bsd/man/man2/flock.2

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  34 .\"     @(#)flock.2     8.2 (Berkeley) 12/11/93
  35 .\"
  36 .Dd December 11, 1993
  37 .Dt FLOCK 2
  38 .Os BSD 4.2
  39 .Sh NAME
  40 .Nm flock
  41 .Nd "apply or remove an advisory lock on an open file"
  42 .Sh SYNOPSIS
  43 .Fd #include <sys/file.h>
  44 .Fd #define     LOCK_SH 1       /* shared lock */
  45 .Fd #define     LOCK_EX 2       /* exclusive lock */
  46 .Fd #define     LOCK_NB 4       /* don't block when locking */
  47 .Fd #define     LOCK_UN 8       /* unlock */
  48 .Ft int
  49 .Fn flock "int fd" "int operation"
  50 .Sh DESCRIPTION
  51 .Fn Flock
  52 applies or removes an
  53 .Em advisory
  54 lock on the file associated with the file descriptor
  55 .Fa fd .
  56 A lock is applied by specifying an
  57 .Fa operation
  58 parameter that is one of
  59 .Dv LOCK_SH
  60 or
  61 .Dv LOCK_EX
  62 with the optional addition of
  63 .Dv LOCK_NB .
  64 To unlock
  65 an existing lock
  66 .Dv operation
  67 should be
  68 .Dv LOCK_UN .
  69 .Pp
  70 Advisory locks allow cooperating processes to perform
  71 consistent operations on files, but do not guarantee
  72 consistency (i.e., processes may still access files
  73 without using advisory locks possibly resulting in
  74 inconsistencies).
  75 .Pp
  76 The locking mechanism allows two types of locks:
  77 .Em shared
  78 locks and
  79 .Em exclusive
  80 locks.
  81 At any time multiple shared locks may be applied to a file,
  82 but at no time are multiple exclusive, or both shared and exclusive,
  83 locks allowed simultaneously on a file.
  84 .Pp
  85 A shared lock may be
  86 .Em upgraded
  87 to an exclusive lock, and vice versa, simply by specifying
  88 the appropriate lock type; this results in the previous
  89 lock being released and the new lock applied (possibly
  90 after other processes have gained and released the lock).
  91 .Pp
  92 Requesting a lock on an object that is already locked
  93 normally causes the caller to be blocked until the lock may be
  94 acquired.  If
  95 .Dv LOCK_NB
  96 is included in
  97 .Fa operation ,
  98 then this will not happen; instead the call will fail and
  99 the error
 100 .Er EWOULDBLOCK
 101 will be returned.
 102 .Sh NOTES
 103 Locks are on files, not file descriptors.  That is, file descriptors
 104 duplicated through
 105 .Xr dup 2
 106 or
 107 .Xr fork 2
 108 do not result in multiple instances of a lock, but rather multiple
 109 references to a single lock.  If a process holding a lock on a file
 110 forks and the child explicitly unlocks the file, the parent will
 111 lose its lock.
 112 .Pp
 113 Processes blocked awaiting a lock may be awakened by signals.
 114 .Sh RETURN VALUES
 115 Zero is returned if the operation was successful;
 116 on an error a -1 is returned and an error code is left in
 117 the global location
 118 .Va errno .
 119 .Sh ERRORS
 120 The
 121 .Fn flock
 122 call fails if:
 123 .Bl -tag -width Er
 124 .It Bq Er EWOULDBLOCK
 125 The file is locked and the
 126 .Dv LOCK_NB
 127 option was specified.
 128 .It Bq Er EBADF
 129 The argument
 130 .Fa fd
 131 is an invalid descriptor.
 132 .It Bq Er EINVAL
 133 The argument
 134 .Fa fd
 135 refers to an object other than a file.
 136 .It Bq Er ENOTSUP
 137 The referenced descriptor is not of the correct type.
 138 .El
 139 .Sh SEE ALSO
 140 .Xr close 2 ,
 141 .Xr dup 2 ,
 142 .Xr execve 2 ,
 143 .Xr fork 2 ,
 144 .Xr open 2
 145 .Sh HISTORY
 146 The
 147 .Fn flock
 148 function call appeared in
 149 .Bx 4.2 .