threadpool — 基于 pthread 实现的简单线程池
线程池介绍
线程池可以说是项目中经常会用到的组件，在这里假设读者都有一定的多线程基础，如果没有的话不妨在这里进行了解：POSIX 多线程基础。
线程池是什么？我的简单理解是有一组预先派生的线程，然后有一个管理员来管理和调度这些线程，你只需不断把需要完成的任务交给他，他就会调度线程的资源来帮你完成。
那么管理员是怎么做的呢？一种简单的方式就是，管理员管理一个任务的队列，如果收到新的任务，就把任务加到队列尾。每个线程盯着队列，如果队列非空，就去队列头拿一个任务来处理（每个任务只能被一个线程拿到），处理完了就继续去队列取任务。如果没有任务了，线程就休眠，直到任务队列不为空。如果这个管理员更聪明一点，他可能会在没有任务或任务少的时候减少线程的数量，任务处理不过来的时候增加线程的数量，这样就实现了资源的动态管理。
那么任务是什么呢？以后台服务器为例，每一个用户的请求就是一个任务，线程不断的在请求队列里取出请求，完成后继续处理下一个请求。
简单图示为：
线程池有一个好处就是减少线程创建和销毁的时间，在任务处理时间比较短的时候这个好处非常显著，可以提升任务处理的效率。
线程池实现
这里介绍的是线程池的一个简单实现，在创建的时候预先派生指定数量的线程，然后去任务队列取添加进来的任务进行处理就好。
作者说之后会添加更多特性，我们作为学习之后就以这个版本为准就好了。
项目主页：threadpool
数据结构
主要有两个自定义的数据结构
threadpool_task_t
用于保存一个等待执行的任务。一个任务需要指明：要运行的对应函数及函数的参数。所以这里的 struct 里有函数指针和 void 指针。
typedef struct {void (*function)(void *);
void *argument;
} threadpool_task_t;thread_pool_t
一个线程池的结构。因为是 C 语言，所以这里任务队列是用数组，并维护队列头和队列尾来实现。
struct threadpool_t {
pthread_mutex_t lock; /* 互斥锁 */
pthread_cond_t notify; /* 条件变量 */
pthread_t *threads; /* 线程数组的起始指针 */
threadpool_task_t *queue; /* 任务队列数组的起始指针 */
int thread_count; /* 线程数量 */
int queue_size; /* 任务队列长度 */
int head; /* 当前任务队列头 */
int tail; /* 当前任务队列尾 */
int count; /* 当前待运行的任务数 */
int shutdown; /* 线程池当前状态是否关闭 */
int started; /* 正在运行的线程数 */
};函数
对外接口
-
threadpool_t *threadpool_create(int thread_count, int queue_size, int flags);创建线程池，用 thread_count 指定派生线程数，queue_size 指定任务队列长度，flags 为保留参数，未使用。 -
int threadpool_add(threadpool_t *pool, void (*routine)(void *),void *arg, int flags);添加需要执行的任务。第二个参数为对应函数指针，第三个为对应函数参数。flags 未使用。 -
int threadpool_destroy(threadpool_t *pool, int flags);销毁存在的线程池。flags 可以指定是立刻结束还是平和结束。立刻结束指不管任务队列是否为空，立刻结束。平和结束指等待任务队列的任务全部执行完后再结束，在这个过程中不可以添加新的任务。
内部辅助函数
-
static void *threadpool_thread(void *threadpool);线程池每个线程所执行的函数。 -
int threadpool_free(threadpool_t *pool);释放线程池所申请的内存资源。
线程池使用
编译
参考项目根目录下的 Makefile, 直接用 make 编译。
测试用例
项目提供了三个测试用例（见 threadpool/test/），我们可以以此来学习线程池的用法并测试是否正常工作。这里提供其中一个：
#define THREAD 32
#define QUEUE 256
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
#include "threadpool.h"
int tasks = 0, done = 0;
pthread_mutex_t lock;
void dummy_task(void *arg) {usleep(10000);
pthread_mutex_lock(&lock);
/* 记录成功完成的任务数 */
done++;
pthread_mutex_unlock(&lock);
}
int main(int argc, char **argv)
{
threadpool_t *pool;
/* 初始化互斥锁 */
pthread_mutex_init(&lock, NULL);
/* 断言线程池创建成功 */
assert((pool = threadpool_create(THREAD, QUEUE, 0)) != NULL);
fprintf(stderr, "Pool started with %d threads and"
"queue size of %d\n", THREAD, QUEUE);
/* 只要任务队列还没满，就一直添加 */
while(threadpool_add(pool, &dummy_task, NULL, 0) == 0) {pthread_mutex_lock(&lock);
tasks++;
pthread_mutex_unlock(&lock);
}
fprintf(stderr, "Added %d tasks\n", tasks);
/* 不断检查任务数是否完成一半以上，没有则继续休眠 */
while((tasks / 2) > done) {usleep(10000);
}
/* 这时候销毁线程池,0 代表 immediate_shutdown */
assert(threadpool_destroy(pool, 0) == 0);
fprintf(stderr, "Did %d tasks\n", done);
return 0;
}源码注释
源码注释一并放在 github, 点我。
threadpool.h
/*
