07-04-2010
linux, glibc 2.5
Right, before waiting on a conditional variable you must hold the mutex. But once within the 'wait' method itself the mutex will be released while waiting on the conditional variable queue (waiting for the conditional variable being signaled or broadcast.)
Once the signal arrives then 'wait' must guaranty that the mutex is acquired again before returning to the 'wait' caller. That's the basic mechanics for a conditional variable. Reacquiring the mutex, as much as I understand, is always something that one would expect implemented with the API at hand, in this case pthread.
My point is that in this pthread implementation relocking the mutex goes without a timed call, that is, the whole method could block forever if something went wrong with the thread that should have released the mutex. Therefore, it is possible that pthread_cond_timedwait would block for ever, which is exactly what one is trying to avoid by using a timed version of 'wait'.
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LEARN ABOUT OSF1
pthread_mutex_trylock
pthread_mutex_trylock(3) Library Functions Manual pthread_mutex_trylock(3)
NAME
pthread_mutex_trylock - Attempts to lock the specified mutex, without waiting if already locked.
LIBRARY
DECthreads POSIX 1003.1c Library (libpthread.so)
SYNOPSIS
#include <pthread.h>
int pthread_mutex_trylock(
pthread_mutex_t *mutex);
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
IEEE Std 1003.1c-1995, POSIX System Application Program Interface
PARAMETERS
Mutex to be locked.
DESCRIPTION
This routine attempts to lock the mutex specified in the mutex argument. When a thread calls this routine, an attempt is made to immedi-
ately lock the mutex. If the mutex is successfully locked, this routine returns zero (0) and the calling thread becomes the mutex's current
owner. If the specified mutex is locked when a thread calls this routine, the calling thread does not wait for the mutex to become avail-
able.
The behavior of this routine is as follows: For a normal, default, or errorcheck mutex: if the mutex is locked by any thread (including the
calling thread) when this routine is called, this routine returns [EBUSY] and the calling thread does not wait to acquire the lock. For a
normal or errorcheck mutex: if the mutex is not owned, this routine returns zero (0) and the mutex becomes locked by the calling thread.
For a recursive mutex: if the mutex is owned by the calling thread, this routine returns zero (0) and the mutex lock count is incremented.
(To unlock a recursive mutex, each call to pthread_mutex_trylock(3) must be matched by a call to pthread_mutex_unlock(3).)
Use the pthread_mutexattr_settype(3) routine to set the mutex type attribute (normal, default, recursive, or errorcheck).
RETURN VALUES
If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Successful completion. The mutex is already locked; therefore, it was not acquired. The value specified by mutex is not a valid mutex.
ERRORS
None
RELATED INFORMATION
Functions: pthread_mutexattr_settype(3), pthread_mutex_destroy(3), pthread_mutex_init(3), pthread_mutex_lock(3), pthread_mutex_unlock(3)
Manuals: Guide to DECthreads and Programmer's Guide
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pthread_mutex_trylock(3)