Pthreads 多线程入门教程

迈不过友情╰ 2022-06-08 04:45 261阅读 0赞

Pthreads 是 IEEE（电子和电气工程师协会）委员会开发的一组线程接口，负责指定便携式操作系统接口（POSIX）. Pthreads 中的 P 表示 POSIX，实际上，Pthreads 有时候也代表 POSIX 线程.

## 前言 ##

--------------------

本文档参考: [https://computing.llnl.gov/tutorials/pthreads/][https_computing.llnl.gov_tutorials_pthreads]

POSIX 委员会定义了一系列基本功能和数据结构，希望能够被大量厂商采用，以便线程代码能够轻松地在各种操作系统上移植. 委员会的梦想由 UNIX 厂商实现了，他们都广泛 Pthreads . 最著名的例外就是 Sun，它继续采用 Solaris 线程作为其主要线程 API .

Pthreads 指定 API 来处理线程要求的大部分行为. 这些行为包括创建和终止线程、等待线程完成、以及管理线程之间的交互. 后面的目录中存在各种锁定机制，能够阻止两个线程同时尝试修改相同的数据值，这些锁定机制包括互斥锁、条件变量.

在 Linux 环境下，可以在 Shell 中通过 man 查询到 Pthreads 的部分函数命令,如： man pthread\_create

## 线程的创建和管理 ##

--------------------

每个线程都有一个在进程中唯一的线程标识符，用一个数据类型 pthread\_t 表示，该数据类型在 Linux 中就是一个无符号长整型数据.

### 创建新的线程 ###

--------------------

int pthread_create (pthread_t *thread,pthread_attr_t *attr,void *(*start_routine)(void *),void *arg)

若创建成功，返回0；若出错，则返回错误编号. thread 是线程标识符，但这个参数不是由用户指定的，而是由 pthread\_create 函数在创建时将新的线程的标识符放到这个变量中. attr 指定线程的属性，可以用 NULL 表示默认属性. start\_routine 指定线程开始运行的函数，arg 是 start\_routine 所需要的参数，是一个无类型指针.

默认地，线程在被创建时要被赋予一定的属性，这个属性存放在数据类型 pthread\_attr\_t 中，它包含了线程的调度策略，堆栈的相关信息，join or detach 的状态等.

pthread\_attr\_init 和 pthread\_attr\_destroy 函数分别用来创建和销毁 pthread\_attr\_t，具体函数声明可参考man帮助.

### 结束线程 ###

--------------------

当发生以下情形之一时，线程就会结束：

*  线程运行的函数return了，也就是线程的任务已经完成；
 *  线程调用了 pthread\_exit 函数；
 *  其他线程调用 pthread\_cancel 结束这个线程；
 *  进程调用 exec() or exit()，结束了；
 *  main() 函数先结束了，而且 main() 自己没有调用 pthread\_exit 来等所有线程完成任务.

当然，一个线程结束，并不意味着它的所有信息都已经消失，后面会看到僵尸线程的问题.

下面介绍这两个函数.

void pthread_exit (void *retval)

retval 是由用户指定的参数， pthread\_exit 完成之后可以通过这个参数获得线程的退出状态.

int pthread_cancel (pthread_t thread)

一个线程可以通过调用 pthread\_cancel 函数来请求取消同一进程中的线程，这个线程由thread 参数指定. 如果操作成功则返回0，失败则返回对应的错误编号.

看一个例子：

/******************************************************************************
     * FILE: hello.c
     * DESCRIPTION:
     * A "hello world" Pthreads program.  Demonstrates thread creation and termination.
     * AUTHOR: Blaise Barney
     * LAST REVISED: 08/09/11
     ******************************************************************************/
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #define NUM_THREADS 5
    
    void *PrintHello(void *threadid) {
     long tid;
     tid = (long) threadid;
     printf("Hello World! It's me, thread #%ld!\n", tid);
     pthread_exit(NULL);
    }
    
    int main(int argc, char *argv[]) {
     pthread_t threads[NUM_THREADS];
     int rc;
     long t;
     for (t = 0; t < NUM_THREADS; t++) { 
     printf("In main: creating thread %ld\n", t);
     rc = pthread_create(&threads[t], NULL, PrintHello, (void *) t);
     if (rc) { 
     printf("ERROR; return code from pthread_create() is %d\n", rc);
     exit(-1);
     }
     }
     /* Last thing that main() should do */
     pthread_exit(NULL);
    }

在 Shell 中输入以下命令编译执行:

gcc -Wall hello.c -lpthread -o hello
    ./hello

输出结果：

In main: creating thread 0
    In main: creating thread 1
    In main: creating thread 2
    Hello World! It's me, thread #0! Hello World! It's me, thread #1!
    In main: creating thread 3
    In main: creating thread 4
    Hello World! It's me, thread #2! Hello World! It's me, thread #3!
    Hello World! It's me, thread #4!

