C Runtime.md

Points To Remember

Some definitions:

• PIC - position independent code (-fPIC)
• PIE - position independent executable (-fPIE -pie)
• crt - C runtime

File	Description
crt0.o	Will contain the _start function that initializes the process
crtbegin.o	GCC uses this to find the start of the constructors(init).
crtend.o	GCC uses this to find the start of the destructors(fini).
crti.o	Header of init & fini (for push in stack)
crtn.o	Footer of init & fini (for pop in stack)
Scrt1.o	Used in place of crt1.o when generating PIEs.

• There could be crt1.o, crt2.o & so on, depending upon implementation, crt0.c is runtime 0 & runs first
• glibc ports call this file 'start.S' while uClibc ports call this crt0.S or crt1.S
• General linking order:

crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o

crt0.s

.text 
.global  _start  
_start : 
       xor %rbp,%rbp 
       mov %rsp,%rdi 
.weak _DYNAMIC 
.hidden _DYNAMIC 
       lea _DYNAMIC(%rip),%rsi 
       andq $-16,%rsp 
       call  _start_c 

Note: Sometime this code could be in crt_arch.h

crt1.c

#include <features.h>

#define START "_start"

#include "crt_arch.h"

int main();
void _init() __attribute__((weak));
void _fini() __attribute__((weak));
_Noreturn int __libc_start_main(int (*)(), int, char **,
        void (*)(), void(*)(), void(*)());

void _start_c(long *p)
{
        int argc = p[0];
        char **argv = (void *)(p+1);
        __libc_start_main(main, argc, argv, _init, _fini, 0);
}

crti.s

.section .init
.global _init
_init:
        push %rax

.section .fini
.global _fini
_fini:
        push %rax

crtn.s

.section .init
        pop %rax
        ret

.section .fini
        pop %rax
        ret

Flow of x86 C program with runtime

Routine	File
_start	./arch/x86_64/crt_arch.h
_start_c	./crt/crt1.c
__libc_start_main	./src/env/__libc_start_main.c
_init (push)	./crt/x86_64/crti.s
main	Our Program
exit	Close process
_fini (pop)	./crt/x86_64/crtn.s
_Exit	Exit system call