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 MOJAVE
pthread_cond_wait
PTHREAD_COND_WAIT(3) BSD Library Functions Manual PTHREAD_COND_WAIT(3)
NAME
pthread_cond_wait -- wait on a condition variable
SYNOPSIS
#include <pthread.h>
int
pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
DESCRIPTION
The pthread_cond_wait() function atomically blocks the current thread waiting on the condition variable specified by cond, and releases the
mutex specified by mutex. The waiting thread unblocks only after another thread calls pthread_cond_signal(3), or pthread_cond_broadcast(3)
with the same condition variable, and the current thread reacquires the lock on mutex.
RETURN VALUES
If successful, the pthread_cond_wait() function will return zero. Otherwise an error number will be returned to indicate the error.
ERRORS
The pthread_cond_wait() function will fail if:
[EINVAL] The value specified by cond or the value specified by mutex is invalid.
SEE ALSO
pthread_cond_broadcast(3), pthread_cond_destroy(3), pthread_cond_init(3), pthread_cond_signal(3), pthread_cond_timedwait(3)
STANDARDS
The pthread_cond_wait() function conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').
Darwin February 16, 2006 Darwin