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NAME
setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal goto
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <setjmp.h>
int setjmp(jmp_buf env);
int sigsetjmp(sigjmp_buf env, int savesigs);
[[noreturn]] void longjmp(jmp_buf env, int val);
[[noreturn]] void siglongjmp(sigjmp_buf env, int val);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
setjmp(): see NOTES.
sigsetjmp():
_POSIX_C_SOURCE
DESCRIPTION
The functions described on this page are used for performing "nonlocal gotos": transferring execution
from one function to a predetermined location in another function. The setjmp() function dynamically
establishes the target to which control will later be transferred, and longjmp() performs the transfer of
execution.
The setjmp() function saves various information about the calling environment (typically, the stack
pointer, the instruction pointer, possibly the values of other registers and the signal mask) in the
buffer env for later use by longjmp(). In this case, setjmp() returns 0.
The longjmp() function uses the information saved in env to transfer control back to the point where
setjmp() was called and to restore ("rewind") the stack to its state at the time of the setjmp() call.
In addition, and depending on the implementation (see NOTES), the values of some other registers and the
process signal mask may be restored to their state at the time of the setjmp() call.
Following a successful longjmp(), execution continues as if setjmp() had returned for a second time.
This "fake" return can be distinguished from a true setjmp() call because the "fake" return returns the
value provided in val. If the programmer mistakenly passes the value 0 in val, the "fake" return will
instead return 1.
sigsetjmp() and siglongjmp()
sigsetjmp() and siglongjmp() also perform nonlocal gotos, but provide predictable handling of the process
signal mask.
If, and only if, the savesigs argument provided to sigsetjmp() is nonzero, the process's current signal
mask is saved in env and will be restored if a siglongjmp() is later performed with this env.
RETURN VALUE
setjmp() and sigsetjmp() return 0 when called directly; on the "fake" return that occurs after longjmp()
or siglongjmp(), the nonzero value specified in val is returned.
The longjmp() or siglongjmp() functions do not return.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│setjmp(), sigsetjmp() │ Thread safety │ MT-Safe │
├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│longjmp(), siglongjmp() │ Thread safety │ MT-Safe │
└──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
setjmp()
longjmp()
C11, POSIX.1-2008.
sigsetjmp()
siglongjmp()
POSIX.1-2008.
HISTORY
setjmp()
longjmp()
POSIX.1-2001, C89.
sigsetjmp()
siglongjmp()
POSIX.1-2001.
POSIX does not specify whether setjmp() will save the signal mask (to be later restored during
longjmp()). In System V it will not. In 4.3BSD it will, and there is a function _setjmp() that will
not. The behavior under Linux depends on the glibc version and the setting of feature test macros.
Before glibc 2.19, setjmp() follows the System V behavior by default, but the BSD behavior is provided if
the _BSD_SOURCE feature test macro is explicitly defined and none of _POSIX_SOURCE, _POSIX_C_SOURCE,
_XOPEN_SOURCE, _GNU_SOURCE, or _SVID_SOURCE is defined. Since glibc 2.19, <setjmp.h> exposes only the
System V version of setjmp(). Programs that need the BSD semantics should replace calls to setjmp() with
calls to sigsetjmp() with a nonzero savesigs argument.
NOTES
setjmp() and longjmp() can be useful for dealing with errors inside deeply nested function calls or to
allow a signal handler to pass control to a specific point in the program, rather than returning to the
point where the handler interrupted the main program. In the latter case, if you want to portably save
and restore signal masks, use sigsetjmp() and siglongjmp(). See also the discussion of program
readability below.
CAVEATS
The compiler may optimize variables into registers, and longjmp() may restore the values of other
registers in addition to the stack pointer and program counter. Consequently, the values of automatic
variables are unspecified after a call to longjmp() if they meet all the following criteria:
• they are local to the function that made the corresponding setjmp() call;
• their values are changed between the calls to setjmp() and longjmp(); and
• they are not declared as volatile.
Analogous remarks apply for siglongjmp().
Nonlocal gotos and program readability
While it can be abused, the traditional C "goto" statement at least has the benefit that lexical cues
(the goto statement and the target label) allow the programmer to easily perceive the flow of control.
Nonlocal gotos provide no such cues: multiple setjmp() calls might employ the same jmp_buf variable so
that the content of the variable may change over the lifetime of the application. Consequently, the
programmer may be forced to perform detailed reading of the code to determine the dynamic target of a
particular longjmp() call. (To make the programmer's life easier, each setjmp() call should employ a
unique jmp_buf variable.)
Adding further difficulty, the setjmp() and longjmp() calls may not even be in the same source code
module.
In summary, nonlocal gotos can make programs harder to understand and maintain, and an alternative should
be used if possible.
Undefined Behavior
If the function which called setjmp() returns before longjmp() is called, the behavior is undefined.
Some kind of subtle or unsubtle chaos is sure to result.
If, in a multithreaded program, a longjmp() call employs an env buffer that was initialized by a call to
setjmp() in a different thread, the behavior is undefined.
POSIX.1-2008 Technical Corrigendum 2 adds longjmp() and siglongjmp() to the list of async-signal-safe
functions. However, the standard recommends avoiding the use of these functions from signal handlers and
goes on to point out that if these functions are called from a signal handler that interrupted a call to
a non-async-signal-safe function (or some equivalent, such as the steps equivalent to exit(3) that occur
upon a return from the initial call to main()), the behavior is undefined if the program subsequently
makes a call to a non-async-signal-safe function. The only way of avoiding undefined behavior is to
ensure one of the following:
• After long jumping from the signal handler, the program does not call any non-async-signal-safe
functions and does not return from the initial call to main().
• Any signal whose handler performs a long jump must be blocked during every call to a non-async-signal-
safe function and no non-async-signal-safe functions are called after returning from the initial call
to main().
SEE ALSO
signal(7), signal-safety(7)