setjmp(3) — Linux manual page


setjmp(3)               Library Functions Manual               setjmp(3)

NAME         top

       setjmp, sigsetjmp, longjmp, siglongjmp  - performing a nonlocal

LIBRARY         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

       setjmp(): see NOTES.


DESCRIPTION         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

       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

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
       │ Interface                           Attribute     Value   │
       │ setjmp(), sigsetjmp()               │ Thread safety │ MT-Safe │
       │ longjmp(), siglongjmp()             │ Thread safety │ MT-Safe │

STANDARDS         top

              C11, POSIX.1-2008.


HISTORY         top

              POSIX.1-2001, C89.


       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 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

       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         top

       signal(7), signal-safety(7)

Linux man-pages (unreleased)     (date)                        setjmp(3)

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