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


       #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, C89, 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.   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 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)


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                                            2021-03-22                                  SETJMP(3)