1/* Startup code compliant to the ELF i386 ABI.
2 Copyright (C) 1995-2024 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
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.
9
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.)
18
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
25 exception.
26
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.
31
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 <https://www.gnu.org/licenses/>. */
35
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:
39
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.
44
45 %esp The stack contains the arguments and environment:
46 0(%esp) argc
47 4(%esp) argv[0]
48 ...
49 (4*argc)(%esp) NULL
50 (4*(argc+1))(%esp) envp[0]
51 ...
52 NULL
53*/
54
55#include <sysdep.h>
56
57ENTRY (_start)
58 /* Clearing frame pointer is insufficient, use CFI. */
59 cfi_undefined (eip)
60 /* Clear the frame pointer. The ABI suggests this be done, to mark
61 the outermost frame obviously. */
62 xorl %ebp, %ebp
63
64 /* Extract the arguments as encoded on the stack and set up
65 the arguments for `main': argc, argv. envp will be determined
66 later in __libc_start_main. */
67 popl %esi /* Pop the argument count. */
68 movl %esp, %ecx /* argv starts just at the current stack top.*/
69
70 /* Before pushing the arguments align the stack to a 16-byte
71 (SSE needs 16-byte alignment) boundary to avoid penalties from
72 misaligned accesses. Thanks to Edward Seidl <seidl@janed.com>
73 for pointing this out. */
74 andl $0xfffffff0, %esp
75 pushl %eax /* Push garbage because we allocate
76 28 more bytes. */
77
78 /* Provide the highest stack address to the user code (for stacks
79 which grow downwards). */
80 pushl %esp
81
82 pushl %edx /* Push address of the shared library
83 termination function. */
84
85#ifdef PIC
86 /* Load PIC register. */
87 call 1f
88 addl $_GLOBAL_OFFSET_TABLE_, %ebx
89
90 /* This used to be the addresses of .fini and .init. */
91 pushl $0
92 pushl $0
93
94 pushl %ecx /* Push second argument: argv. */
95 pushl %esi /* Push first argument: argc. */
96
97# ifdef SHARED
98 pushl main@GOT(%ebx)
99# else
100 /* Avoid relocation in static PIE since _start is called before
101 it is relocated. This also avoid rely on linker optimization to
102 transform 'movl main@GOT(%ebx), %eax' to 'leal main@GOTOFF(%ebx)'
103 if main is defined locally. */
104 leal __wrap_main@GOTOFF(%ebx), %eax
105 pushl %eax
106# endif
107
108 /* Call the user's main function, and exit with its value.
109 But let the libc call main. */
110 call __libc_start_main@PLT
111#else
112 /* This used to be the addresses of .fini and .init. */
113 pushl $0
114 pushl $0
115
116 pushl %ecx /* Push second argument: argv. */
117 pushl %esi /* Push first argument: argc. */
118
119 pushl $main
120
121 /* Call the user's main function, and exit with its value.
122 But let the libc call main. */
123 call __libc_start_main
124#endif
125
126 hlt /* Crash if somehow `exit' does return. */
127
128#ifdef PIC
1291: movl (%esp), %ebx
130 ret
131#endif
132
133#if defined PIC && !defined SHARED
134__wrap_main:
135 jmp main@PLT
136#endif
137END (_start)
138
139/* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
140 meaningless since we don't support machines < 80386. */
141 .section .rodata
142 .globl _fp_hw
143_fp_hw: .long 3
144 .size _fp_hw, 4
145 .type _fp_hw,@object
146
147/* Define a symbol for the first piece of initialized data. */
148 .data
149 .globl __data_start
150__data_start:
151 .long 0
152 .weak data_start
153 data_start = __data_start
154

source code of glibc/sysdeps/i386/start.S