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
       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, pos-
       sibly 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 val-
       ues 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 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 |
       +------------------------+---------------+---------+
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 nonzero 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 following 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 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.

   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
       recommends avoiding the use of these functions from signal handlers and goes on to point out that if these functions are called from a sig-
       nal 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)

COLOPHON

       This page is part of release 4.15 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)