longjmp

SETJMP(3)                  Linux Programmer's Manual                 SETJMP(3)

NAME
       setjmp, sigsetjmp, longjmp, siglongjmp  - performing a nonlocal goto

SYNOPSIS
       #include <setjmp.h>

       int setjmp(jmp_buf env);
       int sigsetjmp(sigjmp_buf env, int savesigs);

       void longjmp(jmp_buf env, int val);
       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 lo-
       cation  in  another function.  The setjmp() function dynamically estab-
       lishes 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 envi-
       ronment (typically, the stack pointer, the instruction pointer,  possi-
       bly  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  distin-
       guished 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  non-
       zero, the process's current signal mask is saved in env and will be re-
       stored 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  at-
       tributes(7).

       +------------------------+---------------+---------+
       |Interface               | Attribute     | Value   |
       +------------------------+---------------+---------+
       |setjmp(), sigsetjmp()   | Thread safety | MT-Safe |
       +------------------------+---------------+---------+
       |longjmp(), siglongjmp() | Thread safety | MT-Safe |
       +------------------------+---------------+---------+
CONFORMING TO
       setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       sigsetjmp(), siglongjmp(): POSIX.1-2001, POSIX.1-2008.

NOTES
       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.   On  Linux  with  glibc  versions  before 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 non-
       zero savesigs argument.

       setjmp() and longjmp() can be useful for  dealing  with  errors  inside
       deeply  nested function calls or to allow a signal handler to pass con-
       trol 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 read-
       ability below.

       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 follow-
       ing criteria:

       o  they are local to the function that made the corresponding  setjmp()
          call;

       o  their   values  are  changed  between  the  calls  to  setjmp()  and
          longjmp(); and

       o  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  dy-
       namic target of a particular longjmp() call.  (To make the programmer's
       life easier, each setjmp() call should employ a  unique  jmp_buf  vari-
       able.)

       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.

   Caveats
       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  recom-
       mends avoiding the use of these functions from signal handlers and goes
       on to point out that if these functions are called from a  signal  han-
       dler  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 unde-
       fined if the program subsequently makes a call to  a  non-async-signal-
       safe  function.   The only way of avoiding undefined behavior is to en-
       sure 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-sig-
          nal-safe functions are called after returning from the initial  call
          to main().

SEE ALSO
       signal(7), signal-safety(7)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

                                  2017-03-13                         SETJMP(3)