boost::mutex vs boost::recursive_mutex

boost::mutex is not re-entrant, a thread can only lock it once, otherwise it’s dead-locked. The following code snippet demonstrates it:

#include "boost/thread/mutex.hpp"

#include <iostream>

boost::mutex mtx;

void bar(){

    boost::mutex::scoped_lock lLock(mtx);//!! dead-locked here

    std::cout << "bar" << std::endl;

}

void foo() {

    boost::mutex::scoped_lock lLock(mtx);

    std::cout << "foo" << std::endl;

    bar();

}

int _tmain(int argc, _TCHAR* argv[]){

    foo();

    return 0;

}

If you need re-entrant mutex, the boost::recursive_mutex is the choice. The following code snippet demonstrates it:

#include "boost/thread/recursive_mutex.hpp"

#include <iostream>

boost::recursive_mutex r_mtx;

void bar(){

    boost::recursive_mutex::scoped_lock lLock(r_mtx);//no problem for a thread to lock more than once

    std::cout << "bar" << std::endl;

}

void foo() {

boost::recursive_mutex::scoped_lock lLock(r_mtx);

    std::cout << "foo" << std::endl;

    bar();

}

int _tmain(int argc, _TCHAR* argv[]){

    foo();

    return 0;

}

I also did a benchmark on my PC. For 1 locking operation, the result is approximately: boost::mutex: 0.043 micro second, boost::recursive_mutex: 0.068 micro second.

Re-entrant mutex is the default in Java and C#. Generally speaking, if a mutex is shared by many modules/classes, it’s recommended to use boost::recursive_mutex; while if it’s only used by a single module/class and no re-entrant feature needed, it’s recommended to use boost::mutex.

Mutex可以分为递归锁(recursive mutex)和非递归锁(non-recursive mutex)。可递归锁也可称为可重入锁(reentrant mutex)，
非递归锁又叫不可重入锁(non-reentrant mutex)。
  二者唯一的区别是，同一个线程可以多次获取同一个递归锁，不会产生死锁。而如果一个线程多次获取同一个非递归锁，则会产生死锁。