man3/pthread_spin_init.3

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  25 .TH PTHREAD_SPIN_INIT 3 2017-09-30 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 pthread_spin_init, pthread_spin_destroy \- initialize or destroy a spin lock
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <pthread.h>
  31 .PP
  32 .BI "int pthread_spin_init(pthread_spinlock_t *" lock ", int " pshared ");"
  33 .BI "int pthread_spin_destroy(pthread_spinlock_t *" lock ");"
  34 .fi
  35 .PP
  36 Compile and link with \fI\-pthread\fP.
  37 .PP
  38 .in -4n
  39 Feature Test Macro Requirements for glibc (see
  40 .BR feature_test_macros (7)):
  41 .in
  42 .PP
  43 .BR pthread_spin_init (),
  44 .BR pthread_spin_destroy ():
  45 .br
  46 .RS 4
  47 .ad l
  48 _POSIX_C_SOURCE >= 200112L
  49 .RE
  50 .ad
  51 .SH DESCRIPTION
  52 .IR "General note" :
  53 Most programs should use mutexes
  54 instead of spin locks.
  55 Spin locks are primarily useful in conjunction with real-time
  56 scheduling policies.
  57 See NOTES.
  58 .PP
  59 The
  60 .BR pthread_spin_init ()
  61 function allocates any resources required for the use of
  62 the spin lock referred to by
  63 .I lock
  64 and initializes the lock to be in the unlocked state.
  65 The
  66 .I pshared
  67 argument must have one of the following values:
  68 .TP
  69 .B PTHREAD_PROCESS_PRIVATE
  70 The spin lock is to be operated on only by threads in the same process
  71 as the thread that calls
  72 .BR pthread_spin_init ().
  73 (Attempting to share the spin lock between processes
  74 results in undefined behavior.)
  75 .TP
  76 .B PTHREAD_PROCESS_SHARED
  77 The spin lock may be operated on by any thread in any process that
  78 has access to the memory containing the lock
  79 (i.e., the lock may be in a shared memory object that is
  80 shared among multiple processes).
  81 .PP
  82 Calling
  83 .BR pthread_spin_init ()
  84 on a spin lock that has already been initialized results
  85 in undefined behavior.
  86 .PP
  87 The
  88 .BR pthread_spin_destroy ()
  89 function destroys a previously initialized spin lock,
  90 freeing any resources that were allocated for that lock.
  91 Destroying a spin lock that has not been previously been initialized
  92 or destroying a spin lock while another thread holds the lock
  93 results in undefined behavior.
  94 .PP
  95 Once a spin lock has been destroyed,
  96 performing any operation on the lock other than
  97 once more initializing it with
  98 .BR pthread_spin_init ()
  99 results in undefined behavior.
 100 .PP
 101 The result of performing operations such as
 102 .BR pthread_spin_lock (3),
 103 .BR pthread_spin_unlock (3),
 104 and
 105 .BR pthread_spin_destroy (3)
 106 on
 107 .I copies
 108 of the object referred to by
 109 .I lock
 110 is undefined.
 111 .SH RETURN VALUE
 112 On success, there functions return zero.
 113 On failure, they return an error number.
 114 In the event that
 115 .BR pthread_spin_init ()
 116 fails, the lock is not initialized.
 117 .SH ERRORS
 118 .BR pthread_spin_init ()
 119 may fail with the following errors:
 120 .\" These errors don't occur on the glibc implementation
 121 .TP
 122 .B EAGAIN
 123 The system has insufficient resources to initialize
 124 a new spin lock.
 125 .TP
 126 .B ENOMEM
 127 Insufficient memory to initialize the spin lock.
 128 .SH VERSIONS
 129 These functions first appeared in glibc in version 2.2.
 130 .SH CONFORMING TO
 131 POSIX.1-2001.
 132 .PP
 133 Support for process-shared spin locks is a POSIX option.
 134 The option is supported in the glibc implementation.
 135 .SH NOTES
 136 Spin locks should be employed in conjunction with
 137 real-time scheduling policies
 138 .RB ( SCHED_FIFO ,
 139 or possibly
 140 .BR SCHED_RR ).
 141 Use of spin locks with nondeterministic scheduling policies such as
 142 .BR SCHED_OTHER
 143 probably indicates a design mistake.
 144 The problem is that if a thread operating under such a policy
 145 is scheduled off the CPU while it holds a spin lock,
 146 then other threads will waste time spinning on the lock
 147 until the lock holder is once more rescheduled and releases the lock.
 148 .PP
 149 If threads create a deadlock situation while employing spin locks,
 150 those threads will spin forever consuming CPU time.
 151 .PP
 152 User-space spin locks are
 153 .I not
 154 applicable as a general locking solution.
 155 They are, by definition,
 156 prone to priority inversion and unbounded spin times.
 157 A programmer using spin locks must be exceptionally careful not
 158 only in the code, but also in terms of system configuration,
 159 thread placement, and priority assignment.
 160 .\" FIXME . When PTHREAD_MUTEX_ADAPTIVE_NP is one day document
 161 .\" make reference to it here
 162 .SH SEE ALSO
 163 .ad l
 164 .nh
 165 .BR pthread_mutex_init (3),
 166 .BR pthread_mutex_lock (3),
 167 .BR pthread_spin_lock (3),
 168 .BR pthread_spin_unlock (3),
 169 .BR pthreads (7)