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### Other APIs

##### 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()`.

[2] ISO/IEC 9899:1999, 2005, 7.13.2.1:2

[3] ISO/IEC 9899:1999, 2005, J.2