1.. _locking:
2
3Locking
4=======
5
6FRR ships two small wrappers around ``pthread_mutex_lock()`` /
7``pthread_mutex_unlock``.  Use ``#include "frr_pthread.h"`` to get these
8macros.
9
10.. c:function:: frr_with_mutex(pthread_mutex_t *mutex)
11
12   Begin a C statement block that is executed with the mutex locked.  Any
13   exit from the block (``break``, ``return``, ``goto``, end of block) will
14   cause the mutex to be unlocked::
15
16      int somefunction(int option)
17      {
18          frr_with_mutex(&my_mutex) {
19              /* mutex will be locked */
20
21              if (!option)
22                  /* mutex will be unlocked before return */
23                  return -1;
24
25              if (something(option))
26                  /* mutex will be unlocked before goto */
27                  goto out_err;
28
29              somethingelse();
30
31              /* mutex will be unlocked at end of block */
32          }
33
34          return 0;
35
36      out_err:
37          somecleanup();
38          return -1;
39      }
40
41   This is a macro that internally uses a ``for`` loop.  It is explicitly
42   acceptable to use ``break`` to get out of the block.  Even though a single
43   statement works correctly, FRR coding style requires that this macro always
44   be used with a ``{ ... }`` block.
45
46.. c:function:: frr_mutex_lock_autounlock(pthread_mutex_t *mutex)
47
48   Lock mutex and unlock at the end of the current C statement block::
49
50      int somefunction(int option)
51      {
52          frr_mutex_lock_autounlock(&my_mutex);
53          /* mutex will be locked */
54
55          ...
56          if (error)
57            /* mutex will be unlocked before return */
58            return -1;
59          ...
60
61          /* mutex will be unlocked before return */
62          return 0;
63      }
64
65   This is a macro that internally creates a variable with a destructor.
66   When the variable goes out of scope (i.e. the block ends), the mutex is
67   released.
68
69   .. warning::
70
71      This macro should only used when :c:func:`frr_with_mutex` would
72      result in excessively/weirdly nested code.  This generally is an
73      indicator that the code might be trying to do too many things with
74      the lock held.  Try any possible venues to reduce the amount of
75      code covered by the lock and move to :c:func:`frr_with_mutex`.
76