1File::FcntlLock::InlineU(s3e)r Contributed Perl DocumentaFtiiloen::FcntlLock::Inline(3)
2
6 File::FcntlLock - File locking with fcntl(2)
7 This text also documents the following sub-packages:
9 File::FcntlLock::XS
11 File::FcntlLock::Pure
12 File::FcntlLock::Inline
13
15 use File::FcntlLock;
16 my $fs = new File::FcntlLock;
18 $fs->l_type( F_RDLCK );
19 $fs->l_whence( SEEK_CUR );
20 $fs->l_start( 100 );
21 $fs->l_len( 123 );
22 open my $fh, '<', 'file_name' or die "Can't open file: $!\n";
24 $fs->lock( $fh, F_SETLK )
25 or print "Locking failed: " . $fs->error . "\n";
26 $fs->l_type( F_UNLCK );
27 $fs->lock( $fh, F_SETLK )
28 or print "Unlocking failed: " . $fs->error . "\n";
29
31 File locking in Perl is usually done using the "flock" function.
32 Unfortunately, this only allows locks on whole files and is often
33 implemented in terms of the flock(2) system function which has some
34 shortcomings (especially concerning locks on remotely mounted file
35 systems) and slightly different behaviour than fcntl(2).
36 Using this module file locking via fcntl(2) can be done (obviously,
38 this restricts the use of the module to systems that have a fcntl(2)
39 system call). Before a file (or parts of a file) can be locked, an
40 object simulating a flock structure, containing information in a binary
41 format to be passed to fcntl(2) for locking requests, must be created
42 and its properties set. Afterwards, by calling the lock() method a lock
43 can be set and removed or it can be determined if and which process
44 currently holds the lock.
45 File::FcntlLock (or its alias File::FcntlLock::XS) uses a shared
47 library, build during installation, to call the fcntl(2) system
48 function directly. If this is unsuitable there are two alternatives,
49 File::FcntlLock::Pure and File::FcntlLock::Inline. Both call the Perl
50 "fcntl" function instead and use Perl code to assemble and disassemble
51 the structure. For this at some time the (system-dependent) binary
52 layout of the flock structure must have been determined via a program
53 written in C. The difference between File::FcntlLock::Pure and
54 File::FcntlLock::Inline is that for the former this happened when the
55 package is installed while for the latter it is done each time the
56 package is loaded (e.g., with "use"). Thus, for File::FcntlLock::Inline
57 to work a C compiler must be available. There are some minor
58 differences in the functionality and the behaviour on passing the
59 method for locking invalid arguments to be described below.
60 Creating objects
62 new()
63 To create a new object, representing a flock structure, call new():
64 $fs = new File::FcntlLock;
66 The object has a number of properties, reflecting the members of
68 the flock structure to be passed to fcntl(2) (see below). Per
69 default on object creation the l_type property is set to "F_RDLCK",
70 l_whence to "SEEK_SET", and both l_start and l_len to 0, i.e., the
71 settings for a read lock on the whole file.
72 These defaults can be overruled by passing the new() method a set
74 of key-value pairs to initialize the objects properties, e.g. use
75 $fs = new File::FcntlLock( l_type => F_WRLCK,
77 l_whence => SEEK_SET,
78 l_start => 0,
79 l_len => 100 );
80 if you intend to obtain a write lock for the first 100 bytes of a
82 file.
83 Object properties
85 Once the object simulating the flock structure has been created the
86 following methods allow to query and, in most cases, to also modify its
87 properties.
88 l_type()
90 If called without an argument the method returns the current
91 setting of the lock type, otherwise the lock type is set to the
92 argument's value which must be either "F_RDLCK", "F_WRLCK" or
93 "F_UNLCK" (for read lock, write lock or unlock).
94 l_whence()
96 This method sets, when called with an argument, the l_whence
97 property of the flock object, determining if the l_start value is
98 relative to the start of the file, to the current position in the
99 file or to the end of the file. These values are "SEEK_SET",
100 "SEEK_CUR" and "SEEK_END" (also see the man page for lseek(2)). If
101 called with no argument the current value of the property is
102 returned.
103 l_start()
105 Queries or sets the start position (offset) of the lock in the file
106 according to the mode selected by the l_whence member. See also the
107 man page for lseek(2).
108 l_len()
110 Queries or sets the length of the region (in bytes) in the file to
111 be locked. A value of 0 is interpreted to mean a lock, starting at
112 "l_start", to the end of the file. E.g., a lock obtained with
113 l_whence set to "SEEK_SET" and both l_start and l_len set to 0
114 locks the complete file.
115 According to SUSv3 support for negative values for l_len are
117 permitted, resulting in a lock ranging from "l_start+l_len" up to
118 and including "l_start-1". But not all systems support negative
119 values for l_len and will return an error when you try to obtain
120 such a lock, so please read the fcntl(2) man page of the system
121 carefully for details.
122 l_pid()
124 If a call of the lock() method with "F_GETLK" indicates that
125 another process is holding the lock (in which case the l_type
126 property will be either "F_WRLCK" or "F_RDLCK") a call of the
127 l_pid() method returns the PID of the process holding the lock.
