trusty (3) pthread_rwlock_rdlock.3posix.gz

NAME

       pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write lock object for reading

SYNOPSIS

       #include <pthread.h>

       int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
       int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

DESCRIPTION

       The pthread_rwlock_rdlock() function shall apply a read lock to the read-write lock referenced by rwlock.
       The calling thread acquires the read lock if a writer does not hold the lock and  there  are  no  writers
       blocked on the lock.

       If  the  Thread  Execution  Scheduling  option  is  supported,  and  the threads involved in the lock are
       executing with the scheduling policies SCHED_FIFO or SCHED_RR, the calling thread shall not  acquire  the
       lock  if  a  writer  holds  the  lock  or if writers of higher or equal priority are blocked on the lock;
       otherwise, the calling thread shall acquire the lock.

       If the Threads Execution Scheduling option is supported,  and  the  threads  involved  in  the  lock  are
       executing  with  the SCHED_SPORADIC scheduling policy, the calling thread shall not acquire the lock if a
       writer holds the lock or if writers of higher or equal priority are blocked on the lock;  otherwise,  the
       calling thread shall acquire the lock.

       If  the  Thread  Execution  Scheduling  option is not supported, it is implementation-defined whether the
       calling thread acquires the lock when a writer does not hold the lock and there are  writers  blocked  on
       the  lock.  If  a  writer holds the lock, the calling thread shall not acquire the read lock. If the read
       lock is not acquired, the calling thread shall block until it can acquire the lock.  The  calling  thread
       may deadlock if at the time the call is made it holds a write lock.

       A   thread  may  hold  multiple  concurrent  read  locks  on  rwlock  (that  is,  successfully  call  the
       pthread_rwlock_rdlock() function n times). If so, the application shall ensure that the  thread  performs
       matching unlocks (that is, it calls the pthread_rwlock_unlock() function n times).

       The maximum number of simultaneous read locks that an implementation guarantees can be applied to a read-
       write lock shall be implementation-defined. The pthread_rwlock_rdlock() function may fail if this maximum
       would be exceeded.

       The  pthread_rwlock_tryrdlock()  function  shall  apply  a  read  lock  as in the pthread_rwlock_rdlock()
       function, with the exception that the function shall fail if the equivalent pthread_rwlock_rdlock()  call
       would  have  blocked  the  calling  thread. In no case shall the pthread_rwlock_tryrdlock() function ever
       block; it always either acquires the lock or fails and returns immediately.

       Results are undefined if any of these functions are called with an uninitialized read-write lock.

       If a signal is delivered to a thread waiting for a read-write lock for  reading,  upon  return  from  the
       signal  handler  the  thread  resumes  waiting  for  the  read-write  lock  for  reading as if it was not
       interrupted.

RETURN VALUE

       If successful, the pthread_rwlock_rdlock() function shall return zero; otherwise, an error  number  shall
       be returned to indicate the error.

       The  pthread_rwlock_tryrdlock() function shall return zero if the lock for reading on the read-write lock
       object referenced by rwlock is acquired. Otherwise, an error number shall be  returned  to  indicate  the
       error.

ERRORS

       The pthread_rwlock_tryrdlock() function shall fail if:

       EBUSY  The  read-write lock could not be acquired for reading because a writer holds the lock or a writer
              with the appropriate priority was blocked on it.

       The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if:

       EINVAL The value specified by rwlock does not refer to an initialized read-write lock object.

       EAGAIN The read lock could not be acquired because the maximum number of read locks for rwlock  has  been
              exceeded.

       The pthread_rwlock_rdlock() function may fail if:

       EDEADLK
              The current thread already owns the read-write lock for writing.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       Applications  using  these  functions  may  be  subject  to  priority inversion, as discussed in the Base
       Definitions volume of IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       pthread_rwlock_destroy()    ,    pthread_rwlock_timedrdlock()    ,     pthread_rwlock_timedwrlock()     ,
       pthread_rwlock_trywrlock()  ,  pthread_rwlock_unlock()  ,  pthread_rwlock_wrlock() , the Base Definitions
       volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc  and  The  Open Group. In the event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .