Museum

     SETJMP(S)                 UNIX System V                 SETJMP(S)



     Name
          setjmp, longjmp - non-local goto

     Syntax
          #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
          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.  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
          identify syntactically those automatic variables on whose
          values they need to rely after the second return from
          setjmp.

     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(S)

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

     Standards Conformance
          longjmp and setjmp are conformant with:
          AT&T SVID Issue 2, Select Code 307-127;
          The X/Open Portability Guide II of January 1987;
          ANSI X3.159-198X C Language Draft Standard, May 13, 1988;
          IEEE POSIX Std 1003.1-1988 with C Standard Language-
          Dependent System Support;
          and NIST FIPS 151-1.


                                             (printed 6/20/89)