linux environment programming-thread synchronization [read-write lock]

1. Read-write lock

Similar to the mutexes introduced earlier, but read-write locks have higher parallelism.

 

2. The status of the read-write lock

1. Read lock [lock in read mode]

2. Write lock [lock in write mode]

3. No lock

 

3. Features of read-write lock

• 1. When the read-write lock is "locked in write mode", all threads that lock the lock will be blocked before unlocking.
• 2. When the read-write lock is "locked in read mode", if the thread locks it in read mode, it will succeed; if the thread locks in write mode, it will block.
• 3. When the read-write lock is "lock in read mode", there are threads that try to lock in write mode, and there are threads that try to lock in read mode. Read-write lock

Will block subsequent read mode lock requests. Satisfy the write mode lock first. Read locks and write locks block in parallel, and write locks with high priority read-write locks are also called shared-exclusive locks. When the read-write lock is locked in read mode, it is locked in shared mode, and when it is locked in write mode, it is locked in exclusive mode. The read-write lock is very suitable for the situation that the number of reads to the data structure is much greater than the writes.
 

Four. Main application functions

pthread_rwlock init, function
pthread_rwlock_destroy function
pthread_rwlock_rdlock function
pthread_rwlock_wrlock, function
pthread_rwlock_tryrdlock, function
pthread_rwlock_trywrlock function
pthread_rwlock_unlock
function The return values ​​of the above 7 functions are: success returns 0, failure directly returns an error number.
The pthread_rwlock_t type uses a sub to define a read-write lock variable.
pthread_rwlock_t rwlock;
 

Five. The principle of read-write lock

Six. Code Demo

#include <iostream>
#include <pthread.h>
#include <string.h>
#include <unistd.h>

using namespace std;

#define THREAD_NUM 8

pthread_rwlock_t rwlock;
int n=0;

void*
thread_func_read(void* arg)
{
	int *index = (int*)arg;
	int i=4;
	while(i--){
		pthread_rwlock_rdlock(&rwlock);
		printf("I am the %dth son thread ID:%lu \n", *index, pthread_self());
		printf("the value of n is %d \n", n);   // 对n读模式
		pthread_rwlock_unlock(&rwlock);
		usleep(100);
	}
	delete index;
}

void*
thread_func_write(void* arg)
{
	int *index = (int*)arg;
	int i=4;
	while(i--){
		pthread_rwlock_wrlock(&rwlock);
		printf("I am the %dth son thread ID:%lu \n", *index, pthread_self());
		n++;  // 写模式
		printf("the value of n is %d \n", n);
		pthread_rwlock_unlock(&rwlock);
		usleep(100);
	}
	delete index;
}

int
main(int argc, char*argv[])
{
	pthread_t tid[THREAD_NUM];
	pthread_attr_t attr;
	int err, i, *value;

	pthread_attr_init(&attr);
	pthread_attr_setdetachstate(&attr,  PTHREAD_CREATE_JOINABLE);

	pthread_rwlock_init(&rwlock, NULL);

	for(i=0; i<THREAD_NUM/2; i++){
		value = new int(i+1);
		if( (err = pthread_create(&tid[i], &attr, thread_func_read, (void*)value)) != 0){
			fprintf(stderr, "%dth thread create error the reason is %s \n", *value, strerror(err));
			delete value;
		}
	}

	for( ; i<THREAD_NUM; i++){
		value = new int(i+1);
		if( (err = pthread_create(&tid[i], &attr, thread_func_write, (void*)value)) != 0){
			fprintf(stderr, "%dth thread create error the reason is %s \n", *value, strerror(err));
			delete value;
		}
	}
	
	for(i=0; i<THREAD_NUM; i++){
		pthread_join(tid[i], NULL);
	}

	pthread_attr_destroy(&attr);
	pthread_rwlock_destroy(&rwlock);
	pthread_exit(NULL);
}