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Thread Locks
Description: Locking threads to protect shared dataKeywords: ecl threads mutex
Tutorial Level: INTERMEDIATE
Contents
Overview
The mutex is fairly standard, and on some systems can be more complex, but here the wrapper simply acts as a closed door with the usual lock, trylock and unlock features. The behaviour is also somewhat different depending on the platform.
On posix, the class is set up to run in two modes. When NDEBUG is not defined, it will do exception handling (via ecl's StandardException) for posix errors as well as configuring the mutexes for deadlock checking (see below for example code). Exception handling and deadlock checking are disabled when NDEBUG is defined.
1 Mutex mutex;
2
3 mutex.lock(); // Locks
4 // do work here
5 mutex.unlock();
6 mutex.trylock(); // Locks
7 mutex.trylock(); // Fails to lock and returns immediately.
8 mutex.unlock();
9
10
11 mutex.lock();
12 // mutex.lock(); // The DEADLOCK! Like this, the program will usually halt forever.
13
14 // If NDEBUG is not defined, then on posix systems, you can catch deadlocks like this.
15 try {
16 mutex.lock();
17 } catch ( StandardException &e ) {
18 std::cout << e.what() << std::endl;
19 }
20
21 // Typical output from a caught deadlock:
22 // Location : /mnt/froody/work/code/cpp/projects/ecl/modules/core/src/lib/threads/mutex_pos.cpp:57
23 // Flag : The object was used incorrectly.
24 // Detail : DEADLOCK! The mutex has already been locked by this thread, it now has to wait on itself.
25
