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General purpose library for delegation locking
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qd_lock_lib is a locking library for multi-threaded programs that targets queue delegation (QD) locking. QD locking is an efficient delegation locking technique that can perform much better than traditional locks but it requires a different interface.

QD locking makes it possible to delegate critical sections to the current lock holder with the possibility to continue without waiting for its actual execution. The lock holding thread can also execute many critical sections in sequence while it has the data protected by the critical section in its fast private cache which makes it very efficent. More information about QD locking can be found here.

qd_lock_lib contains a C (C11) implementation of QD locking as well as a few other locks (TATAS, MCS, DR-MCS, CC-Synch) that can be used with the same generic API. The corresponding library for C++ can be found here.

Supported Platforms

The library is written with the intention of being as platform independent as possible. The library tries to follow the C11 standard so it should work on platforms that have compiler support for this standard. Unfortunately, C11 is not fully supported by all major compilers yet so some tweaking might be required to make it work on some platforms. So far, only clang and gcc has been tested under Linux and x86.


The paper Queue Delegation Locking is a good place to start if you have not heard about delegation locking before or if you want to learn more. This tutorial is a good place to start if you want to use qd_lock_lib. The full locking API together with documentation can be found in this file. Compilation instructions can be found below.

Compile and test

qd_lock_lib uses the Scons build system so you have to install that first. On Debian based systems, you can run sudo apt-get install scons.


qd_lock_lib requires a recent version of clang. It has been tested with a version from the clang svn repository based on clang 3.4. Type the following in the root directory of the repository to compile with clang:



qd_lock_lib has been tested with gcc 4.7.3. Type the following in the root directory of the repository to compile with gcc:

scons --use_gcc

Run tests

You first need to compile with Scons as explained above. When everything has compiled, you should be able to run the tests with the following commands:

./bin/test_lock QD_LOCK
./bin/test_lock MRQD_LOCK
./bin/test_lock TATAS_LOCK
./bin/test_lock MCS_LOCK
./bin/test_lock DRMCS_LOCK
./bin/test_lock CCSYNCH_LOCK

If this fails it might be because you are using an old version of clang. clang had a bug in its atomics API so it is not safe to use an old version of clang even if it compiles. If that is not the case, report the bug in the issue tracker.

How to use

This tutorial is a good place to start. The API is documented in the file src/c/locks/locks.h. You might also want to have a look at the examples:

src/c/examples/shared_int_example.c - shows how to use the functions LL_delegate, LL_delegate_wait, LL_lock, LL_unlock, LL_rlock and LL_runlock.

src/c/examples/concurrent_queue_example.c - shows how to use the functions LL_delegate_or_lock, LL_delegate_unlock and LL_close_delegate_buffer. It also shows how to implement a concurrent queue with QD locking.

src/c/examples/qd_lock_delegate_example.c - starts up multiple threads that issues delegated critical sections.

How to compile with your program

  1. Include the qd_lock_lib/src/c directory in your source path.
  2. Set the flag -std=gnu11 for both clang and gcc.
  3. Run scons or scons --use_gcc to produce the static and shared library files.
  4. Add the static or shared library when compiling the program as explained below.

The following command shows how you compile the shared int example program with clang and link with the static library. Replace clang with gcc to compile with gcc instead. Also note that the name of the static library differ between platforms.

clang -o int_example -std=gnu11 -pthread  -Isrc/c src/c/examples/shared_int_example.c bin/libqd_lock_lib.a

Test the compiled program with:


Use the following command to let clang compile the concurrent queue example with the shared library:

clang -o q_example -std=gnu11 -pthread -Isrc/c src/c/examples/concurrent_queue_example.c -L bin -l qd_lock_lib

To run the program with the shared library, you first have to add the shared library to your system's library path:




Open Source BSD License. See the LICENSE file for more information.

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