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


       Standard C library (libc, -lc)


       #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.



       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

       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.


       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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │setjmp(), sigsetjmp()                                          │ Thread safety │ MT-Safe │
       │longjmp(), siglongjmp()                                        │ Thread safety │ MT-Safe │


       setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C99.

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


       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.

       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.

       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.

       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().


       signal(7), signal-safety(7)