setjmp(3C)          (C Programming Language Utilities)           setjmp(3C)


   NAME
         setjmp, longjmp - non-local goto

   SYNOPSIS
         #include <setjmp.h>

         int setjmp (jmp_buf env);

         void longjmp (jmp_buf env, int val);

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

         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();
               /*NOTREACHED*/


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   setjmp(3C)          (C Programming Language Utilities)           setjmp(3C)


         }

         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.
































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