是否可以在多个条件下使用 pthread 条件变量（pthread_mutex 和 pthread_cond_t）？

Question

I have a multithreaded program and eacch thread focuses on its own work. When a thread finishes its work, it will drive the event to the main thread.

So I want to use the pthread condition variable to implement the idea (ie pthread_cond_t and pthread_mutex_t). The data structure will be like:

typedef struct peer_info {
    int ip; 
    int port;
    pthread_mutex_t peer_mutex;
    pthread_cond_t peer_cond;
    bool write_v;
    bool read_v;
    bool create_v;
    bool destroy_v;
} peer_info;

Suppose that:
thread1 changes the write_v and signals main thread,
thread2 changes the read_v and signals the main thread,
thread3 changes the create_v and signals the main thread,
thread4 changes the destroy_v and signals the main thread.

Is it possible to use only one pthread_mutex and pthread_cond_t to implement the above scenario? Will it cause a deadlock?

Answer 1

Yes, you can safely wait on a "compound predicate," where one signal means "this state changed or that state changed or some other state changed or ..."

In your case, those states are the event_v flags you set. Presuming your main thread is in a loop, and that the event_v flags are initialized to false , it'd look something like this:

// main loop:
pthread_mutex_lock(&pi.peer_mutex);
while (! (pi.write_v || pi.read_v || ... )) {
  pthread_cond_wait(&pi.peer_cond, &pi.peer_mutex);
}
if (pi.write_v) {
  pi.write_v = false; // clear this so we're not confused next time we wake up
  handle_write_event(&pi);
}
if (pi.read_v) {
  pi.read_v = false;
  handle_read_event(&pi);
}
if (...) ...          // handle other events

Note that this will handle events while the mutex is locked, which may or may not be OK for you.

As an aside, I find all those boolean flags a bit cumbersome, and might use bit flags instead:

#define PEER_EVENT_READ  1 << 0
#define PEER_EVENT_WRITE 1 << 1
#define PEER_EVENT_ ...

struct peer_info {
  short events_pending; // initialized to zero
  ...
};

So that I could concisely write:

while (! pi.events_pending) {
  pthread_cond_wait(&pi.peer_cond, &pi.peer_mutex);
}
if (pi.events_pending & PEER_EVENT_READ) { ... }
if ....
pi.events_pending = 0; // clear them all

But it's semantically the same thing, so up to you.