注意输出的顺序可能不同， 要特别注意的是， main() 显示地调用了 pthread\_exit() 来等待其他线程的结束.

下面这个例子传给线程一些初始化用的参数.

/****************************************************************************** * FILE: hello_arg2.c * DESCRIPTION: * A "hello world" Pthreads program which demonstrates another safe way * to pass arguments to threads during thread creation. In this case, * a structure is used to pass multiple arguments. * AUTHOR: Blaise Barney * LAST REVISED: 01/29/09 ******************************************************************************/
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #define NUM_THREADS 8
    
    char *messages[NUM_THREADS];
    
    struct thread_data {
        int thread_id;
        int sum;
        char *message;
    };
    
    struct thread_data thread_data_array[NUM_THREADS];
    
    void *PrintHello(void *threadarg) {
        int taskid, sum;
        char *hello_msg;
        struct thread_data *my_data;
    
        sleep(1);
        my_data = (struct thread_data *) threadarg;
        taskid = my_data->thread_id;
        sum = my_data->sum;
        hello_msg = my_data->message;
        printf("Thread %d: %s Sum=%d\n", taskid, hello_msg, sum);
        pthread_exit(NULL);
    }
    
    int main(int argc, char *argv[]) {
        pthread_t threads[NUM_THREADS];
        int *taskids[NUM_THREADS];
        int rc, t, sum;
    
        sum = 0;
        messages[0] = "English: Hello World!";
        messages[1] = "French: Bonjour, le monde!";
        messages[2] = "Spanish: Hola al mundo";
        messages[3] = "Klingon: Nuq neH!";
        messages[4] = "German: Guten Tag, Welt!";
        messages[5] = "Russian: Zdravstvytye, mir!";
        messages[6] = "Japan: Sekai e konnichiwa!";
        messages[7] = "Latin: Orbis, te saluto!";
    
        for (t = 0; t < NUM_THREADS; t++) {
            sum = sum + t;
            thread_data_array[t].thread_id = t;
            thread_data_array[t].sum = sum;
            thread_data_array[t].message = messages[t];
            printf("Creating thread %d\n", t);
            rc = pthread_create(&threads[t], NULL, PrintHello,
                    (void *) &thread_data_array[t]);
            if (rc) {
                printf("ERROR; return code from pthread_create() is %d\n", rc);
                exit(-1);
            }
        }
        pthread_exit(NULL);
    }

输出结果：

Creating thread 0
    Creating thread 1
    Creating thread 2
    Creating thread 3
    Creating thread 4
    Creating thread 5
    Creating thread 6
    Creating thread 7
    Thread 5: Russian: Zdravstvytye, mir!  Sum=15
    Thread 0: English: Hello World!  Sum=0
    Thread 6: Japan: Sekai e konnichiwa!  Sum=21
    Thread 3: Klingon: Nuq neH!  Sum=6
    Thread 4: German: Guten Tag, Welt!  Sum=10
    Thread 1: French: Bonjour, le monde!  Sum=1
    Thread 7: Latin: Orbis, te saluto!  Sum=28
    Thread 2: Spanish: Hola al mundo  Sum=3

### 对线程的阻塞（Joining and Detaching Threads） ###

--------------------

阻塞是线程之间同步的一种方法.

int pthread_join(pthread_t threadid, void **value_ptr)

pthread\_join 函数会让调用它的线程等待 threadid 线程运行结束之后再运行. value\_ptr 存放了其他线程的返回值. 一个可以被join的线程，仅仅可以被别的一个线程 join，如果同时有多个线程尝试 join 同一个线程时，最终结果是未知的. 另外，线程不能 join 自己.

