pthread_rwlock_rdlock.3p (6330B)
- '\" et
- .TH PTHREAD_RWLOCK_RDLOCK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- pthread_rwlock_rdlock,
- pthread_rwlock_tryrdlock
- \(em lock a read-write lock object for reading
- .SH SYNOPSIS
- .LP
- .nf
- #include <pthread.h>
- .P
- int pthread_rwlock_rdlock(pthread_rwlock_t *\fIrwlock\fP);
- int pthread_rwlock_tryrdlock(pthread_rwlock_t *\fIrwlock\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIpthread_rwlock_rdlock\fR()
- function shall apply a read lock to the read-write lock referenced by
- .IR 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.
- .P
- 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.
- .P
- If the Thread 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.
- .P
- 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.
- .P
- A thread may hold multiple concurrent read locks on
- .IR rwlock
- (that is, successfully call the
- \fIpthread_rwlock_rdlock\fR()
- function
- .IR n
- times). If so, the application shall ensure that the thread performs
- matching unlocks (that is, it calls the
- \fIpthread_rwlock_unlock\fR()
- function
- .IR n
- times).
- .P
- The maximum number of simultaneous read locks that an implementation
- guarantees can be applied to a read-write lock shall be
- implementation-defined. The
- \fIpthread_rwlock_rdlock\fR()
- function may fail if this maximum would be exceeded.
- .P
- The
- \fIpthread_rwlock_tryrdlock\fR()
- function shall apply a read lock as in the
- \fIpthread_rwlock_rdlock\fR()
- function, with the exception that the function shall fail if the
- equivalent
- \fIpthread_rwlock_rdlock\fR()
- call would have blocked the calling thread. In no case shall the
- \fIpthread_rwlock_tryrdlock\fR()
- function ever block; it always either acquires the lock or fails and
- returns immediately.
- .P
- Results are undefined if any of these functions are called with an
- uninitialized read-write lock.
- .P
- 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.
- .SH "RETURN VALUE"
- If successful, the
- \fIpthread_rwlock_rdlock\fR()
- function shall return zero; otherwise, an error number shall be
- returned to indicate the error.
- .P
- The
- \fIpthread_rwlock_tryrdlock\fR()
- function shall return zero if the lock for reading on the read-write
- lock object referenced by
- .IR rwlock
- is acquired. Otherwise, an error number shall be returned to indicate
- the error.
- .SH ERRORS
- The
- \fIpthread_rwlock_tryrdlock\fR()
- function shall fail if:
- .TP
- .BR 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.
- .P
- The
- \fIpthread_rwlock_rdlock\fR()
- and
- \fIpthread_rwlock_tryrdlock\fR()
- functions may fail if:
- .TP
- .BR EAGAIN
- The read lock could not be acquired because the maximum number of read
- locks for
- .IR rwlock
- has been exceeded.
- .P
- The
- \fIpthread_rwlock_rdlock\fR()
- function may fail if:
- .TP
- .BR EDEADLK
- A deadlock condition was detected or the current thread already owns
- the read-write lock for writing.
- .P
- These functions shall not return an error code of
- .BR [EINTR] .
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- Applications using these functions may be subject to priority inversion,
- as discussed in the Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 3.291" ", " "Priority Inversion".
- .SH RATIONALE
- If an implementation detects that the value specified by the
- .IR rwlock
- argument to
- \fIpthread_rwlock_rdlock\fR()
- or
- \fIpthread_rwlock_tryrdlock\fR()
- does not refer to an initialized read-write lock object, it is
- recommended that the function should fail and report an
- .BR [EINVAL]
- error.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .ad l
- .IR "\fIpthread_rwlock_destroy\fR\^(\|)",
- .IR "\fIpthread_rwlock_timedrdlock\fR\^(\|)",
- .IR "\fIpthread_rwlock_timedwrlock\fR\^(\|)",
- .IR "\fIpthread_rwlock_trywrlock\fR\^(\|)",
- .IR "\fIpthread_rwlock_unlock\fR\^(\|)"
- .ad b
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 3.291" ", " "Priority Inversion",
- .IR "Section 4.12" ", " "Memory Synchronization",
- .IR "\fB<pthread.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 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 .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .