POSIX 多线程编程及理解

最近开发基于ZYNQ的嵌入式linux程序，涉及到多线程使用，将一些内容整理如下：

 POSIX多线程编程最为基础和重要的可以分为两部分：

1. 线程操作-Thread Management
2. 线程同步-Synchronization

线程同步主要是由于线程共享同一进程里的资源，因而需要程序员自己对资源进行同步来避免竞争产生

1.线程操作

pthread_create (thread,attr,start_routine,arg) 

pthread_exit (status)

pthread_cancel (thread)

pthread_attr_init (attr)

pthread_attr_destroy (attr) 

具体函数使用见参考文献1，现将参考文献1中示例代码贴出

/******************************************************************************
* 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!
", 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
", t);
     rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
     if (rc){
       printf("ERROR; return code from pthread_create() is %d
", rc);
       exit(-1);
       }
     }

   /* Last thing that main() should do */
   pthread_exit(NULL);
}

2.线程同步

线程同步POSIX主要提供两种结构：

• 互斥量
• 条件变量

互斥量的使用较为简单，具体参见参考文献1，现贴出参考文献1中示例代码

 #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.  
 */

 typedef struct 
  {
    double      *a;
    double      *b;
    double     sum; 
    int     veclen; 
  } DOTDATA;

 /* Define globally accessible variables and a mutex */

 #define NUMTHRDS 4
 #define VECLEN 100
    DOTDATA dotstr; 
    pthread_t callThd[NUMTHRDS];
    pthread_mutex_t mutexsum;

 /*
 The function dotprod is activated when the thread is created.
 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;
    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]=1.0;
      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 
", dotstr.sum);
    free (a);
    free (b);
    pthread_mutex_destroy(&mutexsum);
    pthread_exit(NULL);
 }   

条件变量和互斥量一般同时使用（原因下面做分析），使用方法参见参考文献1，现贴出参考文献1示例代码

/******************************************************************************
* 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.
");
      }
    printf("inc_count(): thread %ld, count = %d, unlocking mutex
", 
       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
", 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...
", my_id,count);
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received. Count= %d
", my_id,count);
    }
  printf("watch_count(): thread %ld Updating the value of count...
", my_id);
  count += 125;
  printf("watch_count(): thread %ld count now = %d.
", my_id, count);
  printf("watch_count(): thread %ld Unlocking mutex.
", 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.
", 
          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);

}

初次接触条件变量很容易疑惑为什么条件变量一定要与互斥量一同使用，很多人认为互斥量是用来保证条件变量的原子性的，其实这是没有真正理解条件变量设计初衷导致的。

条件变量设计也是为了各个线程同步数据，并非单单做等待。如果不使用条件变量，程序就需要在代码关键区内不断进行查询操作，浪费CPU时间片，为此设计了条件变量来消除查询操作的代价。因此把条件变量看做互斥量的一个补充就好理解为什么两者要同时出现了，而不是把互斥量看做条件变量的补充。

也可以理解为线程数据同步都是靠互斥量进行的，条件变量只是做了一个消息通知机制而已。如果仅仅为了等待，而不存在数据保护，条件变量其实无需互斥量。

参考文献2也做了深入分析，可作参考

参考文献：

1.https://computing.llnl.gov/tutorials/pthreads/

2.http://www.cnblogs.com/Dahaka/archive/2012/02/19/2358528.html