线程同步 - POSIX互斥锁
概括
本文讲解POSIX中互斥量的基本用法,从而能达到简单的线程同步。互斥量
是一种特殊的变量,它有两种状态:锁定以及解锁。如果互斥量是锁定的,就有一个特定的线程持有或者拥有这个互斥量;如果没有线程持有这个互斥量,我们就说这个互斥量是解锁的、可用的。同时,互斥量还有一个等待持有该互斥量的线程队列。互斥队列中的线程获得互斥量的顺序由线程调度所决定,但POSIX没有要求实现任何特定的策略。
程序描述
现在我们尝试写一个程序来体会互斥量的基本应用,用程序来模拟验证:\[\int_0^1sin(x) = 1.0 - cos(1)\] 利用多个子线程来产生[0,1]之间的随机数,每产生一次count
则增加一,并且将产生的数加入sum
,然后用sum/count
来模拟等式左边,最后计算等式右边作为标准值,然后计算误差。
程序用法:从命令行参数中接受要创建的线程数目以及运行时间,即$ ./程序名 线程数 等待时间
。
全局变量设计:
static int doneflag = 0;
static int count = 0;
static double sum = 0;
static pthread_mutex_t flaglock = PTHREAD_MUTEX_INITIALIZER;
static pthread_mutex_t sumlock = PTHREAD_MUTEX_INITIALIZER;
用sumlock
来守护count
和sum
,用flaglock
来守护doneflag
。
main
函数里创建线程数组,然后用pthread_create
来创建线程,用pthread_join
来等待线程。关键语句:
num_threads = atoi(argv[1]);
tids = (pthread_t *)calloc(num_threads, sizeof(pthread_t);
for (i = 0; i < num_threads; ++i) /* 创建num_threads个compute_thread线程 */
pthread_create(tids + i, NULL, compute_thread, NULL);
线程函数
注意创建的线程处理函数的形式:void *(*start_routine) (void *)
, 将函数指针(函数名)传递给pthread_create
即可。子进程循环计算,直到doneflag == 1
。
其它函数
randsafe()
:用一个互斥量来保护rand()
,要确保不会有两个线程同时调用rand,因为它在多线程中是不安全的。其次,rand不是一个特别好的伪随机数生成器,所以应该在实际程序中避免使用它,这里只是用作demo示范。
set_done
和get_done
分别用于设置flag和获得flag。
最后show_results
来整理数据并进行输出。
运行情况
$ gcc -o demo mutex_demo.c -lm -lpthread
$ ./demo 5 1
The sum is 232.913662 and the count is 500
The average is 0.465827 and error is 0.006130 or 1.333405%
$ ./demo 10 1
The sum is 467.382538 and the count is 1000
The average is 0.467383 and error is 0.007685 or 1.671717%
$ ./demo 10 2
The sum is 919.177364 and the count is 1990
The average is 0.461898 and error is 0.002200 or 0.478680%
源代码
/**
* @Description: 利用子线程计算0-1范围内正弦函数的平均值,并与实际值进行误差比较。
*/
#include <math.h>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include <unistd.h>
#define TEM_MILLION 10000000L
static int doneflag = 0;
static int count = 0;
static double sum = 0;
static pthread_mutex_t flaglock = PTHREAD_MUTEX_INITIALIZER;
static pthread_mutex_t sumlock = PTHREAD_MUTEX_INITIALIZER;
/* 线程函数, 计算随机和 */
void *compute_thread(void *arg1);
int set_done(void);
int get_done(int *flag);
int randsafe(double *valp);
int add(double x);
int show_results(void);
int main(int argc, char *argv[])
{
int i;
int num_threads;
int sleep_time;
pthread_t *tids;
if (argc != 3) {
fprintf(stderr, "Usage: %s num_threads sleep_time\n", argv[0]);
return 1;
}
num_threads = atoi(argv[1]);
sleep_time = atoi(argv[2]);
if ((tids = (pthread_t *)calloc(num_threads, sizeof(pthread_t))) == NULL) {
perror("Failed to allocate space for thread IDs");
return 1;
}
for (i = 0; i < num_threads; ++i) /* 创建num_threads个compute_thread线程 */
pthread_create(tids + i, NULL, compute_thread, NULL);
sleep(sleep_time);
set_done();
for (i = 0; i < num_threads; ++i) /* 等待线程完成 */
pthread_join(tids[i], NULL);
if (show_results())
return 1;
return 0;
}
/* 线程函数, 计算随机和 */
void *compute_thread(void *arg1) {
int localdone = 0;
struct timespec sleep_local;
double val;
sleep_local.tv_sec = 0;
sleep_local.tv_nsec = TEM_MILLION; /* 10ms */
while (!localdone) {
randsafe(&val);
add(sin(val));
get_done(&localdone);
nanosleep(&sleep_local, NULL); /* 让其他线程进入 */
}
}
int set_done(void) {
pthread_mutex_lock(&flaglock);
doneflag = 1;
return pthread_mutex_unlock(&flaglock);
}
int get_done(int *flag) {
pthread_mutex_lock(&flaglock);
*flag = doneflag;
return pthread_mutex_unlock(&flaglock);
}
int randsafe(double *valp) {
static pthread_mutex_t randlock = PTHREAD_MUTEX_INITIALIZER;
*valp = (double)rand() / (double)RAND_MAX;
return pthread_mutex_unlock(&randlock);
}
int add(double x) {
pthread_mutex_lock(&sumlock);
sum += x;
count++;
return pthread_mutex_unlock(&sumlock);
}
int show_results(void) {
int res_count;
double res_sum;
double calculated;
double average;
double err;
double perr;
pthread_mutex_lock(&sumlock);
res_sum = sum;
res_count = count;
pthread_mutex_unlock(&sumlock);
if (count == 0)
printf("No values were summed.\n");
else {
calculated = 1.0 - cos(1.0);
average = sum/count;
err = average - calculated;
perr = 100.0*err/calculated;
printf("The sum is %f and the count is %d\n", sum, count);
printf("The average is %f and error is %f or %f%%\n", average, err, perr);
}
return 0;
}