1/* Startup code compliant to the ELF i386 ABI.
2 Copyright (C) 1995-2022 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. Don't use "leal main@GOTOFF(%ebx), %eax"
102 since main may be in a shared object. Linker will convert
103 "movl main@GOT(%ebx), %eax" to "leal main@GOTOFF(%ebx), %eax"
104 if main is defined locally. */
105 movl main@GOT(%ebx), %eax
106 pushl %eax
107# endif
108
109 /* Call the user's main function, and exit with its value.
110 But let the libc call main. */
111 call __libc_start_main@PLT
112#else
113 /* This used to be the addresses of .fini and .init. */
114 pushl $0
115 pushl $0
116
117 pushl %ecx /* Push second argument: argv. */
118 pushl %esi /* Push first argument: argc. */
119
120 pushl $main
121
122 /* Call the user's main function, and exit with its value.
123 But let the libc call main. */
124 call __libc_start_main
125#endif
126
127 hlt /* Crash if somehow `exit' does return. */
128
129#ifdef PIC
1301: movl (%esp), %ebx
131 ret
132#endif
133END (_start)
134
135/* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
136 meaningless since we don't support machines < 80386. */
137 .section .rodata
138 .globl _fp_hw
139_fp_hw: .long 3
140 .size _fp_hw, 4
141 .type _fp_hw,@object
142
143/* Define a symbol for the first piece of initialized data. */
144 .data
145 .globl __data_start
146__data_start:
147 .long 0
148 .weak data_start
149 data_start = __data_start
150

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