128 This method does not accept any arguments.
129 Locking
131 After having set up the object representing a flock structure one can
132 then try to obtain a lock, release it or determine the current holder
133 of the lock by invoking the lock() method:
134 lock()
136 This method expects two arguments. The first one is a file handle
137 (or typeglob). File::FcntlLock, and thus File::FcntlLock::XS (but
138 neither File::FcntlLock::Pure nor File::FcntlLock::Inline), also
139 accepts a "raw" integer file descriptor. The second argument is a
140 flag indicating the action to be taken. So call it as in
141 $fs->lock( $fh, F_SETLK );
143 There are three values that can be used as the second argument:
145 "F_SETLK"
147 With "F_SETLK" the lock() method tries to obtain a lock (when
148 l_type is set to either "F_WRLCK" or "F_RDLCK") or releases it
149 (if l_type is set to "F_UNLCK"). If an attempt is made to
150 obtain a lock but a lock is already being held by some other
151 process the method returns "undef" and "errno" is set to
152 "EACCESS" or "EAGAIN" (please see the the man page for fcntl(2)
153 for more details).
154 "F_SETLKW"
156 is similar to "F_SETLK", but instead of returning an error if
157 the lock can't be obtained immediately it puts the calling
158 process to sleep, i.e., it blocks, until the lock is obtained
159 at some later time. If a signal is received while waiting for
160 the lock the method returns "undef" and "errno" is set to
161 "EINTR".
162 "F_GETLK"
164 With "F_GETLK" the lock() method determines if and which
165 process currently is holding the lock. If there's no other
166 lock the l_type property will be set to "F_UNLCK". Otherwise
167 the flock structure object is set to the values that would
168 prevent us from obtaining a lock. There may be several
169 processes that keep us from getting a lock, including some that
170 themselves are blocked waiting to obtain a lock. "F_GETLK" will
171 only make details of one of these processes visible, and one
172 has no control over which process this is.
173 On success the lock() method returns the string "0 but true", i.e.,
175 a value that is true in boolean but 0 in numeric context. If the
176 method fails (as indicated by an "undef" return value) you can
177 either immediately evaluate the error number (using $!, $ERRNO or
178 $OS_ERROR) or check for it via the methods discussed below at some
179 later time.
180 Error handling
182 There are minor differences between File::FcntlLock on the one hand and
183 File::FcntlLock::Pure and File::FcntlLock::Inline on the other, due to
184 the first calling the system function fcntl(2) directly while the
185 latter two invoke the Perl "fcntl" function. Perl's "fcntl" function
186 already returns a Perl error on some types of invalid arguments. In
187 contrast File::FcntlLock passes them on to the fcntl(2) system call and
188 then returns the systems response to the caller.
189 There are three methods for obtaining information about the reason the
191 a call of the lock() method failed:
192 lock_errno()
194 Returns the "errno" error number from the latest call of lock().
195 If the last call did not result in an error "undef" is returned.
196 error()
198 Returns a short description of the error that happened during the
199 latest call of lock(). Please take the messages with a grain of
200 salt, they represent what SUSv3 (IEEE 1003.1-2001) and the Linux,
201 TRUE64, OpenBSD3 and Solaris8 man pages tell what the error numbers
202 mean. There could be differences (and additional error numbers) on
203 other systems. If there was no error the method returns "undef".
204 system_error()
206 While the error() method tries to return a string with some direct
207 relevance to the locking operation (i.e., "File or segment already
208 locked by other process(es)" instead of "Permission denied") this
209 method returns the "normal" system error message associated with
210 "errno". The method returns "undef" if there was no error.
211 EXPORT
213 The package exports the following constants:
214 F_GETLK F_SETLK F_SETLKW
216 F_RDLCK F_WRLCK F_UNLCK
217 SEEK_SET SEEK_CUR SEEK_END
218
220 Obviously, this module requires that there's a fcntl(2) system call.
221 Note also that under certain circumstances the File::FcntlLock::Pure
222 and File::FcntlLock::Inline modules may not have been installed. This
223 happens on 32-bit systems that use 64-bit integers in their flock
224 structure but where the installed Perl version doesn't support the 'q'
225 format for its "pack" and "unpack" functions.
226
228 Thanks to Mark Jason Dominus and Benjamin Goldberg for helpful
229 discussions, code examples and encouragement. Glenn Herteg pointed out
230 several problems and also helped improve the documentation. Julian
231 Moreno Patino helped correcting the documentation and pointed out
232 problems arising on GNU Hurd which seems to have only very rudimentary
233 support for locking with fcntl(2). Niko Tyni and Guillem Jover
234 encouraged and helped with implementing alternatives to an XS-only
235 approach which hopefully will make the module more useful under certain
236 circumstances.
237
239 Jens Thoms Toerring <jt@toerring.de>
240
242 perl(1), fcntl(2), lseek(2).
243
245 This library is free software. You can redistribute it and/or modify it
246 under the same terms as Perl itself.
247perl v5.36.0 2023-01-20 File::FcntlLock::Inline(3)