[Linux operating system] Read-write locks in Linux system programming

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1. The principle of read-write lock

Read-Write Lock (Read-Write Lock) is a special lock mechanism for reading and writing shared resources in a multi-threaded environment. Unlike mutexes, read-write locks allow multiple threads to read a shared resource at the same time, but only allow one thread to write. Read-write locks can improve concurrency performance and are suitable for scenarios where read operations are more frequent than write operations.

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2. Provided operation function

In Linux system programming, the related functions of the read-write lock are also provided through the pthread library. Commonly used read-write lock related functions are:

1. pthread_rwlock_initThe role of the function is to initialize the read-write lock.
   • prototype:int pthread_rwlock_init(pthread_rwlock_t *rwlock, const pthread_rwlockattr_t *attr);
   • parameter:
     • rwlock: Pointer to the read-write lock variable to be initialized.
     • attr: Pointer to the read-write lock attribute, usually set to NULL.
   • Return value: Returns 0 on success, returns an error code on failure.

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2. pthread_rwlock_destroyThe function of the function is to destroy the read-write lock.
   • prototype:int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
   • parameter:
     • rwlock: Pointer to the read-write lock variable to be destroyed.
   • Return value: Returns 0 on success, returns an error code on failure.

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3. pthread_rwlock_rdlockThe role of the function is to acquire a read lock.
   • prototype:int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
   • parameter:
     • rwlock: Pointer to the read-write lock variable to acquire the read lock.
   • Return value: Returns 0 on success, returns an error code on failure.

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4. pthread_rwlock_wrlockThe role of the function is to acquire a write lock.
   • prototype:int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
   • parameter:
     • rwlock: Pointer to the read-write lock variable to acquire the write lock.
   • Return value: Returns 0 on success, returns an error code on failure.

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5. pthread_rwlock_unlockThe role of the function is to release the read lock or write lock.
   • prototype:int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
   • parameter:
     • rwlock: Pointer to the read-write lock variable to be released.
   • Return value: Returns 0 on success, returns an error code on failure.

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sample code

#include <stdio.h>
#include <pthread.h>

pthread_rwlock_t rwlock;
int shared_data = 0;

void* reader_func(void* arg) {
    
    
    pthread_rwlock_rdlock(&rwlock);
    printf("Reader %ld read shared data: %d\n", (long)arg, shared_data);
    pthread_rwlock_unlock(&rwlock);
    return NULL;
}

void* writer_func(void* arg) {
    
    
    pthread_rwlock_wrlock(&rwlock);
    shared_data += 1;
    printf("Writer %ld wrote shared data: %d\n", (long)arg, shared_data);
    pthread_rwlock_unlock(&rwlock);
    return NULL;
}

int main() {
    
    
    pthread_rwlock_init(&rwlock, NULL);

    pthread_t reader_thread1, reader_thread2, writer_thread;
    pthread_create(&reader_thread1, NULL, reader_func, (void*)1);
    pthread_create(&reader_thread2, NULL, reader_func, (void*)2);
    pthread_create(&writer_thread, NULL, writer_func, (void*)1);

    pthread_join(reader_thread1, NULL);
    pthread_join(reader_thread2, NULL);
    pthread_join(writer_thread, NULL);

    pthread_rwlock_destroy(&rwlock);

    return 0;
}

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4. Example explanation

In the above code, we use pthread_rwlock_initthe function to initialize a read-write lock, and then create two read threads and one write thread. The reading thread pthread_rwlock_rdlockobtains the read lock through the function, and the writing thread obtains the write lock through pthread_rwlock_wrlockthe function, and then performs corresponding read and write operations. Finally, pthread_rwlock_unlockthe lock is released through the function, and pthread_rwlock_destroythe read-write lock is destroyed through the function.

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