What we have discovered is not just an understanding of how the Linux kernel works, but also a solution for creating a compile-time list of function pointers which call be called in the order they are initialized. Taking the code from the previous section and making it more stand-alone and general purpose we end up with the following: (notice how no linker script is required)
/*
* Copyright (C) 2006 Trevor Woerner
*/
#include <stdio.h>
typedef void (*funcptr_t)(void);
extern funcptr_t __start_newsect, __stop_newsect;
#define data_attr __attribute__ ((section ("newsect")))
#define create_entry(fn) funcptr_t _##fn data_attr = fn
void my_init1 (void) { printf ("my_init1() #1\n"); }
void my_init2 (void) { printf ("my_init2() #2\n"); }
create_entry (my_init1);
create_entry (my_init2);
int
main (void)
{
funcptr_t *call_p;
call_p = &__start_newsect;
do {
printf ("call_p: %p\n", call_p);
(*call_p)();
++call_p;
} while (call_p < &__stop_newsect);
return 0;
}Which generates:
[trevor@trevor code]$ make mycalls cc mycalls.c -o mycalls [trevor@trevor code]$ ./mycalls call_p: 0x804960c my_init1() #1 call_p: 0x8049610 my_init2() #2
and
0804830c g F .text 00000000 _start 0804830c l d .text 00000000 .text 08048330 l F .text 00000000 call_gmon_start 08048354 l F .text 00000000 __do_global_dtors_aux 08048388 l F .text 00000000 frame_dummy 080483b0 g F .text 00000018 my_init1 080483c8 g F .text 00000018 my_init2 080483e0 g F .text 00000053 main 08048434 g F .text 0000004f __libc_csu_init 08048484 g F .text 00000005 __libc_csu_fini 0804848c l F .text 00000000 __do_global_ctors_aux ... 0804960c g *ABS* 00000000 __start_newsect 0804960c g O newsect 00000004 _my_init1 0804960c l d newsect 00000000 newsect 08049610 g O newsect 00000004 _my_init2 08049614 g *ABS* 00000000 __bss_start 08049614 g *ABS* 00000000 __stop_newsect
Notice also how the my_initN() functions were put in the general .text segment since I don't really care where they end up (unlike the kernel guys).