上面提到过，创建一个线程时，要赋予它一定的属性，这其中就包括joinable or detachable 的属性，只有被声明成joinable的线程，可以被其他线程join. POSIX标准的最终版本指出线程应该被设置成joinable的. 显式地设置一个线程为joinable,需要以下四个步骤：

*  Declare a pthread attribute variable of the pthread\_attr\_t data type
 *  Initialize the attribute variable with pthread\_attr\_init()
 *  Set the attribute detached status with pthread\_attr\_setdetachstate()
 *  When done, free library resources used by the attribute with pthread\_attr\_destroy()

值得注意的的是：僵尸线程 ( “zombie” thread) 是一种已经退出了的 joinable 的线程，但是等待其他线程调用 pthread\_join 来 join 它，以收集它的退出信息(exit status). 如果没有其他线程调用 pthread\_join 来 join 它的话，它占用的一些系统资源不会被释放，比如堆栈. 如果main()函数需要长时间运行，并且创建大量 joinable 的线程，就有可能出现堆栈不足的 error . 对于那些不需要 join 的线程，最好利用 pthread\_detach，这样它运行结束后，资源就会及时得到释放. 注意一个线程被使用 pthread\_detach 之后，它就不能再被改成 joinable 的了.

总而言之，创建的每一个线程都应该使用 pthread\_join 或者 pthread\_detach 其中一个，以防止僵尸线程的出现.

相关函数：

pthread_detach (threadid) pthread_attr_setdetachstate (attr,detachstate) pthread_attr_getdetachstate (attr,detachstate)

下面看一个例子，可以具体体会显式地设置一个线程为 joinable 的过程.

#include<stdlib.h>
    #include <pthread.h>
    #include <stdio.h>
    #include <math.h>
    #define NUM_THREADS 4
    void *BusyWork(void *t) {
        int i;
        long tid;
        double result = 0.0;
        tid = (long) t;
        printf("Thread %ld starting...\n", tid);
        for (i = 0; i < 1000000; i++) {
            result = result + sin(i) * tan(i);
        }
        printf("Thread %ld done. Result = %e\n", tid, result);
        pthread_exit((void*) t);
    }
    
    int main(int argc, char *argv[]) {
        pthread_t thread[NUM_THREADS];
        pthread_attr_t attr;
        int rc;
        long t;
        void *status;
        /* Initialize and set thread detached attribute */
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
        for (t = 0; t < NUM_THREADS; t++) {
            printf("Main: creating thread %ld\n", t);
            rc = pthread_create(&thread[t], &attr, BusyWork, (void *) t);
            if (rc) {
                printf("ERROR; return code from pthread_create() is %d\n", rc);
                exit(-1);
            }
        }
    
        /* Free attribute and wait for the other threads */
        pthread_attr_destroy(&attr);
    
        for (t = 0; t < NUM_THREADS; t++) {
            rc = pthread_join(thread[t], &status);
            if (rc) {
                printf("ERROR; return code from pthread_join() is %d\n", rc);
                exit(-1);
            }
            printf("Main: completed join with thread %ld having a status of %ld\n",
                    t, (long) status);
        }
        printf("Main: program completed. Exiting.\n");
    
        pthread_exit(NULL);
    }

输出结果：

Main: creating thread 0
    Main: creating thread 1
    Thread 0 starting...
    Main: creating thread 2
    Thread 1 starting...
    Main: creating thread 3
    Thread 2 starting...
    Thread 3 starting...
    Thread 1 done. Result = -3.153838e+06
    Thread 2 done. Result = -3.153838e+06
    Thread 0 done. Result = -3.153838e+06
    Main: completed join with thread 0 having a status of 0
    Main: completed join with thread 1 having a status of 1
    Main: completed join with thread 2 having a status of 2
    Thread 3 done. Result = -3.153838e+06
    Main: completed join with thread 3 having a status of 3
    Main: program completed. Exiting.

### 堆栈管理 （Stack Management） ###

--------------------

pthread_attr_getstacksize (attr, stacksize) pthread_attr_setstacksize (attr, stacksize) pthread_attr_getstackaddr (attr, stackaddr) pthread_attr_setstackaddr (attr, stackaddr)

POSIX标准没有规定一个线程的堆栈大小. 安全可移植的程序不会依赖于具体实现默认的堆栈限制，而是显式地调用 pthread\_attr\_setstacksize 来分配足够的堆栈空间.

