PTHREADRWLOCKRDLOCK(3) BSD Library Functions Manual PTHREADRWLOCKRDLOCK(3)

NAME
     pptthhrreeaaddrrwwlloocckkrrddlloocckk, pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk - acquire a read/write
     lock for reading

SYNOPSIS
     ##iinncclluuddee <>

     int
     pptthhrreeaaddrrwwlloocckkrrddlloocckk(pthreadrwlockt *lock);

     int
     pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk(pthreadrwlockt *lock);

DESCRIPTION
     The pptthhrreeaaddrrwwlloocckkrrddlloocckk() function acquires a read lock on lock pro-
     vided that lock is not presently held for writing and no writer threads
     are presently blocked on the lock.  If the read lock cannot be immedi-
     ately acquired, the calling thread blocks until it can acquire the lock.

     The pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk() function performs the same action, but
     does not block if the lock cannot be immediately obtained (i.e. the lock
     is held for writing or there are waiting writers).

     A thread may hold multiple concurrent read locks.  If so,
     pptthhrreeaaddrrwwlloocckkuunnlloocckk() must be called once for each lock obtained.

     The results of acquiring a read lock while the calling thread holds a
     write lock are undefined.

IIMMPPLLEEMMEENNTTAATTIIOONN NNOOTTEESS
     To prevent writer starvation, writers are favored over readers.

RETURN VALUES
     If successful, the pptthhrreeaaddrrwwlloocckkrrddlloocckk() and pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk()
     functions will return zero.  Otherwise an error number will be returned
     to indicate the error.

SEE ALSO
     pthreadrwlockinit(3), pthreadrwlocktrywrlock(3),
     pthreadrwlockunlock(3), pthreadrwlockwrlock(3)

STANDARDS     The pptthhrreeaaddrrwwlloocckkrrddlloocckk() and pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk() functions are
     expected to conform to Version 2 of the Single UNIX Specification
     (``SUSv2'').

EERRRROORRSS
     The pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk() function will fail if:

     [EBUSY]            The lock could not be acquired because a writer holds
                        the lock or was blocked on it.

     The pptthhrreeaaddrrwwlloocckkrrddlloocckk() and pptthhrreeaaddrrwwlloocckkttrryyrrddlloocckk() functions may
     fail if:

     [EAGAIN]           The lock could not be acquired because the maximum
                        number of read locks against lock has been exceeded.

     [EDEADLK]          The current thread already owns lock for writing.

     [EINVAL]           The value specified by lock is invalid.

     [ENOMEM]           Insufficient memory exists to initialize the lock
                        (applies to statically initialized locks only).

HISTORY     The pptthhrreeaaddrrwwlloocckkrrddlloocckk() function first appeared in FreeBSD 3.0.

BSD                             August 4, 1998                             BSD