setjmp(3) - Linux manual page (original) (raw)
setjmp(3) Library Functions Manual setjmp(3)
NAME top
setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal
gotoLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 HISTORY.
**sigsetjmp**():
_POSIX_C_SOURCEDESCRIPTION top
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 and HISTORY), 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 top
**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 top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **setjmp**(), **sigsetjmp**() │ Thread safety │ MT-Safe │
├──────────────────────────────────────┼───────────────┼─────────┤
│ **longjmp**(), **siglongjmp**() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘STANDARDS top
**setjmp**()
**longjmp**()
C11, POSIX.1-2008.
**sigsetjmp**()
**siglongjmp**()
POSIX.1-2008.HISTORY top
**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 top
**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 top
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 jmpbuf 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 jmpbuf 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)](../man3/exit.3.html) 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 top
[signal(7)](../man7/signal.7.html), [signal-safety(7)](../man7/signal-safety.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 setjmp(3)
Pages that refer to this page:sigaltstack(2), abort(3), alloca(3), atexit(3), exit(3), ftw(3), getcontext(3), libexpect(3), makecontext(3), pthread_cleanup_push(3), sleep(3), signal(7), signal-safety(7), ld.so(8)