Malloc cannot use pthread_mutex_init() to initialize a mutex because the mutex initialization process does a malloc! libc_r internals skip the malloc and assign an initializer to a static structure and point the opaque type (pthread_mutex_t in this case) to that structure. This is done on the assumption that the mutex will never be destroyed. This style of initialization is only valid inside libc_r because the structure that is assigned is opaque to the user. This fix allows a simple program to get to main() again. 8-)