Assuming that 'I' can receive a payload, and that it doesn't need to call 'F' directly, you could write a function 'G' that takes a pointer to the locked state

Assuming that 'I' can receive a payload, and that it doesn't need to call 'F' directly, you could write a function 'G' that takes a pointer to the locked state of 'F'. Then, 'F' can lock a non-recursive mutex and call 'G', 'G' can pass that pointer to 'I', and 'I' can pass the pointer back to 'G'. (If 'I' cannot receive a payload, you could also write a variant where 'G' simply assumes that the state is locked, and accesses it directly.) That's basically what this article is suggesting. Otherwise, the only thing I could think of is a thread-local boolean to tell 'F' that it is being called from 'I'. Which is, as I understand it, basically what a recursive mutex does, by remembering the thread that locked it. But it's still technically a different construct. The tricky part of the problem under this condition is that only the recursive call of 'F' from 'I' can proceed, so somehow that call must be distinguished from an identical call to 'F' from a different thread. Overall, things are much trickier, especially in multithreaded code, when you can't directly thread state through your functions. If there's one point I'll give in favor of the FP philosophy, it's that.