Simple Deadlock Examples

Question

I would like to explain threading deadlocks to newbies. I have seen many examples for deadlocks in the past, some using code and some using illustrations (like the famous 4

Answer 1

Here's a simple example in C++11.

#include     // mutex
#include  // cout 
#include    // getchar
#include    // this_thread, yield
#include    // async
#include    // seconds

using namespace std;
mutex _m1;
mutex _m2;

// Deadlock will occur because func12 and func21 acquires the two locks in reverse order

void func12()
{
    unique_lock l1(_m1);
    this_thread::yield(); // hint to reschedule
    this_thread::sleep_for( chrono::seconds(1) );
    unique_lock l2(_m2 );
}

void func21()
{
    unique_lock l2(_m2);
    this_thread::yield(); // hint to reschedule
    this_thread::sleep_for( chrono::seconds(1) );
    unique_lock l1(_m1);
}

int main( int argc, char* argv[] )
{
    async(func12);
    func21();
    cout << "All done!"; // this won't be executed because of deadlock
    getchar();
}