setjmp(3C)                                                       setjmp(3C)

NAME
     setjmp, longjmp - non-local goto

SYNOPSIS
     #include <setjmp.h>

     int setjmp(jmpbuf env);

     void longjmp(jmpbuf env, int val);

DESCRIPTION
     These functions enable errors and interrupts, encountered in a low-
     level subroutine of a program, to be handled.

     setjmp() saves its stack environment in env (whose type, jmpbuf, is
     defined in the setjmp.h header file) for later use by longjmp(). It
     returns the value 0.

     longjmp() restores the environment saved by the last call of setjmp()
     with the corresponding env argument. If there is no such invocation,
     or if the function containing the invocation of setjmp() has ter-
     minated execution in the interim, the behavior is undefined. It is
     unspecified whether longjmp() restores the signal mask, leaves the
     signal mask unchanged or restores it to its value at the time setjmp()
     was called.

     All accessible objects have values as of the time longjmp() was
     called, except that the values of objects of automatic storage dura-
     tion are indeterminate if they meet all the following conditions:

     -  They are local to the function containing the corresponding
        setjmp() invocation.

     -  They do not have volatile-qualified type.

     -  They are changed between the setjmp() invocation and longjmp()
        call.

     As it bypasses the usual function call and return mechanisms,
     longjmp() will execute correctly in contexts of interrupts, signals
     and any of their associated functions. However, if longjmp() is
     invoked from a nested signal handler (that is, from a function invoked
     as a result of a signal raised during the handling of another signal),
     the behavior is undefined.











Page 1                       Reliant UNIX 5.44                Printed 11/98

setjmp(3C)                                                       setjmp(3C)

EXAMPLE
     #include <stdio.h>
     #include <stdlib.h>
     #include <setjmp.h>

     jmpbuf env;
     int i = 0;
     main ()
     {
           void exit();

           if(setjmp(env) != 0) {
                   (void) printf("value of i on 2nd return from setjmp:
                   %d\n", i);
                   exit(0);
           }
           (void) printf("value of i on 1st return from setjmp: %d\n", i);
           i = 1;
           g();              /*NOTREACHED*/
     }
     g()
     {
           longjmp(env, 1);  /*NOTREACHED*/
     }

     If the a.out resulting from this C language code is run, the output
     will be:

          value of i on 1st return from setjmp():0

          value of i on 2nd return from setjmp():1

NOTES
     Applications whose behavior depends on the value of the signal mask
     should not use longjmp() and setjmp(), since their effect on the sig-
     nal mask is unspecified, but should instead use the following alterna-
     tives:

     -  The longjmp() and setjmp() functions (which never modify the sig-
        nal mask)

     -  The siglongjmp() and sigsetjmp() functions (which can save and
        restore the signal mask under application control).

     It is advisable to use the sigsetjmp(3C) function to examine errors
     and interrupts in a subroutine of a program.

SEE ALSO
     signal(2), setjmp(3C-ucb), sigsetjmp(3C), setjmp(5).





Page 2                       Reliant UNIX 5.44                Printed 11/98