(not Windows) no pthreads support
Most modern desktop or server operating systems support POSIX streams. There are even variants of Windows (although Windows does not natively support pthreads, and open source implementations are usually a subset of pthread functions).
The only non-Microsoft platforms I've seen that don't directly support pthreads are usually embedded systems. Threading support on embedded OS platforms is often configured for this system hardware and does not use standard thread libraries.
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