下面是关于堆栈大小的一个例子.

#include<stdlib.h>
    #include <pthread.h>
    #include <stdio.h>
    #define NTHREADS 4
    #define N 1000
    #define MEGEXTRA 1000000
    
    pthread_attr_t attr;
    
    void *dowork(void *threadid) {
        double A[N][N];
        int i, j;
        long tid;
        size_t mystacksize;
    
        tid = (long) threadid;
        pthread_attr_getstacksize(&attr, &mystacksize);
        printf("Thread %ld: stack size = %li bytes \n", tid, mystacksize);
        for (i = 0; i < N; i++)
            for (j = 0; j < N; j++)
                A[i][j] = ((i * j) / 3.452) + (N - i);
        pthread_exit(NULL);
    }
    
    int main(int argc, char *argv[]) {
        pthread_t threads[NTHREADS];
        size_t stacksize;
        int rc;
        long t;
    
        pthread_attr_init(&attr);
        pthread_attr_getstacksize(&attr, &stacksize);
        printf("Default stack size = %li\n", stacksize);
        stacksize = sizeof(double) * N * N + MEGEXTRA;
        printf("Amount of stack needed per thread = %li\n", stacksize);
        pthread_attr_setstacksize(&attr, stacksize);
        printf("Creating threads with stack size = %li bytes\n", stacksize);
        for (t = 0; t < NTHREADS; t++) {
            rc = pthread_create(&threads[t], &attr, dowork, (void *) t);
            if (rc) {
                printf("ERROR; return code from pthread_create() is %d\n", rc);
                exit(-1);
            }
        }
        printf("Created %ld threads.\n", t);
        pthread_exit(NULL);
    }

输出结果：

Default stack size = 8388608
    Amount of stack needed per thread = 9000000
    Creating threads with stack size = 9000000 bytes
    Thread 0: stack size = 9000000 bytes 
    Created 4 threads.
    Thread 1: stack size = 9000000 bytes 
    Thread 2: stack size = 9000000 bytes 
    Thread 3: stack size = 9000000 bytes

### 其他重要函数 ###

--------------------

pthread_self () pthread_equal (thread1,thread2)

调用pthread\_self 可以返回它的 thread ID. pthread\_equal 比较两个线程的 ID,如果不同则返回0,否则返回一个非零值.

## 互斥锁 Mutex ##

--------------------

Mutex 常常被用来保护那些可以被多个线程访问的共享资源，比如可以防止多个线程同时更新同一个数据时出现混乱.

使用互斥锁的一般步骤是：

*  创建一个互斥锁，即声明一个pthread\_mutex\_t类型的数据,然后初始化，只有初始化之后才能使用；
 *  多个线程尝试锁定这个互斥锁；
 *  只有一个成功锁定互斥锁，成为互斥锁的拥有者，然后进行一些指令；
 *  拥有者解锁互斥锁；
 *  其他线程尝试锁定这个互斥锁，重复上面的过程；
 *  最后互斥锁被显式地调用 pthread\_mutex\_destroy 来进行销毁.

有两种方式初始化一个互斥锁：第一种，利用已经定义的常量初始化，例如

pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

第二种方式是调用 pthread\_mutex\_init (mutex,attr) 进行初始化.

当多个线程同时去锁定同一个互斥锁时，失败的那些线程，如果是用 pthread\_mutex\_lock 函数，那么会被阻塞，直到这个互斥锁被解锁，它们再继续竞争；如果是用 pthread\_mutex\_trylock 函数，那么失败者只会返回一个错误.

最后需要指出的是，保护共享数据是程序员的责任. 程序员要负责所有可以访问该数据的线程都使用mutex这种机制，否则，不使用 mutex 的线程还是有可能对数据造成破坏.

相关函数 (具体声明可以用 man 查看 )

pthread_mutex_init (mutex,attr) pthread_mutex_destroy (pthread_mutex_t *mutex) pthread_mutexattr_init (attr) pthread_mutexattr_destroy (attr) phtread_mutex_lock(pthread_mutex_t *mutex) phtread_mutex_trylock(pthread_mutex_t *mutex) phtread_mutex_unlock(pthread_mutex_t *mutex)

下面是一个利用多线程进行向量点乘的程序，其中需要对 dotstr.sum 这个共同读写的数据进行保护.

