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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 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
     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).

     allbox;  lbw23  lb  lb  l  l l.  Interface Attribute Value T{ setjmp(),
     sigsetjmp()  T}   Thread  safety  MT-Safe  T{  longjmp(),  siglongjmp()
     T}   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 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
     dynamic  target  of a particular longjmp() call.  (To make the program-
     mer'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  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
     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-sig-

        nal-safe functions are called after returning from the initial  call
        to main().