SETJMP(3C-SVR4)     RISC/os Reference Manual      SETJMP(3C-SVR4)



NAME
     setjmp, longjmp - non-local goto

SYNOPSIS
     #include <setjmp.h>

     int setjmp (jmpbuf env);

     void longjmp (jmpbuf env, int val);

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

     setjmp saves its stack environment in env (whose type,
     jmp_buf, 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.  After longjmp
     is completed, program execution continues as if the
     corresponding call of setjmp had just returned the value
     val.  (The caller of setjmp must not have returned in the
     interim.)  longjmp 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 example).  The values of
     register and automatic variables are undefined.

     Register or automatic variables whose value must be relied
     upon must be declared as volatile.

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

     jmp_buf 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();



                        Printed 11/19/92                   Page 1

SETJMP(3C-SVR4)     RISC/os Reference Manual      SETJMP(3C-SVR4)



         /* 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

SEE ALSO
     signal(2), sigsetjmp(3C).

NOTES
     If longjmp is called even though env was never primed by a
     call to setjmp, or when the last such call was in a function
     that has since returned, absolute chaos is guaranteed.


































 Page 2                 Printed 11/19/92