SDL_mutexP man page
SDL_mutexP — Lock a mutex
#include "SDL.h" #include "SDL_thread.h"
int SDL_mutexP(SDL_mutex *mutex);
Locks the mutex, which was previously created with SDL_CreateMutex. If the mutex is already locked then SDL_mutexP will not return until it is unlocked. Returns 0 on success, or -1 on an error.
SDL also defines a macro #define SDL_LockMutex(m) SDL_mutexP(m).