* Copyright (c) 2013, Mathias Brossard <mathias@brossard.org>.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#ifndef _THREADPOOL_H_
#define _THREADPOOL_H_
#ifdef __cplusplus
/* 对于 C++ 编译器，指定用 C 的语法编译 */
extern "C" {
#endif
/**
* @file threadpool.h
* @brief Threadpool Header File
*/
/**
* Increase this constants at your own risk
* Large values might slow down your system
*/
#define MAX_THREADS 64
#define MAX_QUEUE 65536
/* 简化变量定义 */
typedef struct threadpool_t threadpool_t;
/* 定义错误码 */
typedef enum {
threadpool_invalid = -1,
threadpool_lock_failure = -2,
threadpool_queue_full = -3,
threadpool_shutdown = -4,
threadpool_thread_failure = -5
} threadpool_error_t;
typedef enum {threadpool_graceful = 1} threadpool_destroy_flags_t;
/* 以下是线程池三个对外 API */
/**
* @function threadpool_create
* @brief Creates a threadpool_t object.
* @param thread_count Number of worker threads.
* @param queue_size Size of the queue.
* @param flags Unused parameter.
* @return a newly created thread pool or NULL
*/
/**
* 创建线程池，有 thread_count 个线程，容纳 queue_size 个的任务队列，flags 参数没有使用
*/
threadpool_t *threadpool_create(int thread_count, int queue_size, int flags);
/**
* @function threadpool_add
* @brief add a new task in the queue of a thread pool
* @param pool Thread pool to which add the task.
* @param function Pointer to the function that will perform the task.
* @param argument Argument to be passed to the function.
* @param flags Unused parameter.
* @return 0 if all goes well, negative values in case of error (@see
* threadpool_error_t for codes).
*/
/**
* 添加任务到线程池, pool 为线程池指针，routine 为函数指针，arg 为函数参数，flags 未使用
*/
int threadpool_add(threadpool_t *pool, void (*routine)(void *),
void *arg, int flags);
/**
* @function threadpool_destroy
* @brief Stops and destroys a thread pool.
* @param pool Thread pool to destroy.
* @param flags Flags for shutdown
*
* Known values for flags are 0 (default) and threadpool_graceful in
* which case the thread pool doesn't accept any new tasks but
* processes all pending tasks before shutdown.
*/
/**
* 销毁线程池，flags 可以用来指定关闭的方式
*/
int threadpool_destroy(threadpool_t *pool, int flags);
#ifdef __cplusplus
}
#endif
#endif /* _THREADPOOL_H_ */threadpool.c
/*
* Copyright (c) 2013, Mathias Brossard <mathias@brossard.org>.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/**
* @file threadpool.c
* @brief Threadpool implementation file
*/
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include "threadpool.h"
/**
* 线程池关闭的方式
*/
typedef enum {
immediate_shutdown = 1,
graceful_shutdown = 2
} threadpool_shutdown_t;
/**
* @struct threadpool_task
* @brief the work struct
*
* @var function Pointer to the function that will perform the task.
* @var argument Argument to be passed to the function.
*/
/**
* 线程池一个任务的定义
*/
typedef struct {void (*function)(void *);
void *argument;
} threadpool_task_t;
/**
* @struct threadpool
* @brief The threadpool struct
*
* @var notify Condition variable to notify worker threads.
