I'm assuming this is from you calling pthread_create, right? This is not a stupid warning, it is complaining that you should give it a function void fn(void* arg) but you gave it void fn(int arg). It will probably work, but you should probably have the thread start function be the expected interface and then just do the void*int conversion in your own code instead of relying on the compiler to do the "right thing" in the face of UB.