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eda078d4 | 1 | .\" Copyright (C) 2008, 2014, Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 2 | .\" |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
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4 | .\" Permission is granted to make and distribute verbatim copies of this |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
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13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
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21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
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24 | .\" |
25 | .\" Created Sat Aug 21 1995 Thomas K. Dyas <tdyas@eden.rutgers.edu> | |
26 | .\" Modified Tue Oct 22 22:09:03 1996 by Eric S. Raymond <esr@thyrsus.com> | |
0678a1d4 | 27 | .\" 2008-06-26, mtk, added some more detail on the work done by sigreturn() |
eda078d4 | 28 | .\" 2014-12-05, mtk, rewrote all of the rest of the original page |
fea681da | 29 | .\" |
29b41e74 | 30 | .TH SIGRETURN 2 2015-12-28 "Linux" "Linux Programmer's Manual" |
fea681da | 31 | .SH NAME |
d893db64 | 32 | sigreturn, rt_sigreturn \- return from signal handler and cleanup stack frame |
fea681da | 33 | .SH SYNOPSIS |
eda078d4 | 34 | .BI "int sigreturn(...);" |
fea681da | 35 | .SH DESCRIPTION |
eda078d4 MK |
36 | If the Linux kernel determines that an unblocked |
37 | signal is pending for a process, then, | |
38 | at the next transition back to user mode in that process | |
39 | (e.g., upon return from a system call or | |
40 | when the process is rescheduled onto the CPU), | |
41 | it saves various pieces of process context | |
42 | (processor status word, registers, signal mask, and signal stack settings) | |
43 | into the user-space stack. | |
44 | .\" See arch/x86/kernel/signal.c::__setup_frame() [in 3.17 source code] | |
45 | ||
46 | The kernel also arranges that, during the transition back to user mode, | |
47 | the signal handler is called, and that, upon return from the handler, | |
48 | control passes to a piece of user-space code commonly called | |
49 | the "signal trampoline". | |
50 | The signal trampoline code in turn calls | |
51 | .BR sigreturn (). | |
0678a1d4 | 52 | |
00ac6ce4 | 53 | This |
e511ffb6 | 54 | .BR sigreturn () |
0678a1d4 | 55 | call undoes everything that was |
eda078d4 | 56 | done\(emchanging the process's signal mask, switching signal stacks (see |
0678a1d4 | 57 | .BR sigaltstack "(2))\(emin " |
eda078d4 MK |
58 | order to invoke the signal handler. |
59 | It restores the process's signal mask, switches stacks, | |
60 | and restores the process's context | |
61 | (processor flags and registers, | |
62 | including the stack pointer and instruction pointer), | |
63 | so that the process resumes execution | |
0678a1d4 | 64 | at the point where it was interrupted by the signal. |
47297adb | 65 | .SH RETURN VALUE |
e511ffb6 | 66 | .BR sigreturn () |
fea681da | 67 | never returns. |
47297adb | 68 | .SH CONFORMING TO |
eda078d4 | 69 | Many UNIX-type systems have a |
2dd578fd | 70 | .BR sigreturn () |
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71 | system call or near equivalent. |
72 | However, this call is not specified in POSIX, | |
73 | and details of its behavior vary across systems. | |
6c36ac81 | 74 | .SH NOTES |
e511ffb6 | 75 | .BR sigreturn () |
eda078d4 | 76 | exists only to allow the implementation of signal handlers. |
c13182ef | 77 | It should |
fea681da | 78 | .B never |
c13182ef | 79 | be called directly. |
eda078d4 MK |
80 | Details of the arguments (if any) passed to |
81 | .BR sigreturn () | |
82 | vary depending on the architecture. | |
83 | ||
84 | Once upon a time, UNIX systems placed the signal trampoline code | |
85 | onto the user stack. | |
86 | Nowadays, pages of the user stack are protected so as to | |
87 | disallow code execution. | |
88 | Thus, on contemporary Linux systems, depending on the architecture, | |
89 | the signal trampoline code lives either in the | |
90 | .BR vdso (7) | |
91 | or in the C library. | |
92 | In the latter case, | |
93 | .\" See, for example, sysdeps/unix/sysv/linux/i386/sigaction.c and | |
94 | .\" sysdeps/unix/sysv/linux/x86_64/sigaction.c in the glibc (2.20) source. | |
95 | the C library supplies the location of the trampoline code using the | |
96 | .I sa_restorer | |
97 | field of the | |
98 | .I sigaction | |
99 | structure that is passed to | |
100 | .BR sigaction (2), | |
101 | and sets the | |
102 | .BR SA_RESTORER | |
103 | flag in the | |
104 | .IR sa_flags | |
105 | field. | |
106 | ||
107 | The saved process context information is placed in a | |
108 | .I ucontext_t | |
109 | structure (see | |
110 | .IR <sys/ucontext.h> ). | |
111 | That structure is visible within the signal handler | |
112 | as the third argument of a handler established with the | |
113 | .BR SA_SIGINFO | |
114 | flag. | |
115 | ||
116 | On some other UNIX systems, | |
117 | the operation of the signal trampoline differs a little. | |
118 | In particular, on some systems, upon transitioning back to user mode, | |
119 | the kernel passes control to the trampoline (rather than the signal handler), | |
120 | and the trampoline code calls the signal handler (and then calls | |
121 | .BR sigreturn () | |
122 | once the handler returns). | |
d893db64 | 123 | .\" |
0722a578 | 124 | .SS C library/kernel differences |
d893db64 MK |
125 | The original Linux system call was named |
126 | .BR sigreturn (). | |
127 | However, with the addition of real-time signals in Linux 2.2, | |
128 | a new system call, | |
129 | .BR rt_sigreturn () | |
130 | was added to support an enlarged | |
131 | .IR sigset_t | |
132 | type. | |
133 | The GNU C library | |
134 | hides these details from us, transparently employing | |
135 | .BR rt_sigreturn () | |
136 | when the kernel provides it. | |
137 | .\" | |
47297adb | 138 | .SH SEE ALSO |
fea681da | 139 | .BR kill (2), |
d806bc05 | 140 | .BR restart_syscall (2), |
e91751c9 | 141 | .BR sigaltstack (2), |
fea681da | 142 | .BR signal (2), |
eda078d4 | 143 | .BR getcontext (3), |
cc158601 MK |
144 | .BR signal (7), |
145 | .BR vdso (7) |