The userspace/kernel interface for invoking syscalls which the OP calls with assembly code is obviously not portable. Not even between similar OSs on one archit

The userspace/kernel interface for invoking syscalls which the OP calls with assembly code is obviously not portable. Not even between similar OSs on one architecture and absolutely not between architectures. POSIX only specifies a set of C library functions. Once you stop using them, you are on your own.