1SETJMP(3)                  Linux Programmer's Manual                 SETJMP(3)
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NAME

6       setjmp, sigsetjmp, longjmp, siglongjmp  - performing a nonlocal goto
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SYNOPSIS

9       #include <setjmp.h>
10
11       int setjmp(jmp_buf env);
12       int sigsetjmp(sigjmp_buf env, int savesigs);
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14       void longjmp(jmp_buf env, int val);
15       void siglongjmp(sigjmp_buf env, int val);
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17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       setjmp(): see NOTES.
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21       sigsetjmp(): _POSIX_C_SOURCE
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DESCRIPTION

24       The  functions described on this page are used for performing "nonlocal
25       gotos": transferring execution from one function to a predetermined lo‐
26       cation  in  another function.  The setjmp() function dynamically estab‐
27       lishes the target to which  control  will  later  be  transferred,  and
28       longjmp() performs the transfer of execution.
29
30       The setjmp() function saves various information about the calling envi‐
31       ronment (typically, the stack pointer, the instruction pointer,  possi‐
32       bly  the  values  of other registers and the signal mask) in the buffer
33       env for later use by longjmp().  In this case, setjmp() returns 0.
34
35       The longjmp() function uses the information saved in  env  to  transfer
36       control  back  to  the  point  where setjmp() was called and to restore
37       ("rewind") the stack to its state at the time of the setjmp() call.  In
38       addition,  and  depending on the implementation (see NOTES), the values
39       of some other registers and the process signal mask may be restored  to
40       their state at the time of the setjmp() call.
41
42       Following  a  successful  longjmp(), execution continues as if setjmp()
43       had returned for a second time.  This  "fake"  return  can  be  distin‐
44       guished from a true setjmp() call because the "fake" return returns the
45       value provided in val.  If the programmer mistakenly passes the value 0
46       in val, the "fake" return will instead return 1.
47
48   sigsetjmp() and siglongjmp()
49       sigsetjmp()  and  siglongjmp() also perform nonlocal gotos, but provide
50       predictable handling of the process signal mask.
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52       If, and only if, the savesigs argument provided to sigsetjmp() is  non‐
53       zero, the process's current signal mask is saved in env and will be re‐
54       stored if a siglongjmp() is later performed with this env.
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RETURN VALUE

57       setjmp() and sigsetjmp() return 0 when called directly; on  the  "fake"
58       return  that  occurs after longjmp() or siglongjmp(), the nonzero value
59       specified in val is returned.
60
61       The longjmp() or siglongjmp() functions do not return.
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ATTRIBUTES

64       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
65       tributes(7).
66
67       ┌────────────────────────┬───────────────┬─────────┐
68Interface               Attribute     Value   
69       ├────────────────────────┼───────────────┼─────────┤
70setjmp(), sigsetjmp()   │ Thread safety │ MT-Safe │
71       ├────────────────────────┼───────────────┼─────────┤
72longjmp(), siglongjmp() │ Thread safety │ MT-Safe │
73       └────────────────────────┴───────────────┴─────────┘

CONFORMING TO

75       setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.
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77       sigsetjmp(), siglongjmp(): POSIX.1-2001, POSIX.1-2008.
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NOTES

80       POSIX  does  not specify whether setjmp() will save the signal mask (to
81       be later restored during longjmp()).  In System  V  it  will  not.   In
82       4.3BSD  it  will, and there is a function _setjmp() that will not.  The
83       behavior under Linux depends on the glibc version and  the  setting  of
84       feature  test  macros.   On  Linux  with  glibc  versions  before 2.19,
85       setjmp() follows the System V behavior by default, but the BSD behavior
86       is provided if the _BSD_SOURCE feature test macro is explicitly defined
87       and none of _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _GNU_SOURCE,
88       or  _SVID_SOURCE is defined.  Since glibc 2.19, <setjmp.h> exposes only
89       the System V version of setjmp().  Programs that need the BSD semantics
90       should  replace calls to setjmp() with calls to sigsetjmp() with a non‐
91       zero savesigs argument.
92
93       setjmp() and longjmp() can be useful for  dealing  with  errors  inside
94       deeply  nested function calls or to allow a signal handler to pass con‐
95       trol to a specific point in the program, rather than returning  to  the
96       point  where  the  handler interrupted the main program.  In the latter
97       case, if you want to  portably  save  and  restore  signal  masks,  use
98       sigsetjmp() and siglongjmp().  See also the discussion of program read‐
99       ability below.
100
101       The compiler may optimize variables into registers, and  longjmp()  may
102       restore  the values of other registers in addition to the stack pointer
103       and program counter.  Consequently, the values of  automatic  variables
104       are  unspecified after a call to longjmp() if they meet all the follow‐
105       ing criteria:
106
107       •  they are local to the function that made the corresponding  setjmp()
108          call;
109
110       •  their   values  are  changed  between  the  calls  to  setjmp()  and
111          longjmp(); and
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113       •  they are not declared as volatile.
114
115       Analogous remarks apply for siglongjmp().
116
117   Nonlocal gotos and program readability
118       While it can be abused, the traditional C "goto" statement at least has
119       the benefit that lexical cues (the goto statement and the target label)
120       allow the programmer to easily perceive the flow of control.   Nonlocal
121       gotos  provide  no  such cues: multiple setjmp() calls might employ the
122       same jmp_buf variable so that the content of the  variable  may  change
123       over the lifetime of the application.  Consequently, the programmer may
124       be forced to perform detailed reading of the code to determine the  dy‐
125       namic target of a particular longjmp() call.  (To make the programmer's
126       life easier, each setjmp() call should employ a  unique  jmp_buf  vari‐
127       able.)
128
129       Adding  further  difficulty,  the  setjmp() and longjmp() calls may not
130       even be in the same source code module.
131
132       In summary, nonlocal gotos can make programs harder to  understand  and
133       maintain, and an alternative should be used if possible.
134
135   Caveats
136       If  the  function  which  called  setjmp()  returns before longjmp() is
137       called, the behavior is undefined.  Some kind  of  subtle  or  unsubtle
138       chaos is sure to result.
139
140       If,  in a multithreaded program, a longjmp() call employs an env buffer
141       that was initialized by a call to setjmp() in a different  thread,  the
142       behavior is undefined.
143
144       POSIX.1-2008 Technical Corrigendum 2 adds longjmp() and siglongjmp() to
145       the list of async-signal-safe functions.  However, the standard  recom‐
146       mends avoiding the use of these functions from signal handlers and goes
147       on to point out that if these functions are called from a  signal  han‐
148       dler  that  interrupted  a call to a non-async-signal-safe function (or
149       some equivalent, such as the steps equivalent  to  exit(3)  that  occur
150       upon  a  return from the initial call to main()), the behavior is unde‐
151       fined if the program subsequently makes a call to  a  non-async-signal-
152       safe  function.   The only way of avoiding undefined behavior is to en‐
153       sure one of the following:
154
155       *  After long jumping from the signal handler,  the  program  does  not
156          call  any  non-async-signal-safe  functions and does not return from
157          the initial call to main().
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159       *  Any signal whose handler performs a long jump must be blocked during
160          every call to a non-async-signal-safe function and no non-async-sig‐
161          nal-safe functions are called after returning from the initial  call
162          to main().
163

SEE ALSO

165       signal(7), signal-safety(7)
166

COLOPHON

168       This  page  is  part of release 5.10 of the Linux man-pages project.  A
169       description of the project, information about reporting bugs,  and  the
170       latest     version     of     this    page,    can    be    found    at
171       https://www.kernel.org/doc/man-pages/.
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175                                  2017-03-13                         SETJMP(3)
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