1File::FcntlLock(3) User Contributed Perl Documentation File::FcntlLock(3)
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6 File::FcntlLock - File locking with fcntl(2)
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9 use File::FcntlLock;
10 my $fs = new File::FcntlLock;
12 $fs->l_type( F_RDLCK );
13 $fs->l_whence( SEEK_CUR );
14 $fs->l_start( 100 );
15 $fs->l_len( 123 );
16 open my $fh, '<', 'file_name' or die "Can't open file: $!\n";
18 $fs->lock( $fh, F_SETLK )
19 or print "Locking failed: " . $fs->error . "\n";
20 $fs->l_type( F_UNLCK );
21 $fs->lock( $fh, F_SETLK )
22 or print "Unlocking failed: " . $fs->error . "\n";
23
25 File locking in Perl is usually done using the flock() function.
26 Unfortunately, this only allows locks on whole files and is often
27 implemented in terms of flock(2), which has some shortcomings.
28 Using this module file locking via fcntl(2) can be done (obviously,
30 this restricts the use of the module to systems that have a fcntl(2)
31 system call). Before a file (or parts of a file) can be locked, an
32 object simulating a flock structure must be created and its properties
33 set. Afterwards, by calling the "lock()" method a lock can be set or it
34 can be determined if and which process currently holds the lock.
35 To create a new object representing a flock structure call "new()":
37 $fs = new File::FcntlLock;
39 You also can pass the "new()" method a set of key-value pairs to
41 initialize the objects properties, e.g. use
42 $fs = new File::FcntlLock l_type => F_WRLCK,
44 l_whence => SEEK_SET,
45 l_start => 0,
46 l_len => 100;
47 if you plan to obtain a write lock for the first 100 bytes of a file.
49 Once you have created the object simulating the flock structure the
51 following methods allow to query and in most cases also to modify the
52 properties of the object.
53 "l_type()"
55 If called without an argument returns the current setting of the
56 lock type, otherwise the lock type is set to the argument, which
57 must be either "F_RDLCK", "F_WRLCK" or "F_UNLCK" (for read lock,
58 write lock or unlock).
59 "l_whence()"
61 Queries or sets the "l_whence" property of the flock object,
62 determining if the "l_start" value is relative to the start of the
63 file, to the current position in the file or to the end of the
64 file. The corresponding values are "SEEK_SET", "SEEK_CUR" and
65 "SEEK_END". See also the man page for lseek(2).
66 "l_start()"
68 Queries or sets the start position (offset) of the lock in the file
69 according to the mode selected by the "l_whence" member. See also
70 the man page for lseek(2).
71 "l_len()"
73 Queries or sets the length of the region (in bytes) in the file to
74 be locked. A value of 0 is interpreted as to mean a lock (starting
75 at "l_start") up to the very end of the file.
76 According to SUSv3 negative values for "l_start" are allowed
78 (resulting in a lock ranging from "l_start + l_len" to "l_start -
79 1") Unfortunately, not all systems allow negative arguments and
80 will return an error when you try to obtain the lock, so please
81 read the fcntl(2) man page of your system carefully for details.
82 "l_pid()"
84 This method allows retrieving the PID of a process currently
85 holding the lock after a call of "lock()" with "F_SETLK" indicated
86 that another process is holding the lock. A call to "lock()" with
87 "F_GETLK" will fill in this value so "l_pid()" can be called.
88 When not initialized the flock objects "l_type" property is set to
90 "F_RDLCK" by default, "l_whence" to "SEEK_SET", and both "l_start" and
91 "l_len" to 0, i.e. the settings for a read lock on the whole file.
92 After having set up the object representing a flock structure you can
94 determine the current holder of a lock or try to obtain a lock by
95 invoking the "lock()" method with two arguments, a file handle (or a
96 file descriptor, the module figures out automatically what it got) and
97 a flag indicating the action to be taken, e.g.
98 $fs->lock( $fh, F_SETLK );
100 There are three values that can be used as the second argument:
102 "F_GETLK"
104 For "F_GETLK" the "lock()" method determines if and who currently
105 is holding the lock. If no other process is holding the lock the
106 "l_type" field is set to "F_UNLCK". Otherwise the flock structure
107 object is set to the values that prevent us from obtaining a lock.
108 There may be multiple such blocking processes, including some that
109 are themselves blocked waiting to obtain a lock. "F_GETLK" will
110 only make details of one of these visible, and one has no control
111 over which process this is.
112 "F_SETLK"
114 For "F_SETLK" the "lock()" method tries to obtain the lock (when
115 "l_type" is set to either "F_WRLCK" or "F_RDLCK") or releases the
116 lock (if "l_type" is set to "F_UNLCK"). If a lock is held by some
117 other proces the method call returns "undef" and errno is set to
118 "EACCESS" or "EAGAIN" (please see the the man page for fcntl(2) for
119 the details).
120 "F_SETLKW"
122 is similar to "F_SETLK" but instead of returning an error if the
123 lock can't be obtained immediately it blocks until the lock is
124 obtained. If a signal is received while waiting for the lock the
125 method returns "undef" and errno is set to "EINTR".
126 On success the method returns the string "0 but true". If the method
128 fails (as indicated by an "undef" return value) you can either
129 immediately evaluate the error number (usingf $!, $ERRNO or $OS_ERROR)
130 or check for it at some later time via the methods discussed below.
131 There are three methods for obtaining information about the reason the
133 the last call of "lock()" for the object failed:
134 "lock_errno()"
136 Returns the error number from the latest call of "lock()". If the
137 last call did not result in an error the method returns "undef".
138 "error()"
140 Returns a short description of the error that happened during the
141 latest call of "lock()" with the object. Please take the messages
142 with a grain of salt, they represent what SUSv3 (IEEE 1003.1-2001)
143 and the Linux, TRUE64, OpenBSD3 and Solaris8 man pages tell what
144 the error numbers mean, there could be differences (and additional
145 error numbers) on other systems. If there was no error the method
146 returns "undef".
147 "system_error()"
149 While the previous method, "error()", tries to return a string with
150 some relevance to the locking operation (i.e. "File or segment
151 already locked by other process(es)" instead of "Permission
152 denied") this method returns the "normal" system error message
153 associated with errno. The method returns "undef" if there was no
154 error.
155 EXPORT
157 F_GETLK F_SETLK F_SETLKW F_RDLCK F_WRLCK F_UNLCK SEEK_SET SEEK_CUR
158 SEEK_END
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161 Thanks to Mark Jason Dominus (MJD) and Benjamin Goldberg (GOLDBB) for
162 helpful discussions, code examples and encouragement. Glenn Herteg
163 pointed out several problems and also helped improve the documentation.
164
166 Jens Thoms Toerring <jt@toerring.de>
167
169 perl(1), fcntl(2), lseek(2).
170perl v5.12.1 2009-10-08 File::FcntlLock(3)