Register Variables and longjmp()
Care must be taken with local variables in functions using setjmp(). In Example 5, for instance, you would expect the following program output:
1: 1
2: 2
3: 3
However, with most optimizing compilers, it will be:
1: 1
2: 2
3: 2
In particular, the incorrect result will be produced whenever the compiler chooses to allocate LocalVar as a register variable. setjmp() will save all registers used for register variables in the given jmp_buf. Even though the register variable has been incremented after the call to setjmp(), longjmp() will restore the value it had at the time of setjmp(). The only way to prevent this problem is to declare such variables as volatile. This is even required by the ANSI C Standard in section 7.6.2.1.
Since the C exception-handling library uses setjmp() in its XTRY macro and calls longjmp() in macro XEND and XRaise(), all parameters and local variables of functions with XTRY blocks must be declared as volatile if their values must be preserved across the XTRY and XEND macros. This is even the case when alternate functions for context saving/restoring are used instead of setjmp()/longjmp(), since they also can only restore register variable values in effect when the context was saved.
-- T.S. and P.P.
