SETJMP(3C-SVR3)     RISC/os Reference Manual      SETJMP(3C-SVR3)



NAME
     setjmp, longjmp - non-local goto

SYNOPSIS
     #include <setjmp.h>

     int setjmp (env)
     jmpbuf env;

     void longjmp (env, val)
     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 (which must not itself have
     returned in the interim) 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
     accessible data have values as of the time longjmp is
     called.  However, global variables will have the expected
     values, i.e. those as of the time of the longjmp (see exam-
     ple).

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*/



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SETJMP(3C-SVR3)     RISC/os Reference Manual      SETJMP(3C-SVR3)



     }

     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
     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
     which has since returned, absolute chaos is guaranteed.

ERRORS
     The values of the registers on the second return from setjmp
     are the register values at the time of the first call to
     setjmp, not those at the time of the longjmp.  This means
     that variables in a given function may behave differently in
     the presence of setjmp, depending on whether they are regis-
     ter or stack variables.


























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