* @var threads Array containing worker threads ID.
* @var thread_count Number of threads
* @var queue Array containing the task queue.
* @var queue_size Size of the task queue.
* @var head Index of the first element.
* @var tail Index of the next element.
* @var count Number of pending tasks
* @var shutdown Flag indicating if the pool is shutting down
* @var started Number of started threads
*/
/**
* 线程池的结构定义
* @var lock 用于内部工作的互斥锁
* @var notify 线程间通知的条件变量
* @var threads 线程数组，这里用指针来表示，数组名 = 首元素指针
* @var thread_count 线程数量
* @var queue 存储任务的数组，即任务队列
* @var queue_size 任务队列大小
* @var head 任务队列中首个任务位置（注：任务队列中所有任务都是未开始运行的）* @var tail 任务队列中最后一个任务的下一个位置（注：队列以数组存储，head 和 tail 指示队列位置）* @var count 任务队列里的任务数量，即等待运行的任务数
* @var shutdown 表示线程池是否关闭
* @var started 开始的线程数
*/
struct threadpool_t {
pthread_mutex_t lock;
pthread_cond_t notify;
pthread_t *threads;
threadpool_task_t *queue;
int thread_count;
int queue_size;
int head;
int tail;
int count;
int shutdown;
int started;
};
/**
* @function void *threadpool_thread(void *threadpool)
* @brief the worker thread
* @param threadpool the pool which own the thread
*/
/**
* 线程池里每个线程在跑的函数
* 声明 static 应该只为了使函数只在本文件内有效
*/
static void *threadpool_thread(void *threadpool);
int threadpool_free(threadpool_t *pool);
threadpool_t *threadpool_create(int thread_count, int queue_size, int flags)
{if(thread_count <= 0 || thread_count > MAX_THREADS || queue_size <= 0 || queue_size > MAX_QUEUE) {return NULL;}
threadpool_t *pool;
int i;
/* 申请内存创建内存池对象 */
if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL) {goto err;}
/* Initialize */
pool->thread_count = 0;
pool->queue_size = queue_size;
pool->head = pool->tail = pool->count = 0;
pool->shutdown = pool->started = 0;
/* Allocate thread and task queue */
/* 申请线程数组和任务队列所需的内存 */
pool->threads = (pthread_t *)malloc(sizeof(pthread_t) * thread_count);
pool->queue = (threadpool_task_t *)malloc
(sizeof(threadpool_task_t) * queue_size);
/* Initialize mutex and conditional variable first */
/* 初始化互斥锁和条件变量 */
if((pthread_mutex_init(&(pool->lock), NULL) != 0) ||
(pthread_cond_init(&(pool->notify), NULL) != 0) ||
(pool->threads == NULL) ||
(pool->queue == NULL)) {goto err;}
/* Start worker threads */
/* 创建指定数量的线程开始运行 */
for(i = 0; i < thread_count; i++) {if(pthread_create(&(pool->threads[i]), NULL,
threadpool_thread, (void*)pool) != 0) {threadpool_destroy(pool, 0);
return NULL;
}
pool->thread_count++;
pool->started++;
}
return pool;
err:
if(pool) {threadpool_free(pool);
}
return NULL;
}
int threadpool_add(threadpool_t *pool, void (*function)(void *),
void *argument, int flags)
{
int err = 0;
int next;
if(pool == NULL || function == NULL) {return threadpool_invalid;}
/* 必须先取得互斥锁所有权 */
if(pthread_mutex_lock(&(pool->lock)) != 0) {return threadpool_lock_failure;}
/* 计算下一个可以存储 task 的位置 */
next = pool->tail + 1;
next = (next == pool->queue_size) ? 0 : next;
do {
/* Are we full ? */
/* 检查是否任务队列满 */
if(pool->count == pool->queue_size) {
err = threadpool_queue_full;
break;
}
/* Are we shutting down ? */
/* 检查当前线程池状态是否关闭 */
if(pool->shutdown) {
err = threadpool_shutdown;
break;
}
/* Add task to queue */
/* 在 tail 的位置放置函数指针和参数，添加到任务队列 */
pool->queue[pool->tail].function = function;
pool->queue[pool->tail].argument = argument;
/* 更新 tail 和 count */
pool->tail = next;
pool->count += 1;
/* pthread_cond_broadcast */
/*
* 发出 signal, 表示有 task 被添加进来了
* 如果由因为任务队列空阻塞的线程，此时会有一个被唤醒
* 如果没有则什么都不做
*/
if(pthread_cond_signal(&(pool->notify)) != 0) {
err = threadpool_lock_failure;
break;
}
/*
* 这里用的是 do {...} while(0) 结构
* 保证过程最多被执行一次，但在中间方便因为异常而跳出执行块
*/
} while(0);
/* 释放互斥锁资源 */
if(pthread_mutex_unlock(&pool->lock) != 0) {err = threadpool_lock_failure;}
return err;
}
int threadpool_destroy(threadpool_t *pool, int flags)
{
int i, err = 0;
if(pool == NULL) {return threadpool_invalid;}
/* 取得互斥锁资源 */
if(pthread_mutex_lock(&(pool->lock)) != 0) {return threadpool_lock_failure;}
do {
/* Already shutting down */
/* 判断是否已在其他地方关闭 */
if(pool->shutdown) {
err = threadpool_shutdown;
break;
}
/* 获取指定的关闭方式 */
pool->shutdown = (flags & threadpool_graceful) ?
