6.4 例程：使用互斥量

        这个例程为使用多线程配合互斥量进行点乘计算，相关的数据通过全局变量的形式存在，因此可以被各个线程访问；每个线程会在相关数据的不同区域上进行处理，主线程等待子线程完成操作后，将最后的结果打印出来。

        代码如下

#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 100DOTDATA 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 resultto 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 sharedstructure, 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 dot product  */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);
}