/*****************************************************************************
     * FILE: dotprod_mutex.c
     * DESCRIPTION:
     *   This example program illustrates the use of mutex variables
     *   in a threads program. The main data is made available to all threads through
     *   a globally accessible  structure. Each thread works on a different
     *   part of the data. The main thread waits for all the threads to complete
     *   their computations, and then it prints the resulting sum.
     * SOURCE: Vijay Sonnad, IBM
     * LAST REVISED: 01/29/09 Blaise Barney
     ******************************************************************************/
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    
    /*
     The following structure contains the necessary information
     to allow the function "dotprod" to access its input data and
     place its output into the structure.  This structure is
     unchanged from the sequential version.
     */
    
    typedef struct {
        double *a;
        double *b;
        double sum;
        int veclen;
    } DOTDATA;
    
    /* Define globally accessible variables and a mutex */
    
    #define NUMTHRDS 4
    #define VECLEN 100000
    DOTDATA dotstr;
    pthread_t callThd[NUMTHRDS];
    pthread_mutex_t mutexsum;
    
    /*
     The function dotprod is activated when the thread is created.
     As before, all input to this routine is obtained from a structure
     of type DOTDATA and all output from this function is written into
     this structure. The benefit of this approach is apparent for the
     multi-threaded program: when a thread is created we pass a single
     argument to the activated function - typically this argument
     is a thread number. All  the other information required by the
     function is accessed from the globally accessible structure.
     */
    
    void *dotprod(void *arg) {
    
        /* Define and use local variables for convenience */
    
        int i, start, end, len;
        long offset;
        double mysum, *x, *y;
        offset = (long) arg;
    
        len = dotstr.veclen;
        start = offset * len;
        end = start + len;
        x = dotstr.a;
        y = dotstr.b;
    
        /*
         Perform the dot product and assign result
         to the appropriate variable in the structure.
         */
        mysum = 0;
        for (i = start; i < end; i++) {
            mysum += (x[i] * y[i]);
        }
    
        /*
         Lock a mutex prior to updating the value in the shared
         structure, and unlock it upon updating.
         */
        pthread_mutex_lock(&mutexsum);
        dotstr.sum += mysum;
        printf("Thread %ld did %d to %d: mysum=%f global sum=%f\n", offset, start,
                end, mysum, dotstr.sum);
        pthread_mutex_unlock(&mutexsum);
        pthread_exit((void*) 0);
    }
    
    /*
     The main program creates threads which do all the work and then print out result
     upon completion. Before creating the threads, The input data is created. Since
     all threads update a shared structure, we need a mutex for mutual exclusion.
     The main thread needs to wait for all threads to complete, it waits for each one
     of the threads. We specify a thread attribute value that allow the main thread to
     join with the threads it creates. Note also that we free up handles  when they
     are no longer needed.
     */
    
    int main(int argc, char *argv[]) {
        long i;
        double *a, *b;
        void *status;
        pthread_attr_t attr;
    
        /* Assign storage and initialize values */
    
        a = (double*) malloc(NUMTHRDS * VECLEN * sizeof(double));
        b = (double*) malloc(NUMTHRDS * VECLEN * sizeof(double));
    
        for (i = 0; i < VECLEN * NUMTHRDS; i++) {
            a[i] = 1;
            b[i] = a[i];
        }
    
        dotstr.veclen = VECLEN;
        dotstr.a = a;
        dotstr.b = b;
        dotstr.sum = 0;
    
        pthread_mutex_init(&mutexsum, NULL);
    
        /* Create threads to perform the dotproduct  */
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    
        for (i = 0; i < NUMTHRDS; i++) {
            /* Each thread works on a different set of data.
             * The offset is specified by 'i'. The size of
             * the data for each thread is indicated by VECLEN.
             */
            pthread_create(&callThd[i], &attr, dotprod, (void *) i);
        }
    
        pthread_attr_destroy(&attr);
        /* Wait on the other threads */
    
        for (i = 0; i < NUMTHRDS; i++) {
            pthread_join(callThd[i], &status);
        }
        /* After joining, print out the results and cleanup */
    
        printf("Sum = %f \n", dotstr.sum);
        free(a);
        free(b);
        pthread_mutex_destroy(&mutexsum);
        pthread_exit(NULL);
    }

