pthread_cond_wait() & pthread_cond_signal() 调用顺序是什么？

Question

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

pthread_mutex_t lockid;
pthread_cond_t cvar = PTHREAD_COND_INITIALIZER;
char name[14];

void *entry()
{
    pthread_mutex_lock(&lockid);

    printf("Enter Name\n");
    scanf("%s",name);

    pthread_cond_signal(&cvar);
    pthread_mutex_unlock(&lockid);
    pthread_exit(NULL);

}

void *display()
{
    pthread_mutex_lock(&lockid);
    pthread_cond_wait(&cvar,&lockid);

    printf("Name: %s\n",name);

    pthread_mutex_unlock(&lockid);
    pthread_exit(NULL);
}

int main()
{
    pthread_t id1,id2;
    if(pthread_mutex_init(&lockid,NULL)== 0){
        pthread_create(&id1,NULL,&entry,NULL);
        pthread_create(&id2,NULL,&display,NULL);

        pthread_join(id1,NULL);
        pthread_join(id2,NULL);
    }
    return 0;
}

我希望线程 id2 打印我在线程 id1 上输入的名称。我知道 pthread_cond_wait() 不允许线程 id2 显示在线程 id1 上输入的名称，但是应该在 wait() 之前调用 signal() 对吗？ wait()和signal()方法的调用方式是否正确？我是 linux 编程的新手，请帮助我。

Answer 1

如果您有顺序任务，请不要使用线程：

#include <stdio.h>

char name[14];

int main() {
    printf("Enter Name\n");
    scanf("%s",name);
    printf("Name: %s\n", name);
    return 0
}

您可以使用 state 变量来确保 entry() 运行在 display():

之前

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

pthread_mutex_t lockid;
pthread_cond_t cvar = PTHREAD_COND_INITIALIZER;
char name[14];
enum { START, ENTRY } state = START;
#define CHECK(e) { int error = (e); if(error) printf("%s:%d %s", __FILE__, __LINE__, strerror(error)); }
#define CHECK2(p, e) { if(p) CHECK(e); }


void *entry() {
    CHECK(pthread_mutex_lock(&lockid));
    printf("Enter Name\n");
    CHECK2(scanf("%s", name) == EOF, errno);
    state = ENTRY;
    pthread_cond_signal(&cvar);
    CHECK(pthread_mutex_unlock(&lockid));
    return 0;
}

void *display() {
    CHECK(pthread_mutex_lock(&lockid));
    while(state != ENTRY) pthread_cond_wait(&cvar, &lockid);
    printf("Name: %s\n", name);
    CHECK(pthread_mutex_unlock(&lockid));
    return 0;
}

int main() {
    pthread_mutex_init(&lockid, NULL);
    pthread_t id1, id2;
    CHECK(pthread_create(&id1, NULL, &entry, NULL));
    CHECK(pthread_create(&id2, NULL, &display, NULL));
    CHECK(pthread_join(id1,NULL));
    CHECK(pthread_join(id2,NULL));
    return 0;
}

pthread_cond_wait() & pthread_cond_signal() 调用顺序是什么？

What is the pthread_cond_wait() & pthread_cond_signal() calling sequence?

c

linux

multithreading

mutex

ipc