多线程访问全局变量(两个线程,一读一写)_两个线程访问同一个全局变量

使用两个线程访问全局字符串,一个线程不停反转字符串,另一个线程读字符串. 

问题: 在反转字符串的过程中,可能另一个线程访问字符串,造成读取错误

解决方案:

使用全局变量控制两个线程执行顺序

#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
 
#define PRINT_ERR(msg)  \
        do {                            \
                printf("%s:%s:%d\n", __FILE__, __func__, __LINE__); \
                perror(msg);    \
                exit(-1);               \
        } while (0)
 
 
char buf[128] = "1234567";
int ready_read = 0;
 
void reverse(char *buf, int len) {
    char *start = buf;
    char *end = buf + len - 1; 
    while (start < end) {
        char c = *start;
        *start = *end;
        *end = c;
        start ++;
        end --;
    }
}
 
void *func2(void *arg) {
    while (1) {
        if (ready_read == 0) {
            reverse(buf, strlen(buf));
            ready_read = 1;
        }
    }
}
 
void *func1(void *arg) {
    while (1) {
        if (ready_read == 1) {
            printf("%s\n", buf);
            ready_read = 0;
        }
    }
}
 
int main(int argc, const char *argv[]) {
    pthread_t tid1;
    pthread_t tid2;
    if (errno = pthread_create(&tid1, NULL, func1, NULL)) {
        PRINT_ERR("error");
    }
    if (errno = pthread_create(&tid2, NULL, func2, NULL)) {
        PRINT_ERR("error");
    }
    while (1) {
        sleep(1);
    }
    return 0;
}

执行结果:

unbuntu@unbuntu:~ $ ./a.out 
7654321
1234567
7654321
1234567
7654321
1234567
7654321
1234567
7654321
1234567
7654321
1234567
^C

加锁防止写过程中被读,或者读过程中被写

#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
 
#define PRINT_ERR(msg)  \
        do {                            \
                printf("%s:%s:%d\n", __FILE__, __func__, __LINE__); \
                perror(msg);    \
                exit(-1);               \
        } while (0)
 
pthread_mutex_t lock;
char buf[128] = "1234567";
 
void reverse(char *buf, int len) {
    char *start = buf;
    char *end = buf + len - 1; 
    while (start < end) {
        char c = *start;
        *start = *end;
        *end = c;
        start ++;
        end --;
    }
}
 
void *func2(void *arg) {
    while (1) {
        pthread_mutex_lock(&lock);
        reverse(buf, strlen(buf));
        pthread_mutex_unlock(&lock);
    }
}
 
void *func1(void *arg) {
    while (1) {
        pthread_mutex_lock(&lock);
        printf("%s\n", buf);
        pthread_mutex_unlock(&lock);
    }
}
 
int main(int argc, const char *argv[]) {
    pthread_mutex_init(&lock, NULL);
    pthread_t tid1;
    pthread_t tid2;
    if (errno = pthread_create(&tid1, NULL, func1, NULL)) {
        PRINT_ERR("error");
    }
    if (errno = pthread_create(&tid2, NULL, func2, NULL)) {
        PRINT_ERR("error");
    }
    while (1) {
        sleep(1);
    }
    return 0;
}

执行结果: (此时线程不是顺序执行, 但读到的数据是正常的)

unbuntu@unbuntu:~ $ ./a.out
7654321
7654321
7654321
7654321
7654321
1234567
1234567
1234567
1234567
1234567
1234567
1234567
1234567
1234567
1234567
7654321
7654321
7654321
7654321
^C