posix线程编程

在共享内存多处理器结构中，线程可用于实现并行性。
在理解线程之前，首先要了解unix进程。进程由操作系统创建，并且需要相当大的“开销”。进程包含有关程序资源和程序执行状态的信息，包括：

• 进程ID，进程组ID，用户ID和组ID
• 工作目录
• 程序指令
• 寄存器
• 栈
• 堆
• 文件描述符
• 信号动作
• 共享库
• 进程间通信工具（消息队列，管道，信号量或者共享内存）
  线程使用并存在于这些进程资源中，但是能够由操作系统进行独立调度并且作为独立实体运行

线程

• 栈指针
• 寄存器
• 调度属性
• 线程数据

共享内存模型

• 所有线程都可以访问相同的全局变量
• 线程也有自己的私有数据
• 程序员负责同步访问全局共享数据

线程安全
线程安全性：简而言之，指的是应用程序同时执行多个线程而不”破坏”共享数据或创建“竞争”条件的能力

创建和终止线程

pthread_create（thread，attr，start_routine，arg）
pthread_exit（状态）
pthread_cancel（线程）
pthread_attr_init（attr）
pthread_attr_destroy（attr）

线程属性
默认情况下，使用某些属性创建一个线程。程序员可以通过线程属性对象更改其中一些属性
属性包括

• 分离或可连接状态
• 调度继承
• 调度策略
• 调度参数
• 堆栈大小
• 堆栈地址
• 堆栈防护（溢出）大小

堆栈管理

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

线程管理

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

线程的链接于分离，体现在我们是否要知道这个线程的返回值和状态

一个线程在我们连接（join）它之前可能就已经终止了。结果是，当一个线程是可连接时，POSIX线程系统必须保持、维护某些信息：至少是该线程的ID和返回值（the returned value）。的确，正在等待连接该线程的线程是需要这些信息的。实际上，POSIX线程系统也可保持和可连接线程相关的其它资源信息，比如线程的栈。从POSIX的角度来看，这是完全合法的。事实上，pthread_join()保证回收与已连接线程的任何存储空间。

那当我的线程不返回任何有用的信息，并且我不必等待它的完成时，我该怎么办？仅仅为了清理的意图，无论如何还是得必须调用pthrad_join()吗？
幸运的是，不必这样做。Pthreads提供了一种机制告诉管理系统：我正在开启这个线程，但我对连接它并没有兴趣；一旦该线程结束，请为我执行清理动作。这个操作叫做分离一个线程。我们可以按照如下方法分离一个线程：

• 在线程的创建期间，采用detachstate线程属性
• 在任何线程处调用pthread_detach()函数

互斥变量

pthread_mutex_init (mutex,attr)
pthread_mutex_destroy (mutex)
pthread_mutexattr_init (attr)
pthread_mutexattr_destroy (attr)
pthread_mutex_lock (mutex)
pthread_mutex_trylock (mutex)
pthread_mutex_unlock (mutex)

条件变量

pthread_cond_init (condition,attr)
pthread_cond_destroy (condition)
pthread_condattr_init (attr)
pthread_condattr_destroy (attr)
pthread_cond_wait (condition,mutex)
pthread_cond_signal (condition)
pthread_cond_broadcast (condition)

例子代码

/******************************************************************************
* 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: 03/07/17  Blaise Barney
******************************************************************************/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.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 += 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);

}

观测，调式，性能分析工具对于线程