Michael Davidson distrusted:
Note that this means that r4, r3 and r2 get restored to
that they had at the time that the next stack frame was established
(ie when the "next" function down was called from the original context)
and that these are not necessarily the values that they had at the
moment when the original call to setjnp returned. Hence the various
caveats about not relying on the value of any register variables
after returning via longjmp.
You can trust register variables when setjmp() returns the second time. Then
the registers are restored to the values they had when the "next" function was
called, that is the "values as of the time longjmp() was called" (quoted from
longjmp(3)'s man page. Thus any variable behaves the same, regardless of its
By the way, this is not the case in V6. The register variables-- and only
the register variables-- are restored to the values as of the time setexit() was
called. (In V6 "setjmp()" is named "setexit()" and
"longjmp()" is named "reset()").
Thus the value of a variable depends on its storage class. If it is register,
you get the value when setexit() was called, if it is not register, you get
the value when reset() was called. This might be considered a bug.