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 There is also a WaitAll method that can be used to block a thread until a set of WaitHandle objects or the resources they protect are available.Īn application can create an instance of the Mutex class using one of several constructors.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. A database can be accessed either by any number of readers or by exactly one writer at the same time. Whereas a reader has only read-access to the database, a writer has read- and write-access. We saw earlier in the chapter (refer to Table 13-1) how asynchronous calls use the WaitOne method to block a thread until the asynchronous operation is completed. A process can open a database as a reader or a writer. This abstract class defines "wait" methods that are used by a thread to gain ownership of a WaitHandle object, such as a mutex.
To understand the Mutex class, it is first necessary to have some familiarity with the WaitHandle class from which it is derived. Even if a deadlock does not occur, performance is likely to suffer. Deadlocks can result when a static method in class A calls static methods in class B, and vice versa. Monitor.Exit(typeof(WorkClass)) īe wary of using synchronization in static methods. To do so, pass the type of object as a command parameter rather than the object itself: Monitor.Enter(typeof(WorkClass)) // Synchronized code. Monitor and lock can also be used with static methods and properties.
Example of Class That Requires Synchronization using System using System.Threading using System.IO public class MyApp In this case, the counter is incorrectly incremented by 1. About half the time, the counter is incremented correctly by 2 other times, the first thread is preempted and the second thread gets in before the first finishes updating.
#Gdbm database as a semaphor code#
Executing this code multiple times produces inconsistent results, which is the pitfall of using code that is not thread-safe. The implementation of the pseudo-code is presented in Listing 13-8. Because server applications may have hundreds of active threads, there is clear need for a mechanism to control access to shared resources.
#Gdbm database as a semaphor update#
Execution path that requires synchronizationīecause the first thread is suspended before it updates the log file, both threads update the file with the same value. As an example, consider the pseudo-code in Figure 13-7 that describes how concurrent threads execute the same code segment.įigure 13-7. However, while suspended, another thread may have executed the same method and altered some global variables or database values that invalidate the results of the original thread. If it does not finish its task, its state is preserved and later restored when the thread resumes execution. Recall from the previous section that a thread executes in time slices. The potential corruption arises from the nature of thread scheduling. A class (or its members) is thread-safe when it can be accessed by multiple threads without having its state corrupted.
The specific objective of these techniques is to ensure thread safety. Thread synchronization refers to the techniques employed to share resources among concurrent threads in an efficient and orderly manner.
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