Why is the pthread_mutex_t struct all zeros in gdb?

What causes pthread_mutex_t struct to be all zeros? Core of a deadlocked ARM test application in a gdb session:

[Switching to thread 3 (LWP 3587)]
#0  __lll_lock_wait (futex=futex@entry=0x2105c <one_mutex>, private=0) at lowlevellock.c:46
46  lowlevellock.c: No such file or directory.
(gdb) f 1
#1  0xaf973818 in __GI___pthread_mutex_lock (mutex=0x2105c <one_mutex>) at pthread_mutex_lock.c:81
81  pthread_mutex_lock.c: No such file or directory.
(gdb) p mutex
$1 = (pthread_mutex_t *) 0x2105c <one_mutex>
(gdb) p (pthread_mutex_t ) 0x2105c
$2 = {
  __data = {
    __lock = 0,
    __count = 0,
    __owner = 0,
    __kind = 0,
    __nusers = 0,
    {
      __spins = 0,
      __list = {
        __next = 0x0
      }
    }
  },
  __size = '00' <repeats 23 times>,
  __align = 0
}

Answer

pthread_mutex_t is used for all mutex types and some of its members are only used for specific mutex types.

owner member is set for at least the following mutex types:

• PTHREAD_MUTEX_ERRORCHECK_NP
• PTHREAD_MUTEX_RECURSIVE_NP

See glibc implementation of pthread_mutex_trylock for full more details.