1 /* Startup code compliant to the ELF x86-64 ABI.
2 Copyright (C) 2001 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Andreas Jaeger <aj@suse.de>, 2001.
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, write to the Free
18 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
21 /* This is the canonical entry point, usually the first thing in the text
22 segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
23 point runs, most registers' values are unspecified, except for:
25 %rdx Contains a function pointer to be registered with `atexit'.
26 This is how the dynamic linker arranges to have DT_FINI
27 functions called for shared libraries that have been loaded
28 before this code runs.
30 %rsp The stack contains the arguments and environment:
35 (8*(argc+1))(%rsp) envp[0]
44 .type _start,@function
46 /* Clear the frame pointer. The ABI suggests this be done, to mark
47 the outermost frame obviously. */
50 /* Extract the arguments as encoded on the stack and set up
51 the arguments for __libc_start_main (int (*main) (int, char **, char **),
53 void (*init) (void), void (*fini) (void),
54 void (*rtld_fini) (void), void *stack_end).
55 The arguments are passed via registers and on the stack:
64 movq %rdx, %r9 /* Address of the shared library termination
66 popq %rsi /* Pop the argument count. */
67 movq %rsp, %rdx /* argv starts just at the current stack top. */
68 /* Align the stack to a 16 byte boundary to follow the ABI. */
71 pushq %rax /* Push garbage because we push 8 more bytes. */
73 /* Provide the highest stack address to the user code (for stacks
74 which grow downwards). */
77 /* Pass address of our own entry points to .fini and .init. */
81 movq $BP_SYM (main), %rdi
83 /* Call the user's main function, and exit with its value.
84 But let the libc call main. */
85 call BP_SYM (__libc_start_main)
87 hlt /* Crash if somehow `exit' does return. */
89 /* Define a symbol for the first piece of initialized data. */
95 data_start = __data_start