will run before main. For example:

#include <stdio.h>

void myinit(int argc, char **argv, char **envp) {
  printf("%s: %s\n", __FILE__, __FUNCTION__);
}
__attribute__((section(".init_array"))) typeof(myinit) *__init = myinit;

By compiling this with GCC and linking it in, the function myinit will be run before main.

Is there way to do this under Mac OSX and MACH-O?

Thanks.

Answer 1

You could place the function in __mod_init_func data section of Mach-O binary.

From Mach-O format reference:

__DATA,__mod_init_func

Module initialization functions. The C++ compiler places static constructors here.

example.c

#include <stdio.h>

void myinit(int argc, char **argv, char **envp) {
  printf("%s: %s\n", __FILE__, __FUNCTION__);
}
__attribute__((section("__DATA,__mod_init_func"))) typeof(myinit) *__init = myinit;

int main() {
  printf("%s: %s\n", __FILE__, __FUNCTION__);
  return 0;
}

I build your example with clang on OS X platform:

$ clang -Wall example.c
$ ./a.out
example.c: myinit
example.c: main