pthread_mutex_t struct: What does lock stand for?

Question

I am looking at the pthread_mutex_t structure in the pthreadtypes.h file. What does the "__lock" stand for? Is it like a lock number assigned to the mutex?

typedef union
{
  struct __pthread_mutex_s
  {
     int __lock;
     unsigned int __count;
     int __owner;
     #if __WORDSIZE == 64
     unsigned int __nusers;
     #endif
    /* KIND must stay at this position in the structure to maintain
     binary compatibility.  */
     int __kind;
     #if __WORDSIZE == 64
     int __spins;
     __pthread_list_t __list;
     # define __PTHREAD_MUTEX_HAVE_PREV 1
     #else
     unsigned int __nusers;
     __extension__ union
    {
      int __spins;
      __pthread_slist_t __list;
    };
    #endif
  } __data;
 char __size[__SIZEOF_PTHREAD_MUTEX_T];
 long int __align;

} pthread_mutex_t;

Answer 1

The __lock member of struct __pthread_mutex_s __data is used as a futex object on Linux. Many of the following details may differ depending on the architecture you're looking at:

See the pthread_mutex_lock.c code for the high level locking function for pthread mutexes - __pthread_mutex_lock(), which generally will end up calling LLL_MUTEX_LOCK() and the definitions of LLL_MUTEX_LOCK() and friends, which end up calling lll_lock(), etc., in lowlevellock.h.

The lll_lock() macro in turn calls __lll_lock_wait_private(), which calls lll_futex_wait(), which makes the sys_futex system call.