输出结果：

Thread 3 did 300000 to 400000: mysum=100000.000000 global sum=100000.000000
    Thread 0 did 0 to 100000: mysum=100000.000000 global sum=200000.000000
    Thread 2 did 200000 to 300000: mysum=100000.000000 global sum=300000.000000
    Thread 1 did 100000 to 200000: mysum=100000.000000 global sum=400000.000000
    Sum =  400000.000000

## 条件变量 Condition Variable ##

--------------------

互斥锁只有两种状态，这限制了它的用途. 条件变量允许线程在阻塞的时候等待另一个线程发送的信号，当收到信号后，阻塞的线程就被唤醒并试图锁定与之相关的互斥锁. 条件变量要和互斥锁结合使用.

### 条件变量的声明和初始化 ###

--------------------

通过声明 pthread\_cond\_t 类型的数据,并且必须先初始化才能使用.

初始化的方法也有两种：

第一种，利用内部定义的常量，例如：

pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;

第二种，利用函数 pthread\_cond\_init(cond,attr),其中 attr 由 pthread\_condattr\_init() 和 pthread\_condattr\_destroy() 创建和销毁.

可以用 pthread\_cond\_destroy() 销毁一个条件变量.

相关函数：

pthread_cond_wait (condition,mutex) pthread_cond_signal (condition) pthread_cond_broadcast (condition)

pthread\_cond\_wait() 会阻塞调用它的线程，直到收到某一信号. 这个函数需要在 mutex 已经被锁之后进行调用，并且当线程被阻塞时，会自动解锁 mutex. 信号收到后，线程被唤醒，这时 mutex 又会被这个线程锁定.

pthread\_cond\_signal() 函数结束时，必须解锁 mutex，以供 pthread\_cond\_wait() 锁定mutex. 当不止一个线程在等待信号时，要用 pthread\_cond\_broadcast() 代替 pthread\_cond\_signal() 来告诉所有被该条件变量阻塞的线程结束阻塞状态.

下面是一个例子，三个线程共同访问 count 变量，thread 2 和 thread 3 竞争地对其进行加 1 的操作，thread 1 等 count 达到 12 的时候，一次性加 125 . 然后 thread 2 和 thread 3 再去竞争 count 的控制权，直到完成自己的对 count 加 10 次的任务.

/******************************************************************************
     * FILE: condvar.c
     * DESCRIPTION:
     *   Example code for using Pthreads condition variables.  The main thread
     *   creates three threads.  Two of those threads increment a "count" variable,
     *   while the third thread watches the value of "count".  When "count"
     *   reaches a predefined limit, the waiting thread is signaled by one of the
     *   incrementing threads. The waiting thread "awakens" and then modifies
     *   count. The program continues until the incrementing threads reach
     *   TCOUNT. The main program prints the final value of count.
     * SOURCE: Adapted from example code in "Pthreads Programming", B. Nichols
     *   et al. O'Reilly and Associates.
     * LAST REVISED: 10/14/10  Blaise Barney
     ******************************************************************************/
    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    
    #define NUM_THREADS 3
    #define TCOUNT 10
    #define COUNT_LIMIT 12
    
    int count = 0;
    pthread_mutex_t count_mutex;
    pthread_cond_t count_threshold_cv;
    
    void *inc_count(void *t) {
        int i;
        long my_id = (long) t;
    
        for (i = 0; i < TCOUNT; i++) {
            pthread_mutex_lock(&count_mutex);
            count++;
    
            /*
             Check the value of count and signal waiting thread when condition is
             reached.  Note that this occurs while mutex is locked.
             */
            if (count == COUNT_LIMIT) {
                printf("inc_count(): thread %ld, count = %d Threshold reached. ",
                        my_id, count);
                pthread_cond_signal(&count_threshold_cv);
                printf("Just sent signal.\n");
            }
            printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", my_id,
                    count);
            pthread_mutex_unlock(&count_mutex);
    
            /* Do some work so threads can alternate on mutex lock */
            sleep(1);
        }
        pthread_exit(NULL);
    }
    
    void *watch_count(void *t) {
        long my_id = (long) t;
    
        printf("Starting watch_count(): thread %ld\n", my_id);
    
