Question

Advanced Programming in the UNIX Environment by W. Richard Stevens states:

“What are the the states of the automatic variables and register
variables in the main function?”

with regard to what happens when you longjmp back to main (or another function) from somewhere lower down the stack.

It goes on to say:

“It depends. Most implementations do not try to roll back these
automatic variables and register variables, but all that the standards
say is that their values are indeterminate. If you have an automatic
variable that you don’t want rolled back, define it with the
volatile attribute. Variables that are declared global or static
are left alone when longjmp is executed.

It seems like he’s saying that normal stack-variables will not have their values set back to what they were at the time of the setjmp – but then the rest of the function couldn’t rely on its stack variables after the longjmp back to it which seems crazy, so I’m guessing I’m wrong.

Can someone define “automatic variables” for me and explain what specifically isn’t set back to its original value and why that is?

Answer 1

All it’s saying is that if

1. you have an automatic (function-local non-static) variable that’s not declared volatile; and
2. you change the value of the variable between setjmp and longjmp

then after the longjmp the value of that variable becomes indeterminate.

I believe this has to do with the possibility of such variables residing in CPU registers rather than in RAM, and the associated difficulty of preserving the values of such variable across the longjmp.

Here is a quote from the gcc manual:

If you use longjmp, beware of automatic variables. ISO C says that
automatic variables that are not declared volatile have undefined
values after a longjmp. And this is all GCC promises to do, because
it is very difficult to restore register variables correctly, and one
of GCC’s features is that it can put variables in registers without
your asking it to.

If the potential loss of variable values is a problem in your use case, declare the relevant variables as volatile.