[英]Strange behavior when using threads in C
我对线程很陌生,很难理解下面代码的行为。 假设我使用命令行输入10,我希望输出为20,因为有两个线程将count的值递增十次。 但是,每次我运行此程序时输出都不是20。 以下是我的一些尝试:
命令行输入:10,预期输出:20,实际输出:15
命令行输入:10,预期输出:20,实际输出:10
命令行输入:10,预期输出:20,实际输出:13
命令行输入:10,例外输出:20,实际输出:20
#include <stdio.h>
#include <pthread.h>
/* The threads will increment this count
* n times (command line input).
*/
int count = 0;
/* The function the threads will perform */
void *increment(void *params);
int main(int argc, char *argv[]) {
/* Take in command line input for number of iterations */
long iterations = (long)atoi(argv[1]);
pthread_t thread_1, thread_2;
/* Create two threads. */
pthread_create(&thread_1, NULL, increment, (void*)iterations);
pthread_create(&thread_2, NULL, increment, (void*)iterations);
/* Wait for both to finish */
pthread_join(thread_1, NULL);
pthread_join(thread_2, NULL);
/* Print the final result for count. */
printf("Count = %d.\n", count);
pthread_exit(NULL);
}
void *increment(void *params) {
long iterations = (long)params;
int i;
/* Increment global count */
for(i = 0; i < iterations; i++) {
count++;
}
pthread_exit(NULL);
}
您的增量不是原子的,并且您没有插入任何同步机制,因此,您的一个线程当然会覆盖count
而另一个线程仍在递增计数,从而导致增量的“损失”。
您需要一个原子增量功能(在Windows上具有InterlockedIncrement
)或显式锁定机制(例如互斥锁)。
要在汇编中对两个数字求和,通常需要以下几个指令:
因此,当操作系统给您的程序系统时间时,它不能保证所有这些操作都不会中断。 这些称为关键部分。 每次输入这样的部分时,都需要同步两个线程。
这应该工作:
#include <stdio.h>
#include <pthread.h>
/* The threads will increment this count
* n times (command line input).
*/
int count = 0;
pthread_mutex_t lock;
/* The function the threads will perform */
void *increment(void *params);
int main(int argc, char *argv[]) {
/* Take in command line input for number of iterations */
long iterations = (long)atoi(argv[1]);
pthread_t thread_1, thread_2;
pthread_mutex_init(&lock); //initialize the mutex
/* Create two threads. */
pthread_create(&thread_1, NULL, increment, (void*)iterations);
pthread_create(&thread_2, NULL, increment, (void*)iterations);
/* Wait for both to finish */
pthread_join(thread_1, NULL);
pthread_join(thread_2, NULL);
/* Print the final result for count. */
printf("Count = %d.\n", count);
pthread_mutex_destroy(&lock); //destroy the mutex, its similar to malloc and free
pthread_exit(NULL);
}
void *increment(void *params) {
long iterations = (long)params;
int i;
int local_count = 0;
/* Increment global count */
for(i = 0; i < iterations; i++) {
local_count++;
}
pthread_mutex_lock(&lock); //enter a critical section
count += local_count;
pthread_mutex_unlock(&lock); //exit a critical section
pthread_exit(NULL);
}
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