graceful_shutdown : immediate_shutdown;
/* Wake up all worker threads */
/* 唤醒所有因条件变量阻塞的线程，并释放互斥锁 */
if((pthread_cond_broadcast(&(pool->notify)) != 0) ||
(pthread_mutex_unlock(&(pool->lock)) != 0)) {
err = threadpool_lock_failure;
break;
}
/* Join all worker thread */
/* 等待所有线程结束 */
for(i = 0; i < pool->thread_count; i++) {if(pthread_join(pool->threads[i], NULL) != 0) {err = threadpool_thread_failure;}
}
/* 同样是 do{...} while(0) 结构 */
} while(0);
/* Only if everything went well do we deallocate the pool */
if(!err) {
/* 释放内存资源 */
threadpool_free(pool);
}
return err;
}
int threadpool_free(threadpool_t *pool)
{if(pool == NULL || pool->started > 0) {return -1;}
/* Did we manage to allocate ? */
/* 释放线程 任务队列 互斥锁 条件变量 线程池所占内存资源 */
if(pool->threads) {free(pool->threads);
free(pool->queue);
/* Because we allocate pool->threads after initializing the
mutex and condition variable, we're sure they're
initialized. Let's lock the mutex just in case. */
pthread_mutex_lock(&(pool->lock));
pthread_mutex_destroy(&(pool->lock));
pthread_cond_destroy(&(pool->notify));
}
free(pool);
return 0;
}
static void *threadpool_thread(void *threadpool)
{threadpool_t *pool = (threadpool_t *)threadpool;
threadpool_task_t task;
for(;;) {
/* Lock must be taken to wait on conditional variable */
/* 取得互斥锁资源 */
pthread_mutex_lock(&(pool->lock));
/* Wait on condition variable, check for spurious wakeups.
When returning from pthread_cond_wait(), we own the lock. */
/* 用 while 是为了在唤醒时重新检查条件 */
while((pool->count == 0) && (!pool->shutdown)) {
/* 任务队列为空，且线程池没有关闭时阻塞在这里 */
pthread_cond_wait(&(pool->notify), &(pool->lock));
}
/* 关闭的处理 */
if((pool->shutdown == immediate_shutdown) ||
((pool->shutdown == graceful_shutdown) &&
(pool->count == 0))) {break;}
/* Grab our task */
/* 取得任务队列的第一个任务 */
task.function = pool->queue[pool->head].function;
task.argument = pool->queue[pool->head].argument;
/* 更新 head 和 count */
pool->head += 1;
pool->head = (pool->head == pool->queue_size) ? 0 : pool->head;
pool->count -= 1;
/* Unlock */
/* 释放互斥锁 */
pthread_mutex_unlock(&(pool->lock));
/* Get to work */
/* 开始运行任务 */
(*(task.function))(task.argument);
/* 这里一个任务运行结束 */
}
/* 线程将结束，更新运行线程数 */
pool->started--;
pthread_mutex_unlock(&(pool->lock));
pthread_exit(NULL);
return(NULL);
}