Hello every one I am making a program using filing I know how to read an write in a file .But please can any one help me about the file read write locks in C programming.Like how to insert lock and how to release it especially in forking .Please any give a small example or a tutorial as i didn't file any thing about file locks in c
Thanks
File locking is not part of C, but is dependent on the operating system. Since you talk abour forking I assume you are using UNIX or a UNIX-like system (e.g. Linux or BSD.)
In that case you can use the flock or lockf functions. These locks are preserved on forking, which means that multiple processes can have an exclusive lock to the same file if the lock was acquired in the parent process before the fork.
On Windows it can be specified in the CreateFilecall, or later with the LockFile or LockFileEx functions.
Related
I need to create named lock that work correctly with multi-thread application for Linux. Each instance of application could use more than one named-lock with different names.
I know about fcntl/flock, but it doesn't work if try to lock twice from different thread of one application or from one thread.
I know about open(..., O_CREATE | O_EXCL), but this file-lock will not be removed if application was killed by signal KILL or was crashed with segmentation fault and there is needed manual removing of lock-files after restart application.
Any another ways?
If you just need to run under modern Linux, you could use file-private locks. If that's not an option, you'll have to build your own thread-safe locking abstraction on top of fcntl locks. SQLite is public domain and has implemented that, so you could look at that for inspiration. If GPLed code is okay: OpenJDK has another, incompatible implementation of the same thing.
O_EXCL does not perform locking (beyond the file creation step), so that's usually not helpful.
Other options are System V and POSIX semaphores, but these usually do not work as well as fcntl locks when processes day. A robust, process-shared mutex in a file mapping could be an option as well, but you need to be careful to stay within the POSIX semantics as far as serialization to disk is concerned (basically, you need to reinitialize the mutex every time the application starts from scratch, after a reboot or libc update).
Lets say 4 simultaneous processes are running on a processor, and data needs to be copied from an HDFS (used with Spark) file system to a local directory. Now I want only one process to copy that data, while the other processes just wait for that data to be copied by the first process.
So, basically, I want some kind of a semaphore mechanism, where every process tries to obtain semaphore to try copying the data, but only one process gets the semaphore. All processes who failed to acquire the semaphore would then just wait for the semaphore to be cleared (the process who was able to acquire the semaphore would clear it after its done with copying), and when its cleared they know the data has already been copied. How can I do that in Linux?
There's a lot of different ways to implement semaphores. The classical, System V semaphore way is described in man semop and more broadly in man sem_overview.
You might still want to do something more easily scalable and modern. Many IPC frameworks (Apache has one or two of those, too!) have atomic IPC operations. These can be used to implement semaphores, but I'd be very very careful.
Generally, I regularly encourage people who write multi-process or multi-threaded applications to use C++ instead of C. It's often simpler to see where a shared state must be protected if your state is nicely encapsulated in an object which might do its own locking. Hence, I urge you to have a look at Boost's IPC synchronization mechanisms.
In addition of Marcus Müller's answer, you could use some file locking mechanism to synchronize.
File locking might not work very well on networked or remote file systems. You should use it on a locally mounted file system (e.g. Ext4, BTRFS, ...) not on a remote one (e.g. NFS)
For example, you might adopt the convention that your directory contains (or else you'll create it) some .lock file and use an advisory lock flock(2) (or a POSIX lockf(3)) on that .lock file before accessing the directory.
If using flock, you could even lock the directory directly....
The advantage of using such a file lock approach is that you could code shell scripts using flock(1)
And on Linux, you might also use inotify(7) (e.g. to be notified when some file is created in that directory)
Notice that most solutions are (advisory, so) presupposing that every process accessing that directory is following some convention (in other words, without more precautions like using flock(1), a careless user could access that directory - e.g. with a plain cp command -, or files under it, while your locking process is accessing the directory). If you don't accept that, you might look for mandatory file locking (which is a feature of some Linux kernels & filesystems, AFAIK it is sort-of deprecated).
BTW, you might read more about ACID properties and consider using some database, etc...
I'm writing a (very small) webserver in C language on Windows.
I need exclusive lock file both on reading and writing files, i've read msdn documentation about locking etc. and I've found the function LockFileEx with the OVERLAPPED structure and an Event hEvent, I read also about how they work but the question is:
- In a web server we have lots of files, when a thread locks for example the file "test.txt"(exclusive lock) because for there was a request of this file, how can I synchronize another thread that wants to get the lock on the same file?
thank you.
Take a look at the use of mutex objects. They should solve that problem for you.
Threads that need access to the lock file can request a lock for it and be queued. When the current thread is done, it releases its lock and the next requesting thread is granted the lock.
According to a man pages the following approaches supports only advisory locking: flock, lockf and fcntl. Is there any way to mandatory lock a some file by a single process, for example with a write lock, so that other process will not able to open this file with the write permissions?
No. Operating systems in the Unix family do not generally support mandatory file locking1. This includes Linux, BSD, and OS X.
On some Unixes, you are prevented from opening files for writing if they are executable images that are currently running; open() will fail with ETXTBSY. However, you can always just unlink (delete) the file and create a new one instead, and nothing will prevent that.
Footnotes
1: This is not entirely true, but mandatory file locks require a bit of work, mandatory locks are platform-specific, and OS X has no support for them.
Looking for information regarding the advantages and disadvantages of both fcntl and lockf for file locking. For example which is better to use for portability? I am currently coding a linux daemon and wondering which is better suited to use for enforcing mutual exclusion.
