term% ls -F
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SETJMP(2)                     System Calls Manual                    SETJMP(2)

       setjmp, longjmp, notejmp - non-local goto

       #include <u.h>
       #include <libc.h>

       int  setjmp(jmp_buf env)

       void longjmp(jmp_buf env, int val)

       void notejmp(void *uregs, jmp_buf env, int val)

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

       Setjmp saves its stack environment in env for later use by longjmp.  It
       returns value 0.

       Longjmp  restores the environment saved by the last call of setjmp.  It
       then causes execution to continue as if the call  of  setjmp  had  just
       returned  with  value  val.  The invoker of setjmp must not itself have
       returned in the interim.  All accessible data have  values  as  of  the
       time longjmp was called.

       Notejmp  is  the  same  as  longjmp except that it is to be called from
       within a note handler (see notify(2)).  The uregs  argument  should  be
       the first argument passed to the note handler.

       Setjmp  and  longjmp can also be used to switch stacks.  Several macros
       are defined in /$objtype/include/u.h that can be used to build jmp_bufs
       by  hand.   The following code establishes a jmp_buf that may be called
       by longjmp to begin execution in a function f with 1024 bytes of stack:

              #include <u.h>
              #include <libc.h>

              jmp_buf label;
              #define NSTACK 1024
              char stack[NSTACK];

                   label[JMPBUFPC] = ((ulong)f+JMPBUFDPC);
                   /* -2 leaves room for old pc and new pc in frame */
                   label[JMPBUFSP] =



       Notejmp cannot recover from an address trap or bus error  (page  fault)
       on the 680x0 architectures.