There is a genuine complication that this post seems to have missed. If a system call is interrupted and returns to user space (due to a signal, most likely), then it is in one of three states: nothing happened, the call finished, or it is part-way done. If nothing happened, then PC can point back to the syscall or EINTR can be returned. (EINTR is a promise that no progress was made.) If it’s all done, it can return success. But, if it’s only part way done, then either it needs to restart transparently or the user code needs to be prepared to handle this.

With recv(), for example, the return value can indicate that some but not all bytes were read. When read()one from a file, most programs aren’t prepared for partial success.

Linux has some ERESTARTSYS mechanisms to handle some of these cases. The code and the semantics are a bit gross.