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When compiling semaphore_osx.c on win32, the following error happened:
/usr/i686-w64-mingw32/include/semaphore.h:160:6: error: unknown type name 'mode_t'
This is because this system header references symbols that are not
not defined anywhere. This is clearly a bug in pthreads-w32, but has
been known and unfixed since 2012, so add a hack to fix it.
We build semaphore_osx.c this way because it saves us an extra configure
check. On win32, Linux, etc. it's empty and contains
"#include <semaphore.h>" only.
Should fix #1108.
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OSX is POSIX conformant, but it's a sad joke: it provides the
<semaphore.h> prototype as the standard demands, but they're empty
wrappers, and all functions just return ENOSYS.
Emulate them similar to how osdep/io.h emulate filesystem functions on
Windows. By including the header, working sem_* functions become
available.
To make it async-signal safe, use a pipe for wakeup (write() is AS-safe,
but mutexes can't be). Actually I'm not sure anymore if we really need
AS-safety, but for now the emulation can do it.
On Linux, the system provides a far more efficient and robust
implementation. We definitely want to avoid using the emulation if
possible, so this code is active on OSX only. For convenience we always
build the source file though, even if the implementation is disabled and
no actual code is generated.
(Linux provides working semaphores, but is formally not POSIX
conformant. On OSX it's the opposite. Is POSIX a complete joke?)
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