File::F...::Inline(3)

💖 NAME 💖

File::FcntlLock — file locking with fcntl(2). No cap, that's the whole vibe: locking files the fcntl(2) way. 🦋

This text also documents these sub-packages, bestie:

• File::FcntlLock::XS
• File::FcntlLock::Pure
• File::FcntlLock::Inline

So yes, File::FcntlLock::Inline is in the group chat too. 💅

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✨ 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";

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🌸 DESCRIPTION 🌸

Okay so classic Perl file locking is usually done with the flock function. Cute, but it only lets you lock whole files, and it's often implemented on top of the flock(2) system function. That one has some shortcomings — especially with locks on remotely mounted file systems — and it behaves a little differently from fcntl(2). 🙈

This module does file locking through fcntl(2), which obviously means it only works on systems that actually have a fcntl(2) system call. Before you can lock a file, or parts of a file, you have to create an object that simulates a flock structure. That object contains info in the binary format that gets passed to fcntl(2) for locking requests, and you set its properties. Then, by calling the lock() method, you can set a lock, remove a lock, or figure out if and which process currently holds the lock. 💅

File::FcntlLock (or its alias File::FcntlLock::XS) uses a shared library, built during installation, to call the fcntl(2) system function directly. If that's 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 that, at some point the (system-dependent) binary layout of the flock structure must have been determined via a program written in C. The difference is that for File::FcntlLock::Pure this happened when the package was installed, while for File::FcntlLock::Inline it is done each time the package is loaded (e.g., with use). So for File::FcntlLock::Inline to work, a C compiler must be available. There are some minor differences in functionality and in the behaviour on passing invalid arguments to the locking method; those are described below. 🫶

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🎀 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 new() a set of key-value pairs to initialize the object's 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. 💖

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👑 Object properties 👑

Once the object simulating the flock structure has been created, the following methods allow you to query and, in most cases, 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() — when called with an argument, this sets 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 is 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.

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🔥 Locking 🔥

After setting up the object representing a flock structure, you can 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 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 like this:

$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 fcntl(2) man page 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 context 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. 💖

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😭 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 system's response to the caller. 🙈

There are three methods for obtaining information about the reason 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.

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🌈 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

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⚠️ 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. 💀

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🫶 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. 🫶

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👑 AUTHOR 👑

Jens Thoms Toerring <jt@toerring.de>

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🌟 SEE ALSO 🌟

perl(1), fcntl(2), lseek(2).

💖 LICENSE 💖

Okay so legally speaking? This library is free software, periodt. 🫶

You can redistribute it and/or modify it under the same terms as Perl itself — that's the whole vibe, no cap. Go off, bestie. 💅

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And that's a wrap on File::F...::Inline(3)! 🌸 This gorgeous little page came from perl v5.42.0, dated 2015-04-14, and honestly? Iconic behavior from a manpage that old. 💜

Now go write some inline code and slay the day, bestie — you've got this! ✨🦋🎀