SETJMP(3C)           INTERACTIVE UNIX System           SETJMP(3C)



          NAME
               setjmp, longjmp - non-local goto

          SYNOPSIS
               #include <setjmp.h>

               int setjmp (env)
               jmp_buf env;

               void longjmp (env, val)
               jmp_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.

               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.  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.

               In a future release, C language users will be able to iden-
               tify syntactically those automatic variables on whose values
               they need to rely after the second return from setjmp.



















          Rev. C Software Development Set                            Page 1

          SETJMP(3C)           INTERACTIVE UNIX System           SETJMP(3C)



          EXAMPLE
          #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*/
          }
          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).

          WARNING
               Problems may occur if longjmp is called before env is primed
               with a call to setjmp.



















          Rev. C Software Development Set                            Page 2