        /*
         Lock mutex and wait for signal.  Note that the pthread_cond_wait routine
         will automatically and atomically unlock mutex while it waits.
         Also, note that if COUNT_LIMIT is reached before this routine is run by
         the waiting thread, the loop will be skipped to prevent pthread_cond_wait
         from never returning.
         */
        pthread_mutex_lock(&count_mutex);
        while (count < COUNT_LIMIT) {
            printf("watch_count(): thread %ld Count= %d. Going into wait...\n",
                    my_id, count);
            pthread_cond_wait(&count_threshold_cv, &count_mutex);
            printf(
                    "watch_count(): thread %ld Condition signal received. Count= %d\n",
                    my_id, count);
            printf("watch_count(): thread %ld Updating the value of count...\n",
                    my_id, count);
            count += 125;
            printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
        }
        printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
        pthread_mutex_unlock(&count_mutex);
        pthread_exit(NULL);
    }
    
    int main(int argc, char *argv[]) {
        int i, rc;
        long t1 = 1, t2 = 2, t3 = 3;
        pthread_t threads[3];
        pthread_attr_t attr;
    
        /* Initialize mutex and condition variable objects */
        pthread_mutex_init(&count_mutex, NULL);
        pthread_cond_init(&count_threshold_cv, NULL);
    
        /* For portability, explicitly create threads in a joinable state */
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
        pthread_create(&threads[0], &attr, watch_count, (void *) t1);
        pthread_create(&threads[1], &attr, inc_count, (void *) t2);
        pthread_create(&threads[2], &attr, inc_count, (void *) t3);
    
        /* Wait for all threads to complete */
        for (i = 0; i < NUM_THREADS; i++) {
            pthread_join(threads[i], NULL);
        }
        printf(
                "Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n",
                NUM_THREADS, count);
    
        /* Clean up and exit */
        pthread_attr_destroy(&attr);
        pthread_mutex_destroy(&count_mutex);
        pthread_cond_destroy(&count_threshold_cv);
        pthread_exit(NULL);
    }

输出结果：

inc_count(): thread 2, count = 1, unlocking mutex
    Starting watch_count(): thread 1
    watch_count(): thread 1 Count= 1. Going into wait...
    inc_count(): thread 3, count = 2, unlocking mutex
    inc_count(): thread 2, count = 3, unlocking mutex
    inc_count(): thread 3, count = 4, unlocking mutex
    inc_count(): thread 2, count = 5, unlocking mutex
    inc_count(): thread 3, count = 6, unlocking mutex
    inc_count(): thread 2, count = 7, unlocking mutex
    inc_count(): thread 3, count = 8, unlocking mutex
    inc_count(): thread 2, count = 9, unlocking mutex
    inc_count(): thread 3, count = 10, unlocking mutex
    inc_count(): thread 2, count = 11, unlocking mutex
    inc_count(): thread 3, count = 12  Threshold reached. Just sent signal.
    inc_count(): thread 3, count = 12, unlocking mutex
    watch_count(): thread 1 Condition signal received. Count= 12
    watch_count(): thread 1 Updating the value of count...
    watch_count(): thread 1 count now = 137.
    watch_count(): thread 1 Unlocking mutex.
    inc_count(): thread 2, count = 138, unlocking mutex
    inc_count(): thread 3, count = 139, unlocking mutex
    inc_count(): thread 2, count = 140, unlocking mutex
    inc_count(): thread 3, count = 141, unlocking mutex
    inc_count(): thread 2, count = 142, unlocking mutex
    inc_count(): thread 3, count = 143, unlocking mutex
    inc_count(): thread 2, count = 144, unlocking mutex
    inc_count(): thread 3, count = 145, unlocking mutex
    Main(): Waited and joined with 3 threads. Final value of count = 145. Done.

转自：[https://hanbingyan.github.io/2016/03/07/pthread\_on\_linux/\#section-2][https_hanbingyan.github.io_2016_03_07_pthread_on_linux_section-2]

[https_computing.llnl.gov_tutorials_pthreads]: https://computing.llnl.gov/tutorials/pthreads/
[https_hanbingyan.github.io_2016_03_07_pthread_on_linux_section-2]: https://hanbingyan.github.io/2016/03/07/pthread_on_linux/#section-2