What is the difference between lockf and fcntl:
On many systems, the lockf() library routine is just a wrapper around fcntl(). That is to say lockf offers a subset of the functionality that fcntl does.
Source
But on some systems, fcntl and lockf locks are completely independent.
Source
Since it is implementation dependent, make sure to always use the same convention. So either always use lockf from both your processes or always use fcntl. There is a good chance that they will be interchangeable, but it's safer to use the same one.
Which one you chose doesn't matter.
Some notes on mandatory vs advisory locks:
Locking in unix/linux is by default advisory, meaning other processes don't need to follow the locking rules that are set. So it doesn't matter which way you lock, as long as your co-operating processes also use the same convention.
Linux does support mandatory locking, but only if your file system is mounted with the option on and the file special attributes set. You can use mount -o mand to mount the file system and set the file attributes g-x,g+s to enable mandatory locks, then use fcntl or lockf. For more information on how mandatory locks work see here.
Note that locks are applied not to the individual file, but to the inode. This means that 2 filenames that point to the same file data will share the same lock status.
In Windows on the other hand, you can actively exclusively open a file, and that will block other processes from opening it completely. Even if they want to. I.e., the locks are mandatory. The same goes for Windows and file locks. Any process with an open file handle with appropriate access can lock a portion of the file and no other process will be able to access that portion.
How mandatory locks work in Linux:
Concerning mandatory locks, if a process locks a region of a file with a read lock, then other processes are permitted to read but not write to that region. If a process locks a region of a file with a write lock, then other processes are not permitted to read nor write to the file. What happens when a process is not permitted to access the part of the file depends on if you specified O_NONBLOCK or not. If blocking is set it will wait to perform the operation. If no blocking is set you will get an error code of EAGAIN.
NFS warning:
Be careful if you are using locking commands on an NFS mount. The behavior is undefined and the implementation widely varies whether to use a local lock only or to support remote locking.
Both interfaces are part of the POSIX standard, and nowadays both interfaces are available on most systems (I just checked Linux, FreeBSD, Mac OS X, and Solaris). Therefore, choose the one that fits better your requirements and use it.
One word of caution: it is unspecified what happens when one process locks a file using fcntl and another using lockf. In most systems these are equivalent operations (in fact under Linux lockf is implemented on top of fcntl), but POSIX says their interaction is unspecified. So, if you are interoperating with another process that uses one of the two interfaces, choose the same one.
Others have written that the locks are only advisory: you are responsible for checking whether a region is locked. Also, don't use stdio functions, if you want the to use the locking functionality.
Your main concerns, in this case (i.e. when "coding a Linux daemon and wondering which is better suited to use for enforcing mutual exclusion"), should be:
will the locked file be local or can it be on NFS?
e.g. can the user trick you into creating and locking your daemon's pid file on NFS?
how will the lock behave when forking, or when the daemon process is terminated with extreme prejudice e.g. kill -9?
The flock and fcntl commands behave differently in both cases.
My recommendation would be to use fcntl. You may refer to the File locking article on Wikipedia for an in-depth discussion of the problems involved with both solutions:
Both flock and fcntl have quirks which
occasionally puzzle programmers from
other operating systems. Whether flock
locks work on network filesystems,
such as NFS, is implementation
dependent. On BSD systems flock calls
are successful no-ops. On Linux prior
to 2.6.12 flock calls on NFS files
would only act locally. Kernel 2.6.12
and above implement flock calls on NFS
files using POSIX byte range locks.
These locks will be visible to other
NFS clients that implement
fcntl()/POSIX locks.1 Lock upgrades
and downgrades release the old lock
before applying the new lock. If an
application downgrades an exclusive
lock to a shared lock while another
application is blocked waiting for an
exclusive lock, the latter application
will get the exclusive lock and the
first application will be locked out.
All fcntl locks associated with a file
for a given process are removed when
any file descriptor for that file is
closed by that process, even if a lock
was never requested for that file
descriptor. Also, fcntl locks are not
inherited by a child process. The
fcntl close semantics are particularly
troublesome for applications which
call subroutine libraries that may
access files.
I came across an issue while using fcntl and flock recently that I felt I should report here as searching for either term shows this page near the top on both.
Be advised BSD locks, as mentioned above, are advisory. For those who do not know OSX (darwin) is BSD. This must be remembered when opening a file to write into.
To use fcntl/flock you must first open the file and get its ID. However if you have opened the file with "w" the file will instantly be zeroed out. If your process then fails to get the lock as the file is in use elsewhere, it will most likely return, leaving the file as 0kb. The process which had the lock will now find the file has vanished from underneath it, catastrophic results normally follow.
To remedy this situation, when using file locking, never open the file "w", but instead open it "a", to append. Then if the lock is successfully acquired, you can then safely clear the file as "w" would have, ie. :
fseek(fileHandle, 0, SEEK_SET);//move to the start
ftruncate(fileno((FILE *) fileHandle), 0);//clear it out
This was an unpleasant lesson for me.
As you're only coding a daemon which uses it for mutual exclusion, they are equivalent, after all, your application only needs to be compatible with itself.
The trick with the file locking mechanisms is to be consistent - use one and stick to it. Varying them is a bad idea.
I am assuming here that the filesystem will be a local one - if it isn't, then all bets are off, NFS / other network filesystems handle locking with varying degrees of effectiveness (in some cases none)