Boost C++ Libraries

setjmp()/longjmp()

C99 defines setjmp()/longjmp() to provide non-local jumps but it does not require that longjmp() preserves the current stack frame. Therefore, jumping into a function which was exited via a call to longjmp() is undefined ^[2].

ucontext_t

Since POSIX.1-2004 ucontext_t is deprecated and was removed in POSIX.1-2008! The function signature of makecontext() is:

void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);

The third argument of makecontext() specifies the number of integer arguments that follow which will require function pointer cast if func will accept those arguments which is undefined in C99 ^[3].

The arguments in the var-arg list are required to be integers, passing pointers in var-arg list is not guaranteed to work, especially it will fail for architectures where pointers are larger than integers.

ucontext_t preserves signal mask between context switches which involves system calls consuming a lot of CPU cycles (ucontext_t is slower; a context switch takes two magnitutes of order more CPU cycles more than fcontext_t).

Windows fibers

A drawback of Windows Fiber API is that CreateFiber() does not accept a pointer to user allocated stack space preventing the reuse of stacks for other context instances. Because the Windows Fiber API requires to call ConvertThreadToFiber() if SwitchFiber() is called for a thread which has not been converted to a fiber. For the same reason ConvertFiberToThread() must be called after return from SwitchFiber() if the thread was forced to be converted to a fiber before (which is inefficient).

if ( ! is_a_fiber() )
{
    ConvertThreadToFiber( 0);
    SwitchToFiber( ctx);
    ConvertFiberToThread();
}

If the condition _WIN32_WINNT >= _WIN32_WINNT_VISTA is met function IsThreadAFiber() is provided in order to detect if the current thread was already converted. Unfortunately Windows XP + SP 2/3 defines _WIN32_WINNT >= _WIN32_WINNT_VISTA without providing IsThreadAFiber().