BSD 2.11 - man page for setjmp (bsd section 3)

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SETJMP(3)										SETJMP(3)

       setjmp, longjmp - non-local goto

       #include <setjmp.h>

       jmp_buf env;

       longjmp(env, val)
       jmp_buf env;

       jmp_buf env;

       _longjmp(env, val)
       jmp_buf env;

       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  returns  in
       such  a	way that execution continues as if the call of setjmp had just returned the value
       val to the function that invoked setjmp, which  must  not  itself  have	returned  in  the
       interim.  All accessible data have values as of the time longjmp was called.

       Setjmp and longjmp save and restore the signal mask sigmask(2), while _setjmp and _longjmp
       manipulate only the C stack and registers.

       If the contents of the jmp_buf are corrupted, or correspond to  an  environment	that  has
       already	returned,  longjmp  calls  the routine longjmperror.  If longjmperror returns the
       program is aborted.  The default version of  longjmperror  prints  the  message	``longjmp
       botch''	to standard error and returns.	User programs wishing to exit more gracefully can
       write their own versions of longjmperror.

       sigvec(2), sigstack(2), signal(3)

       On the PDP-11, longjmperror is called as _ljerr.  This difference stems from  the  limited
       name  size  of  the  PDP-11 that requires all external names to be unique within the first
       seven characters.  However, <setjmp.h> automatically translates	longjmperror  to  ljerror
       and should be included before any definition longjmperror.

       The PDP-11 implementation also contains a subtle bug that occurs when a routine containing
       a setjmp has register variables.  The bug sometimes causes those  variables  to	be  given
       invalid	values	when  a  longjmp  is made back to the routine.	Register variables should
       therefore be avoided in routines containing setjmps.

       And finally, _longjmp may sometimes die fatally.  Sorry.

4th Berkeley Distribution		 January 9, 1986				SETJMP(3)
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