Manual Pages for UNIX Operating System command usage for man longjmp

Standard C Library Functions                           setjmp(3C)



NAME
     setjmp, sigsetjmp, longjmp, siglongjmp - non-local goto

SYNOPSIS
     #include 

     int setjmp(jmp_buf env);


     int sigsetjmp(sigjmp_buf env, int savemask);


     void longjmp(jmp_buf env, int val);


     void siglongjmp(sigjmp_buf env, int val);


DESCRIPTION
     These functions are useful  for  dealing  with  errors   and
     interrupts  encountered  in a low-level subroutine of a pro-
     gram.


     The setjmp() function saves its stack environment in env for
     later use by longjmp().


     The sigsetjmp() function saves the calling process's  regis-
     ters  and stack environment (see sigaltstack(2)) in  env for
     later use by siglongjmp(). If   savemask  is  non-zero,  the
     calling  process's  signal  mask  (see  sigprocmask(2))  and
     scheduling parameters (see priocntl(2)) are also saved.


     The longjmp() function restores the environment saved by the
     last  call  of setjmp() with the corresponding env argument.
     After longjmp() completes, program execution continues as if
     the  corresponding  call  to  setjmp() had just returned the
     value val. The caller of  setjmp() must not have returned in
     the  interim.   The longjmp() function cannot cause setjmp()
     to return the value 0.   If  longjmp()  is  invoked  with  a
     second argument of 0, setjmp() will return 1. At the time of
     the second return from setjmp(),  all  external  and  static
     variables  have  values  as  of the time longjmp() is called
     (see EXAMPLES).


     The siglongjmp() function restores the environment saved  by
     the  last  call  of  sigsetjmp()  with the corresponding env
     argument. After siglongjmp()  completes,  program  execution
     continues  as  if  the corresponding call to sigsetjmp() had



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Standard C Library Functions                           setjmp(3C)



     just returned the value val. The siglongjmp() function  can-
     not   cause   sigsetjmp()   to   return  the  value  0.   If
     siglongjmp() is invoked with a second argument  of  0,  sig-
     setjmp()  will  return  1.  At the time of the second return
     from sigsetjmp(), all external  and  static  variables  have
     values as of the time siglongjmp() was called.


     If a signal-catching function interrupts sleep(3C) and calls
     siglongjmp()  to  restore  an environment saved prior to the
     sleep() call, the action associated with SIGALRM and time it
     is  scheduled  to  be  generated are unspecified. It is also
     unspecified whether the SIGALRM signal  is  blocked,  unless
     the  process's  signal  mask  is  restored  as  part  of the
     environment.


     The  siglongjmp() function restores the saved signal mask if
     and  only  if the  env argument was initialized by a call to
     the sigsetjmp() function with a non-zero  savemask argument.


     The values of register and  automatic  variables  are  unde-
     fined.  Register  or automatic variables whose value must be
     relied upon must be declared as volatile.

RETURN VALUES
     If the return is from a direct invocation, setjmp() and sig-
     setjmp()  return  0.  If  the  return  is  from  a  call  to
     longjmp(), setjmp() returns a non-zero value. If the  return
     is  from  a call to siglongjmp(), sigsetjmp() returns a non-
     zero value.


     After longjmp() is completed, program execution continues as
     if   the  corresponding  invocation  of  setjmp()  had  just
     returned the value specified by val. The longjmp()  function
     cannot  cause  setjmp()  to  return 0; if val is 0, setjmp()
     returns 1.


     After siglongjmp() is completed, program execution continues
     as  if  the corresponding invocation of sigsetjmp() had just
     returned the value specified by val. The siglongjmp()  func-
     tion cannot cause sigsetjmp() to return 0; if val is 0, sig-
     setjmp() returns 1.

EXAMPLES
     Example 1 Example of setjmp() and longjmp() functions.






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Standard C Library Functions                           setjmp(3C)



     The following example uses both setjmp()  and  longjmp()  to
     return  the  flow  of control to the appropriate instruction
     block:


       #include 
       #include 
       #include 
       #include 
       jmp_buf env; static void signal_handler();

       main()  {
               int returned_from_longjump, processing = 1;
               unsigned int time_interval = 4;
               if ((returned_from_longjump = setjmp(env)) != 0)
                   switch (returned_from_longjump)     {
                     case SIGINT:
                       printf("longjumped from interrupt %d\n",SIGINT);
                       break;
                     case SIGALRM:
                       printf("longjumped from alarm %d\n",SIGALRM);
                       break;
                   }
               (void) signal(SIGINT, signal_handler);
               (void) signal(SIGALRM, signal_handler);
               alarm(time_interval);
               while (processing)        {
                 printf(" waiting for you to INTERRUPT (cntrl-C) ...\n");
                 sleep(1);
               }       /* end while forever loop */
       }

       static void signal_handler(sig)
       int sig; {
               switch (sig)     {
                 case SIGINT:   ...    /* process for interrupt */
                                longjmp(env,sig);
                                       /* break never reached */
                 case SIGALRM:  ...    /* process for alarm */
                                longjmp(env,sig);
                                      /* break never reached */
                 default:       exit(sig);
               }
       }



     When this example is compiled and  executed,  and  the  user
     sends an interrupt signal, the output will be:


       longjumped from interrupt



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Standard C Library Functions                           setjmp(3C)



     Additionally, every 4 seconds the alarm will expire, signal-
     ling this process, and the output will be:


       longjumped from alarm



ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:



     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | Interface Stability         | Committed                   |
    |_____________________________|_____________________________|
    | MT-Level                    | Unsafe                      |
    |_____________________________|_____________________________|
    | Standard                    | See standards(5).           |
    |_____________________________|_____________________________|


SEE ALSO
     getcontext(2),  priocntl(2),  sigaction(2),  sigaltstack(2),
     sigprocmask(2), signal(3C), attributes(5), standards(5)

WARNINGS
     If longjmp() or siglongjmp() are called even though env  was
     never  primed  by a call to setjmp() or sigsetjmp(), or when
     the last  such  call  was  in  a  function  that  has  since
     returned, the results are undefined.





















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