glibc/sysdeps/x86_64/elf/start.S

/* Startup code compliant to the ELF x86-64 ABI.
   Copyright (C) 2001 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Andreas Jaeger <aj@suse.de>, 2001.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

/* This is the canonical entry point, usually the first thing in the text
   segment.  The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
   point runs, most registers' values are unspecified, except for:

   %rdx		Contains a function pointer to be registered with `atexit'.
		This is how the dynamic linker arranges to have DT_FINI
		functions called for shared libraries that have been loaded
		before this code runs.

   %rsp		The stack contains the arguments and environment:
		0(%rsp)			argc
		8(%rsp)			argv[0]
		...
		(8*argc)(%rsp)		NULL
		(8*(argc+1))(%rsp)	envp[0]
		...
					NULL
*/

#include "bp-sym.h"

	.text
	.globl _start
	.type _start,@function
_start:
	/* Clear the frame pointer.  The ABI suggests this be done, to mark
	   the outermost frame obviously.  */
	xorq %rbp, %rbp

	/* Extract the arguments as encoded on the stack and set up
	   the arguments for __libc_start_main (int (*main) (int, char **, char **),
		   int argc, char *argv,
		   void (*init) (void), void (*fini) (void),
		   void (*rtld_fini) (void), void *stack_end).
	   The arguments are passed via registers and on the stack:
	main:		%rdi
	argc:		%rsi
	argv:		%rdx
	init:		%rcx
	fini:		%r8
	rtld_fini:	%r9
	stack_end:	stack.	*/

	movq %rdx, %r9		/* Address of the shared library termination
				   function.  */
	popq %rsi		/* Pop the argument count.  */
	movq %rsp, %rdx		/* argv starts just at the current stack top.  */
	/* Align the stack to a 16 byte boundary to follow the ABI.  */
	andq  $~15, %rsp

	pushq %rax		/* Push garbage because we push 8 more bytes.  */

	/* Provide the highest stack address to the user code (for stacks
	   which grow downwards).  */
	pushq %rsp

	/* Pass address of our own entry points to .fini and .init.  */
	movq $_fini, %r8
	movq $_init, %rcx

	movq $BP_SYM (main), %rdi

	/* Call the user's main function, and exit with its value.
	   But let the libc call main.	  */
	call BP_SYM (__libc_start_main)

	hlt			/* Crash if somehow `exit' does return.	 */

/* Define a symbol for the first piece of initialized data.  */
	.data
	.globl __data_start
__data_start:
	.long 0
	.weak data_start
	data_start = __data_start