What is the difference between condition variables and semaphores in linux

Difference: 1. Using condition variables can wake up all waiters at once, but semaphores cannot wake up; 2. Semaphores always have a value (status), but condition variables have no value, and there is no place to record the transmission. The number of signals, and there is no place to record the number of times returned by wait; 3. The purpose of the semaphore is to synchronize between processes, and the purpose of the condition variable is to synchronize between threads.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

Condition variable

Condition variable (cond) is used in multi-threaded programs to implement "wait--->wake up" logic A commonly used method is a mechanism for synchronization between processes. Condition variables are used to block a thread until the condition is met and is triggered. Usually condition variables and mutexes are used at the same time.

Generally, condition variables have two states:

(1) One/multiple threads are suspended waiting for "the condition of the condition variable is established";

(2) Another thread notifies other threads "when the condition variable condition is true."

Use of condition variables:

#include <pthread.h>

struct msg {
    struct msg *m_next;
    /* ... more stuff here ... */
};
struct msg *workq;
pthread_cond_t qready = PTHREAD_COND_INITIALIZER;
pthread_mutex_t qlock = PTHREAD_MUTEX_INITIALIZER;

void
process_msg(void)
{
    struct msg *mp;

    for (;;) {
        pthread_mutex_lock(&qlock);
        while (workq == NULL)
            pthread_cond_wait(&qready, &qlock);
        mp = workq;
        workq = mp->m_next;
        pthread_mutex_unlock(&qlock);
        /* now process the message mp */
    }
}

void
enqueue_msg(struct msg *mp)
{
    pthread_mutex_lock(&qlock);
    mp->m_next = workq;
    workq = mp;
    pthread_mutex_unlock(&qlock);
    pthread_cond_signal(&qready);
}

Of course, you can also use the following code when triggering condition variables. Both methods have their own advantages and disadvantages

void
enqueue_msg(struct msg *mp)
{
    pthread_mutex_lock(&qlock);
    mp->m_next = workq;
    workq = mp;
    pthread_cond_signal(&qready);
    pthread_mutex_unlock(&qlock);
}

Semaphore

The semaphore is a special variable whose access is atomic.

Only two operations are allowed on it:

(1) Wait for the semaphore

When the semaphore value is 0, the program waits; when the semaphore value is greater than 0 When , the semaphore is decremented by 1 and the program continues to run.

(2) Send the semaphore

Add 1 to the semaphore value.

Note: Linux provides a set of semaphore APIs, which are declared in the header file sys/sem.h.

The difference between linux condition variables and semaphores:

(1) Using condition variables can wake up all waiters at once, and this semaphore The lack of functions and feeling is the biggest difference.

(2) The semaphore always has a value (status), but the condition variable does not. There is no place to record how many times it has been woken up (sending a signal), and there is no place to record the number of times it has been woken up (wait returns). how many times. From an implementation perspective, a semaphore can be implemented using a mutex counter condition variable. Because the semaphore has a state, if you want precise synchronization, the semaphore may have special features. Semaphores can solve the problem of lost wakeup in condition variables.

(3) The purpose of semaphores is to synchronize between processes, and the purpose of mutexes and condition variables is to synchronize between threads, but semaphores can also be used between threads, and mutexes and condition variables can also be used in processes between. Decisions should be made based on actual circumstances. The most useful scenario for semaphores is to indicate the amount of available resources.

Classic sentence:

A mutex is a special case of a semaphore. The essence of a mutex is a lock.

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