Threading in C, cross platform

Try OpenMP API, it's multi-platform and you can compile it with GCC.

Brief description from the wikipedia:

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran,[3] on most platforms, processor architectures and operating systems, including Solaris, AIX, HP-UX, Linux, macOS, and Windows. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

I would use the POSIX thread API - pthread. This article has some hints for implementing it on Windows, and a header-file-only download (BSD license):

http://locklessinc.com/articles/pthreads_on_windows/

Edit: I used the sourceforge pthreads-win32 project in the past for multi-platform threading and it worked really nicely. Things have moved on since then and the above link seems more up-to-date, though I haven't tried it. This answer assumes of course that pthreads are available on your non-Windows targets (for Mac / Linux I should think they are, probably even embedded)

Windows threading has sufficiently different functionality when compared to that of Linux such that perhaps you should consider two different implementations, at least if application performance could be an issue. On the other hand, simply implementing multi-threading may well make your app slower than it was before. Lets assume that performance is an issue and that multi-threading is the best option.

With Windows threads I'm specifically thinking of I/O Completion Ports (IOCPs) which allow implementing I/O-event driven threads that make the most efficient use of the hardware.

Many "classic" applications are constructed along one thread/one socket (/one user or similar) concept where the number of simultaneous sessions will be limited by the scheduler's ability to handle large numbers of threads (>1000). The IOCP concept allows limiting the number of threads to the number of cores in your system which means that the scheduler will have very little to do. The threads will only execute when the IOCP releases them after an I/O event has occurred. The thread services the IOC, (typically) initiates a new I/O and returns to wait at the IOCP for the next completion. Before releasing a thread the IOCP will also provide the context of the completion such that the thread will "know" what processing context the IOC belongs to.

The IOCP concept completely does away with polling which is a great resource waster although "wait on multiple object" polling is somewhat of an improvement. The last time I looked Linux had nothing remotely like IOCPs so a Linux multi-threaded application would be constructed quite differently compared to a Windows app with IOCPs.

In really efficient IOCP apps there is a risk that so many IOs (or rather Outputs) are queued to the IO resource involved that the system runs out of non-paged memory to store them. Conversely, in really inefficient IOCP apps there is a risk that so many Inputs are queued (waiting to be serviced) that the non-paged memory is exhausted when trying to temporarily buffer them.