1SETJMP(3) Linux Programmer's Manual SETJMP(3)
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6 setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal goto
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9 #include <setjmp.h>
10 int setjmp(jmp_buf env);
12 int sigsetjmp(sigjmp_buf env, int savesigs);
13 noreturn void longjmp(jmp_buf env, int val);
15 noreturn void siglongjmp(sigjmp_buf env, int val);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 setjmp(): see NOTES.
20 sigsetjmp():
22 _POSIX_C_SOURCE
23
25 The functions described on this page are used for performing "nonlocal
26 gotos": transferring execution from one function to a predetermined lo‐
27 cation in another function. The setjmp() function dynamically estab‐
28 lishes the target to which control will later be transferred, and
29 longjmp() performs the transfer of execution.
30 The setjmp() function saves various information about the calling envi‐
32 ronment (typically, the stack pointer, the instruction pointer, possi‐
33 bly the values of other registers and the signal mask) in the buffer
34 env for later use by longjmp(). In this case, setjmp() returns 0.
35 The longjmp() function uses the information saved in env to transfer
37 control back to the point where setjmp() was called and to restore
38 ("rewind") the stack to its state at the time of the setjmp() call. In
39 addition, and depending on the implementation (see NOTES), the values
40 of some other registers and the process signal mask may be restored to
41 their state at the time of the setjmp() call.
42 Following a successful longjmp(), execution continues as if setjmp()
44 had returned for a second time. This "fake" return can be distin‐
45 guished from a true setjmp() call because the "fake" return returns the
46 value provided in val. If the programmer mistakenly passes the value 0
47 in val, the "fake" return will instead return 1.
48 sigsetjmp() and siglongjmp()
50 sigsetjmp() and siglongjmp() also perform nonlocal gotos, but provide
51 predictable handling of the process signal mask.
52 If, and only if, the savesigs argument provided to sigsetjmp() is non‐
54 zero, the process's current signal mask is saved in env and will be re‐
55 stored if a siglongjmp() is later performed with this env.
56
58 setjmp() and sigsetjmp() return 0 when called directly; on the "fake"
59 return that occurs after longjmp() or siglongjmp(), the nonzero value
60 specified in val is returned.
61 The longjmp() or siglongjmp() functions do not return.
63
65 For an explanation of the terms used in this section, see at‐
66 tributes(7).
67 ┌────────────────────────────────────────────┬───────────────┬─────────┐
69 │Interface │ Attribute │ Value │
70 ├────────────────────────────────────────────┼───────────────┼─────────┤
71 │setjmp(), sigsetjmp() │ Thread safety │ MT-Safe │
72 ├────────────────────────────────────────────┼───────────────┼─────────┤
73 │longjmp(), siglongjmp() │ Thread safety │ MT-Safe │
74 └────────────────────────────────────────────┴───────────────┴─────────┘
75
77 setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.
78 sigsetjmp(), siglongjmp(): POSIX.1-2001, POSIX.1-2008.
80
82 POSIX does not specify whether setjmp() will save the signal mask (to
83 be later restored during longjmp()). In System V it will not. In
84 4.3BSD it will, and there is a function _setjmp() that will not. The
85 behavior under Linux depends on the glibc version and the setting of
86 feature test macros. On Linux with glibc versions before 2.19,
87 setjmp() follows the System V behavior by default, but the BSD behavior
88 is provided if the _BSD_SOURCE feature test macro is explicitly defined
89 and none of _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _GNU_SOURCE,
90 or _SVID_SOURCE is defined. Since glibc 2.19, <setjmp.h> exposes only
91 the System V version of setjmp(). Programs that need the BSD semantics
92 should replace calls to setjmp() with calls to sigsetjmp() with a non‐
93 zero savesigs argument.
94 setjmp() and longjmp() can be useful for dealing with errors inside
96 deeply nested function calls or to allow a signal handler to pass con‐
97 trol to a specific point in the program, rather than returning to the
98 point where the handler interrupted the main program. In the latter
99 case, if you want to portably save and restore signal masks, use
100 sigsetjmp() and siglongjmp(). See also the discussion of program read‐
101 ability below.
102 The compiler may optimize variables into registers, and longjmp() may
104 restore the values of other registers in addition to the stack pointer
105 and program counter. Consequently, the values of automatic variables
106 are unspecified after a call to longjmp() if they meet all the follow‐
107 ing criteria:
108 • they are local to the function that made the corresponding setjmp()
110 call;
111 • their values are changed between the calls to setjmp() and
113 longjmp(); and
114 • they are not declared as volatile.
116 Analogous remarks apply for siglongjmp().
118 Nonlocal gotos and program readability
120 While it can be abused, the traditional C "goto" statement at least has
121 the benefit that lexical cues (the goto statement and the target label)
122 allow the programmer to easily perceive the flow of control. Nonlocal
123 gotos provide no such cues: multiple setjmp() calls might employ the
124 same jmp_buf variable so that the content of the variable may change
125 over the lifetime of the application. Consequently, the programmer may
126 be forced to perform detailed reading of the code to determine the dy‐
127 namic target of a particular longjmp() call. (To make the programmer's
128 life easier, each setjmp() call should employ a unique jmp_buf vari‐
129 able.)
130 Adding further difficulty, the setjmp() and longjmp() calls may not
132 even be in the same source code module.
133 In summary, nonlocal gotos can make programs harder to understand and
135 maintain, and an alternative should be used if possible.
136 Caveats
138 If the function which called setjmp() returns before longjmp() is
139 called, the behavior is undefined. Some kind of subtle or unsubtle
140 chaos is sure to result.
141 If, in a multithreaded program, a longjmp() call employs an env buffer
143 that was initialized by a call to setjmp() in a different thread, the
144 behavior is undefined.
145 POSIX.1-2008 Technical Corrigendum 2 adds longjmp() and siglongjmp() to
147 the list of async-signal-safe functions. However, the standard recom‐
148 mends avoiding the use of these functions from signal handlers and goes
149 on to point out that if these functions are called from a signal han‐
150 dler that interrupted a call to a non-async-signal-safe function (or
151 some equivalent, such as the steps equivalent to exit(3) that occur up‐
152 on a return from the initial call to main()), the behavior is undefined
153 if the program subsequently makes a call to a non-async-signal-safe
154 function. The only way of avoiding undefined behavior is to ensure one
155 of the following:
156 * After long jumping from the signal handler, the program does not
158 call any non-async-signal-safe functions and does not return from
159 the initial call to main().
160 * Any signal whose handler performs a long jump must be blocked during
162 every call to a non-async-signal-safe function and no non-async-sig‐
163 nal-safe functions are called after returning from the initial call
164 to main().
165
167 signal(7), signal-safety(7)
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170 This page is part of release 5.12 of the Linux man-pages project. A
171 description of the project, information about reporting bugs, and the
172 latest version of this page, can be found at
173 https://www.kernel.org/doc/man-pages/.
174 2021-03-22 SETJMP(3)