| File::FcntlLock::Pure(3pm) - phpMan
File::FcntlLock::Pure(3pm) User Contributed Perl Documentation File::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 AT 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 v5.20.0 2014-05-27 File::FcntlLock::Pure(3pm)
|