Trying to understand deadlocks in c

Question

I'm studying for my systems programming examination, but am having some trouble coming up with a simple deadlock example in c. I understand deadlocks occur whenever the waiting process is still holding on to another resource that the first needs, however I am having a difficult time understanding it in terms of c code. Are there any simple examples someone can provide?

Answer 1

Are there any simple examples someone can provide?

Here is a simple example: a piece of code might need to acquire two locks, A and B, eg so it can atomically modify two pieces of data, one protected by A and the other by B. For example:

pthread_mutex_t a, b;

pthread_mutex_lock(&a);
pthread_mutex_lock(&b);
// ... process data protected by A and B
pthread_mutex_unlock(&b);
pthread_mutex_unlock(&a);

At the same time, another piece of code can have the same requirement, but merely acquire the locks in the opposite order:

pthread_mutex_lock(&b);
pthread_mutex_lock(&a);
// ... process data protected by A and B
pthread_mutex_unlock(&a);
pthread_mutex_unlock(&b);

In isolation, either approach works just fine. But when the two pieces of code execute concurrently, the following can occur:

// neither lock is held

// thread 1                      // thread 2
pthread_mutex_lock(&a);          pthread_mutex_lock(&b);
// thread 1 now holds A          // thread 2 now holds B
// thread 1 is waiting for B     // thread 2 is waiting for A
pthread_mutex_lock(&b);          pthread_mutex_lock(&a);

The last line constitutes a deadlock : thread 1 is blocked waiting for lock B, which is held by thread 2. Thread 2 is blocked waiting for lock A, which is held by thread 1. As written, the threads are blocked indefinitely and neither can proceed.