Statement expression with function-call variable lifetime

[Russell Shaw]

Shaun Jackman wrote:
> Hello,
> 
> I'm implementing a tiny vfork/exit implementation using setjmp and
> longjmp. Since the function calling setjmp can't return (if you still
> want to longjmp to its jmp_buf) I implemented vfork using a statement
> expression macro. Here's my implementation of vfork.
> 
> jmp_buf *vfork_jmp_buf;
> #define vfork() ({ \
> 	int setjmp_ret; \
> 	jmp_buf *prev_jmp_buf = vfork_jmp_buf, new_jmp_buf; \
> 	vfork_jmp_buf = &new_jmp_buf; \
> 	if( (setjmp_ret = setjmp(*vfork_jmp_buf)) != 0 ) \
> 		vfork_jmp_buf = prev_jmp_buf; \
> 	setjmp_ret; \
> })
> 
> Unfortunately, after tracing a nasty bug I found that the same problem
> applies to leaving a statement expression as does returning from a
> function. The storage allocated for prev_jmp_buf and new_jmp_buf is
> unallocated as soon as we leave the scope of the statement expression.
> gcc quickly reuses that stack space later in the same function,
> overwriting the saved jmp_buf. Does anyone have a suggestion how I can
> allocate some storage space here for prev_jmp_buf and new_jmp_buf that
> will last the lifetime of the function call instead of the lifetime of
> the statement expression macro? My best idea was to use alloca, but it
> wouldn't look pretty. Can someone confirm that memory allocated with
> alloca would last the lifetime of the function call, and not the
> lifetime of the statement expression?
> 
> Please cc me in your reply. Thanks,
> Shaun

Alloca is like creating a stack variable, except it just gives you some
generic bytes that don't mean anything. Exiting the local scope will
trash the local variables and anything done with alloca(). You'll need
to store some information in a global variable. This C exceptions method
stores things as a linked list in nested stack frames and keeps a pointer
to the list in a global variable. Well worth studying:

   http://ldeniau.home.cern.ch/ldeniau/html/exception/exception.html