This behaviour seems to be present all the way back to 4.0.6 from the little additional checking I made. I tried to trace this out between 4.1.2 and 4.2.2 (4.2.2 does not exhibit this behaviour) but I can find no obvious differances in the codepaths between versions. This becomes bad news because the ipc system on linux is a global resource and not based per user, so it's possible for a local user to DOS the box by running php -v test]$ ipcs > test]$ php -vĬontent-type: test]$ ipcs > test]$ diff l ll Synchronization using a counting semaphore: Let's say you have 3 tasks - #1 and 2 you want executed after 3.I've been contacted by a php user in the wild who told me that by simply issuing 'php -v', a semaphore that php opens for session management is not closed on exit.
Of what use is a multi-stage-lock in mutual exclusion or synchronization? Headers to include: gdbm.bi Header version: from 2010. Let's look at what this is implying - a lock with more than one value? So open, closed, and. Provides database functions using extensible hashing, primarily for storing key/data pairs to data files. hint hint : do i necessarily only need to use one semaphore?)Ĭounting semaphore: A semaphore with more than one value. (Think about how else one could use semaphores to achieve mutual exclusion. So a database named 'aliases' resides in the files 'aliases.dir' and 'aliases.pag', and name is specified as 'aliases'. pag, so name refers to the common name without extensions. Now, is it possible to have greater concurrency? Depends on the critical sections. In gdbm mode, the file is named normally in dbm and ndbm modes each database consists of two files with extensions. So each thread is using the same semaphore to provide mutual exclusion for its two critical sections. The mutual exclusion is quite simple as well - m1 and m2 cannot enter the critical section at the same time. It is used to store key/value pairs in a file in a manner similar to the Unix dbm library and provides interfaces to the traditional file format. GDBM is a library for manipulating hashed databases. Public static Semaphore Bouncer //constructor Hash library of database functions compatible with traditional dbm. If the database has no entries, the return value. (GDBM:GDBM-FIRSTKEY db &KEY (TYPE (GDBM:GDBM-DEFAULT-KEY-TYPE db))) Return the key of the first entry, as :TYPE. (GDBM:GDBM-EXISTS db key) Search data without retrieving it. A gdbm database thus provides the same functionality as a hash. It furthermore allows a non-sorted traversal of all key-value pairs. Gdbm allows the user to store, retrieve, and delete data by key. A database is a file that stores key-value pairs.
(GDBM:GDBM-DELETE db key) Delete key and its contents. GNU dbm is a library for simple databases. Here is a very pedagogic example in C# :-) using System The :TYPE argument specifies the return type. For example, to limit the number of simultaneous calls to a database in an application. This is not restricted to only one thread.
For all writers (GDBMWRITER, GDBMWRCREAT and GDBMNEWDB) the value GDBMFAST can be added to the flags field using logical or. This keeps two threads from concurrently accesing a resource, which can cause problems. If flags is set to GDBMNEWDB, the user want a new database created, regardless of whether one existed, and wants read and write access to the new database.
Gdbm database as a semaphor code#
It's simply a way to limit the number of consumers for a specific resource. A semaphore is a way to lock a resource so that it is guaranteed that while a piece of code is executed, only this piece of code has access to that resource.
Gdbm database as a semaphor full#
If the club is full no one is allowed to enter, but as soon as one person leaves another person might enter. There are a dedicated number of people that are allowed in the club at once. Think of semaphores as bouncers at a nightclub.
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