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24 .TH SETJMP 3 2017-03-13 "" "Linux Programmer's Manual"
26 setjmp, sigsetjmp, longjmp, siglongjmp \- performing a nonlocal goto
29 .B #include <setjmp.h>
31 .BI "int setjmp(jmp_buf " env );
32 .BI "int sigsetjmp(sigjmp_buf " env ", int " savesigs );
34 .BI "void longjmp(jmp_buf " env ", int " val );
35 .BI "void siglongjmp(sigjmp_buf " env ", int " val );
39 Feature Test Macro Requirements for glibc (see
40 .BR feature_test_macros (7)):
51 The functions described on this page are used for performing "nonlocal gotos":
52 transferring execution from one function to a predetermined location
56 function dynamically establishes the target to which control
57 will later be transferred, and
59 performs the transfer of execution.
63 function saves various information about the calling environment
64 (typically, the stack pointer, the instruction pointer,
65 possibly the values of other registers and the signal mask)
76 function uses the information saved in
78 to transfer control back to the point where
80 was called and to restore ("rewind") the stack to its state at the time of the
83 In addition, and depending on the implementation (see NOTES),
84 the values of some other registers and the process signal mask
85 may be restored to their state at the time of the
89 Following a successful
91 execution continues as if
93 had returned for a second time.
94 This "fake" return can be distinguished from a true
96 call because the "fake" return returns the value provided in
98 If the programmer mistakenly passes the value 0 in
100 the "fake" return will instead return 1.
101 .SS sigsetjmp() and siglongjmp()
105 also perform nonlocal gotos, but provide predictable handling of
106 the process signal mask.
112 is nonzero, the process's current signal mask is saved in
114 and will be restored if a
116 is later performed with this
122 return 0 when called directly;
123 on the "fake" return that occurs after
127 the nonzero value specified in
135 functions do not return.
137 For an explanation of the terms used in this section, see
145 Interface Attribute Value
149 T} Thread safety MT-Safe
153 T} Thread safety MT-Safe
161 POSIX.1-2001, POSIX.1-2008, C89, C99.
165 POSIX.1-2001, POSIX.1-2008.
167 POSIX does not specify whether
169 will save the signal mask
170 (to be later restored during
172 In System V it will not.
173 In 4.3BSD it will, and there
177 The behavior under Linux depends on the glibc version
178 and the setting of feature test macros.
179 On Linux with glibc versions before 2.19,
181 follows the System V behavior by default,
182 but the BSD behavior is provided if the
184 feature test macro is explicitly defined
185 .\" so that _FAVOR_BSD is triggered
188 .BR _POSIX_C_SOURCE ,
190 .\" .BR _XOPEN_SOURCE_EXTENDED ,
197 exposes only the System V version of
199 Programs that need the BSD semantics should replace calls to
210 can be useful for dealing with errors inside deeply nested function calls
211 or to allow a signal handler to pass control to
212 a specific point in the program,
213 rather than returning to the point where the handler interrupted
216 if you want to portably save and restore signal masks, use
220 See also the discussion of program readability below.
222 The compiler may optimize variables into registers, and
224 may restore the values of other registers in addition to the
225 stack pointer and program counter.
226 Consequently, the values of automatic variables are unspecified
229 if they meet all the following criteria:
231 they are local to the function that made the corresponding
235 their values are changed between the calls to
241 they are not declared as
244 Analogous remarks apply for
247 .SS Nonlocal gotos and program readability
248 While it can be abused,
249 the traditional C "goto" statement at least has the benefit that lexical cues
250 (the goto statement and the target label)
251 allow the programmer to easily perceive the flow of control.
252 Nonlocal gotos provide no such cues: multiple
254 calls might employ the same
256 variable so that the content of the variable may change
257 over the lifetime of the application.
258 Consequently, the programmer may be forced to perform detailed
259 reading of the code to determine the dynamic target of a particular
262 (To make the programmer's life easier, each
264 call should employ a unique
268 Adding further difficulty, the
272 calls may not even be in the same source code module.
274 In summary, nonlocal gotos can make programs harder to understand
275 and maintain, and an alternative should be used if possible.
278 If the function which called
282 is called, the behavior is undefined.
283 Some kind of subtle or unsubtle chaos is sure to result.
285 If, in a multithreaded program, a
289 buffer that was initialized by a call to
291 in a different thread, the behavior is undefined.
293 .\" The following statement appeared in versions up to POSIX.1-2008 TC1,
294 .\" but is set to be removed in POSIX.1-2008 TC2:
296 .\" According to POSIX.1, if a
298 .\" call is performed from a nested signal handler
299 .\" (i.e., from a handler that was invoked in response to a signal that was
300 .\" generated while another signal was already in the process of being
301 .\" handled), the behavior is undefined.
303 POSIX.1-2008 Technical Corrigendum 2 adds
304 .\" http://austingroupbugs.net/view.php?id=516#c1195
308 to the list of async-signal-safe functions.
309 However, the standard recommends avoiding the use of these functions
310 from signal handlers and goes on to point out that
311 if these functions are called from a signal handler that interrupted
312 a call to a non-async-signal-safe function (or some equivalent,
313 such as the steps equivalent to
315 that occur upon a return from the initial call to
317 the behavior is undefined if the program subsequently makes a call to
318 a non-async-signal-safe function.
319 The only way of avoiding undefined behavior is to ensure one of the following:
321 After long jumping from the signal handler,
322 the program does not call any non-async-signal-safe functions
323 and does not return from the initial call to
326 Any signal whose handler performs a long jump must be blocked during
328 call to a non-async-signal-safe function and
329 no non-async-signal-safe functions are called after
330 returning from the initial call to
334 .BR signal\-safety (7)