[英]Strange behavior when using threads in C
我對線程很陌生,很難理解下面代碼的行為。 假設我使用命令行輸入10,我希望輸出為20,因為有兩個線程將count的值遞增十次。 但是,每次我運行此程序時輸出都不是20。 以下是我的一些嘗試:
命令行輸入:10,預期輸出:20,實際輸出:15
命令行輸入:10,預期輸出:20,實際輸出:10
命令行輸入:10,預期輸出:20,實際輸出:13
命令行輸入:10,例外輸出:20,實際輸出: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,並且您沒有插入任何同步機制,因此,您的一個線程當然會覆蓋count
而另一個線程仍在遞增計數,從而導致增量的“損失”。
您需要一個原子增量功能(在Windows上具有InterlockedIncrement
)或顯式鎖定機制(例如互斥鎖)。
要在匯編中對兩個數字求和,通常需要以下幾個指令:
因此,當操作系統給您的程序系統時間時,它不能保證所有這些操作都不會中斷。 這些稱為關鍵部分。 每次輸入這樣的部分時,都需要同步兩個線程。
這應該工作:
#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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