File::FcntlLock::Pure(User Contributed Perl DocumentFile::FcntlLock::Pure(3pm)
NAME
File::FcntlLock - File locking with fcntl(2)
This text also documents the following sub-packages:
File::FcntlLock::XS
File::FcntlLock::Pure
File::FcntlLock::Inline
SYNOPSIS
use File::FcntlLock;
my $fs = new File::FcntlLock;
$fs->l_type( F_RDLCK );
$fs->l_whence( SEEK_CUR );
$fs->l_start( 100 );
$fs->l_len( 123 );
open my $fh, '<', 'file_name' or die "Can't open file: $!\n";
$fs->lock( $fh, F_SETLK )
or print "Locking failed: " . $fs->error . "\n";
$fs->l_type( F_UNLCK );
$fs->lock( $fh, F_SETLK )
or print "Unlocking failed: " . $fs->error . "\n";
DESCRIPTION
File locking in Perl is usually done using the "flock" function.
Unfortunately, this only allows locks on whole files and is often
implemented in terms of the flock(2) system function which has some
shortcomings (especially concerning locks on remotely mounted file
systems) and slightly different behaviour than fcntl(2).
Using this module file locking via fcntl(2) can be done (obviously,
this restricts the use of the module to systems that have a fcntl(2)
system call). Before a file (or parts of a file) can be locked, an
object simulating a flock structure, containing information in a binary
format to be passed to fcntl(2) for locking requests, must be created
and its properties set. Afterwards, by calling the lock() method a lock
can be set and removed or it can be determined if and which process
currently holds the lock.
File::FcntlLock (or its alias File::FcntlLock::XS) uses a shared
library, build during installation, to call the fcntl(2) system
function directly. If this is unsuitable there are two alternatives,
File::FcntlLock::Pure and File::FcntlLock::Inline. Both call the Perl
"fcntl" function instead and use Perl code to assemble and disassemble
the structure. For this at some time the (system-dependent) binary
layout of the flock structure must have been determined via a program
written in C. The difference between File::FcntlLock::Pure and
File::FcntlLock::Inline is that for the former this happened when the
package is installed while for the latter it is done each time the
package is loaded (e.g., with "use"). Thus, for File::FcntlLock::Inline
to work a C compiler must be available. There are some minor
differences in the functionality and the behaviour on passing the
method for locking invalid arguments to be described below.
Creating objects
"new()"
To create a new object, representing a flock structure, call new():
$fs = new File::FcntlLock;
The object has a number of properties, reflecting the members of
the flock structure to be passed to fcntl(2) (see below). Per
default on object creation the l_type property is set to "F_RDLCK",
l_whence to "SEEK_SET", and both l_start and l_len to 0, i.e., the
settings for a read lock on the whole file.
These defaults can be overruled by passing the new() method a set
of key-value pairs to initialize the objects properties, e.g. use
$fs = new File::FcntlLock( l_type => F_WRLCK,
l_whence => SEEK_SET,
l_start => 0,
l_len => 100 );
if you intend to obtain a write lock for the first 100 bytes of a
file.
Object properties
Once the object simulating the flock structure has been created the
following methods allow to query and, in most cases, to also modify its
properties.
"l_type()"
If called without an argument the method returns the current
setting of the lock type, otherwise the lock type is set to the
argument's value which must be either "F_RDLCK", "F_WRLCK" or
"F_UNLCK" (for read lock, write lock or unlock).
"l_whence()"
This method sets, when called with an argument, the l_whence
property of the flock object, determining if the l_start value is
relative to the start of the file, to the current position in the
file or to the end of the file. These values are "SEEK_SET",
"SEEK_CUR" and "SEEK_END" (also see the man page for lseek(2)). If
called with no argument the current value of the property is
returned.
"l_start()"
Queries or sets the start position (offset) of the lock in the file
according to the mode selected by the l_whence member. See also the
man page for lseek(2).
"l_len()"
Queries or sets the length of the region (in bytes) in the file to
be locked. A value of 0 is interpreted to mean a lock, starting at
"l_start", to the end of the file. E.g., a lock obtained with
l_whence set to "SEEK_SET" and both l_start and l_len set to 0
locks the complete file.
According to SUSv3 support for negative values for l_len are
permitted, resulting in a lock ranging from "l_start+l_len" up to
and including "l_start-1". But not all systems support negative
values for l_len and will return an error when you try to obtain
such a lock, so please read the fcntl(2) man page of the system
carefully for details.
"l_pid()"
If a call of the lock() method with "F_GETLK" indicates that
another process is holding the lock (in which case the l_type
property will be either "F_WRLCK" or "F_RDLCK") a call of the
l_pid() method returns the PID of the process holding the lock.
