1 /* Startup code compliant to the ELF i386 ABI.
2 Copyright (C) 1995-2012 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
10 In addition to the permissions in the GNU Lesser General Public
11 License, the Free Software Foundation gives you unlimited
12 permission to link the compiled version of this file with other
13 programs, and to distribute those programs without any restriction
14 coming from the use of this file. (The GNU Lesser General Public
15 License restrictions do apply in other respects; for example, they
16 cover modification of the file, and distribution when not linked
17 into another program.)
19 Note that people who make modified versions of this file are not
20 obligated to grant this special exception for their modified
21 versions; it is their choice whether to do so. The GNU Lesser
22 General Public License gives permission to release a modified
23 version without this exception; this exception also makes it
24 possible to release a modified version which carries forward this
27 The GNU C Library is distributed in the hope that it will be useful,
28 but WITHOUT ANY WARRANTY; without even the implied warranty of
29 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
30 Lesser General Public License for more details.
32 You should have received a copy of the GNU Lesser General Public
33 License along with the GNU C Library; if not, see
34 <http://www.gnu.org/licenses/>. */
36 /* This is the canonical entry point, usually the first thing in the text
37 segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
38 point runs, most registers' values are unspecified, except for:
40 %edx Contains a function pointer to be registered with `atexit'.
41 This is how the dynamic linker arranges to have DT_FINI
42 functions called for shared libraries that have been loaded
43 before this code runs.
45 %esp The stack contains the arguments and environment:
50 (4*(argc+1))(%esp) envp[0]
59 .type _start,@function
61 /* Clear the frame pointer. The ABI suggests this be done, to mark
62 the outermost frame obviously. */
65 /* Extract the arguments as encoded on the stack and set up
66 the arguments for `main': argc, argv. envp will be determined
67 later in __libc_start_main. */
68 popl %esi /* Pop the argument count. */
69 movl %esp, %ecx /* argv starts just at the current stack top.*/
71 /* Before pushing the arguments align the stack to a 16-byte
72 (SSE needs 16-byte alignment) boundary to avoid penalties from
73 misaligned accesses. Thanks to Edward Seidl <seidl@janed.com>
74 for pointing this out. */
75 andl $0xfffffff0, %esp
76 pushl %eax /* Push garbage because we allocate
79 /* Provide the highest stack address to the user code (for stacks
80 which grow downwards). */
83 pushl %edx /* Push address of the shared library
84 termination function. */
87 /* Load PIC register. */
89 addl $_GLOBAL_OFFSET_TABLE_, %ebx
91 /* Push address of our own entry points to .fini and .init. */
92 leal __libc_csu_fini@GOTOFF(%ebx), %eax
94 leal __libc_csu_init@GOTOFF(%ebx), %eax
97 pushl %ecx /* Push second argument: argv. */
98 pushl %esi /* Push first argument: argc. */
100 pushl BP_SYM (main)@GOT(%ebx)
102 /* Call the user's main function, and exit with its value.
103 But let the libc call main. */
104 call BP_SYM (__libc_start_main)@PLT
106 /* Push address of our own entry points to .fini and .init. */
107 pushl $__libc_csu_fini
108 pushl $__libc_csu_init
110 pushl %ecx /* Push second argument: argv. */
111 pushl %esi /* Push first argument: argc. */
115 /* Call the user's main function, and exit with its value.
116 But let the libc call main. */
117 call BP_SYM (__libc_start_main)
120 hlt /* Crash if somehow `exit' does return. */
127 /* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
128 meaningless since we don't support machines < 80386. */
135 /* Define a symbol for the first piece of initialized data. */
141 data_start = __data_start