man3/pthread_rwlockattr_setkind_np.3

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  25 .TH PTHREAD_RWLOCKATTR_SETKIND_NP 3 2019-03-06 "Linux Programmer's Manual"
  26 .SH NAME
  27 pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np \- set/get
  28 the read-write lock kind of the thread read-write lock attribute object
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <pthread.h>
  32 .PP
  33 .BI "int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *" attr ,
  34 .BI "                                   int " pref );
  35 .BI "int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *" attr ,
  36 .BI "                                   int *" pref );
  37 .PP
  38 Compile and link with \fI\-pthread\fP.
  39 .PP
  40 .fi
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .PP
  46 .BR pthread_rwlockattr_setkind_np (),
  47 .BR pthread_rwlockattr_getkind_np ():
  48 .br
  49 .RS 4
  50 .ad l
  51 _XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE >= 200809L
  52 .RE
  53 .ad
  54 .SH DESCRIPTION
  55 The
  56 .BR pthread_rwlockattr_setkind_np ()
  57 function sets the "lock kind" attribute of the
  58 read-write lock attribute object referred to by
  59 .I attr
  60 to the value specified in
  61 .IR pref .
  62 The argument
  63 .I pref
  64 may be set to one of the following:
  65 .TP
  66 .B PTHREAD_RWLOCK_PREFER_READER_NP
  67 This is the default.
  68 A thread may hold multiple read locks; that is, read locks are recursive.
  69 According to The Single Unix Specification, the behavior is unspecified when a
  70 reader tries to place a lock, and there is no write lock but writers are
  71 waiting.
  72 Giving preference to the reader, as is set by
  73 .BR PTHREAD_RWLOCK_PREFER_READER_NP ,
  74 implies that the reader will receive the requested lock, even if
  75 a writer is waiting.
  76 As long as there are readers, the writer will be
  77 starved.
  78 .TP
  79 .B PTHREAD_RWLOCK_PREFER_WRITER_NP
  80 This is intended as the write lock analog of
  81 .BR PTHREAD_RWLOCK_PREFER_READER_NP .
  82 This is ignored by glibc because the POSIX requirement to support
  83 recursive writer locks would cause this option to create trivial
  84 deadlocks; instead use
  85 .B PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
  86 which ensures the application developer will not take recursive
  87 read locks thus avoiding deadlocks.
  88 .\" ---
  89 .\" Here is the relevant wording:
  90 .\"
  91 .\"     A thread may hold multiple concurrent read locks on rwlock (that is,
  92 .\"     successfully call the pthread_rwlock_rdlock() function n times). If
  93 .\"     so, the thread must perform matching unlocks (that is, it must call
  94 .\"     the pthread_rwlock_unlock() function n times).
  95 .\"
  96 .\" By making write-priority work correctly, I broke the above requirement,
  97 .\" because I had no clue that recursive read locks are permissible.
  98 .\"
  99 .\" If a thread which holds a read lock tries to acquire another read lock,
 100 .\" and now one or more writers is waiting for a write lock, then the algorithm
 101 .\" will lead to an obvious deadlock. The reader will be suspended, waiting for
 102 .\" the writers to acquire and release the lock, and the writers will be
 103 .\" suspended waiting for every existing read lock to be released.
 104 .\" ---
 105 .\" http://sources.redhat.com/ml/libc-alpha/2000-01/msg00055.html
 106 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=7057
 107 .TP
 108 .B PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
 109 Setting the lock kind to this
 110 avoids writer starvation as long as any read locking is not done in a
 111 recursive fashion.
 112 .PP
 113 The
 114 .BR pthread_rwlockattr_getkind_np ()
 115 function returns the value of the lock kind attribute of the
 116 read-write lock attribute object referred to by
 117 .IR attr
 118 in the pointer
 119 .IR pref .
 120 .SH RETURN VALUE
 121 On success, these functions return 0.
 122 Given valid pointer arguments,
 123 .BR pthread_rwlockattr_getkind_np ()
 124 always succeeds.
 125 On error,
 126 .BR pthread_rwlockattr_setkind_np ()
 127 returns a nonzero error number.
 128 .SH ERRORS
 129 .TP
 130 .BR EINVAL
 131 .I pref
 132 specifies an unsupported value.
 133 .SH VERSIONS
 134 The
 135 .BR pthread_rwlockattr_getkind_np ()
 136 and
 137 .BR pthread_rwlockattr_setkind_np ()
 138 functions first appeared in glibc 2.1.
 139 .SH CONFORMING TO
 140 These functions are non-standard GNU extensions;
 141 hence the suffix "_np" (nonportable) in the names.
 142 .SH SEE ALSO
 143 .BR pthreads (7)