This method does not accept any arguments.
Locking
After having set up the object representing a flock structure one can
then try to obtain a lock, release it or determine the current holder
of the lock by invoking the lock() method:
"lock()"
This method expects two arguments. The first one is a file handle
(or typeglob). File::FcntlLock, and thus File::FcntlLock::XS (but
neither File::FcntlLock::Pure nor File::FcntlLock::Inline), also
accepts a "raw" integer file descriptor. The second argument is a
flag indicating the action to be taken. So call it as in
$fs->lock( $fh, F_SETLK );
There are three values that can be used as the second argument:
"F_SETLK"
With "F_SETLK" the lock() method tries to obtain a lock (when
l_type is set to either "F_WRLCK" or "F_RDLCK") or releases it
(if l_type is set to "F_UNLCK"). If an attempt is made to
obtain a lock but a lock is already being held by some other
process the method returns "undef" and "errno" is set to
"EACCESS" or "EAGAIN" (please see the the man page for fcntl(2)
for more details).
"F_SETLKW"
is similar to "F_SETLK", but instead of returning an error if
the lock can't be obtained immediately it puts the calling
process to sleep, i.e., it blocks, until the lock is obtained
at some later time. If a signal is received while waiting for
the lock the method returns "undef" and "errno" is set to
"EINTR".
"F_GETLK"
With "F_GETLK" the lock() method determines if and which
process currently is holding the lock. If there's no other
lock the l_type property will be set to "F_UNLCK". Otherwise
the flock structure object is set to the values that would
prevent us from obtaining a lock. There may be several
processes that keep us from getting a lock, including some that
themselves are blocked waiting to obtain a lock. "F_GETLK" will
only make details of one of these processes visible, and one
has no control over which process this is.
On success the lock() method returns the string "0 but true", i.e.,
a value that is true in boolean but 0 in numeric context. If the
method fails (as indicated by an "undef" return value) you can
either immediately evaluate the error number (using $!, $ERRNO or
$OS_ERROR) or check for it via the methods discussed below at some
later time.
Error handling
There are minor differences between File::FcntlLock on the one hand and
File::FcntlLock::Pure and File::FcntlLock::Inline on the other, due to
the first calling the system function fcntl(2) directly while the
latter two invoke the Perl "fcntl" function. Perl's "fcntl" function
already returns a Perl error on some types of invalid arguments. In
contrast File::FcntlLock passes them on to the fcntl(2) system call and
then returns the systems response to the caller.
There are three methods for obtaining information about the reason the
a call of the lock() method failed:
"lock_errno()"
Returns the "errno" error number from the latest call of lock().
If the last call did not result in an error "undef" is returned.
"error()"
Returns a short description of the error that happened during the
latest call of lock(). Please take the messages with a grain of
salt, they represent what SUSv3 (IEEE 1003.1-2001) and the Linux,
TRUE64, OpenBSD3 and Solaris8 man pages tell what the error numbers
mean. There could be differences (and additional error numbers) on
other systems. If there was no error the method returns "undef".
"system_error()"
While the error() method tries to return a string with some direct
relevance to the locking operation (i.e., "File or segment already
locked by other process(es)" instead of "Permission denied") this
method returns the "normal" system error message associated with
"errno". The method returns "undef" if there was no error.
EXPORT
The package exports the following constants:
F_GETLK F_SETLK F_SETLKW
F_RDLCK F_WRLCK F_UNLCK
SEEK_SET SEEK_CUR SEEK_END
INCOMPATIBILITIES
Obviously, this module requires that there's a fcntl(2) system call.
Note also that under certain circumstances the File::FcntlLock::Pure
and File::FcntlLock::Inline modules may not have been installed. This
happens on 32-bit systems that use 64-bit integers in their flock
structure but where the installed Perl version doesn't support the 'q'
format for its "pack" and "unpack" functions.
CREDITS
Thanks to Mark Jason Dominus and Benjamin Goldberg for helpful
discussions, code examples and encouragement. Glenn Herteg pointed out
several problems and also helped improve the documentation. Julian
Moreno Patino helped correcting the documentation and pointed out
problems arising on GNU Hurd which seems to have only very rudimentary
support for locking with fcntl(2). Niko Tyni and Guillem Jover
encouraged and helped with implementing alternatives to an XS-only
approach which hopefully will make the module more useful under certain
circumstances.
AUTHOR
Jens Thoms Toerring <jt@toerring.de>
SEE ALSO
perl(1), fcntl(2), lseek(2).
LICENSE
This library is free software. You can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.36.0 2022-10-19 File::FcntlLock::Pure(3pm)
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