mutexes

[Ross Johnson]

uGAH man wrote:
> 
> i was testing this pthread for win32 in my prog.
> when i checked if it was blocking mutex_lock calls, i was surprised when it
> didnt lock
> 
> pthread_mutex_t DBlock;
> 
> pthread_mutex_init( &DBlock, NULL );
> pthread_mutex_lock( &DBlock );
> pthread_mutex_lock( &DBlock );
> 
> ^^ these two calls didnt block

This appears to be a FAQ.

POSIX leaves the result "undefined" for a thread that tries
to recursively lock the same mutex (one that it owns already).
That means the actual semantics are left up to the
implementation, but should not be relied upon for code that
will be ported to different POSIX threads implementations.

In the pthreads-win32 implementation a thread won't deadlock
itself by relocking the mutex. Subsequent calls to
pthread_mutex_lock() as in your example above increment
the lock count but the thread continues on. Consequently,
the thread must ensure that it unlocks the mutex once for
each lock operation. That is, pthreads-win32 mutexes are
always recursive.

You may want to look at the other synchronisation devices
available in the library, such as condition variables or
read-write locks.

Ross