私は C でのマルチスレッドが初めてで、この質問がありました。次のコードを書きました。
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t m=PTHREAD_MUTEX_INITIALIZER;
pthread_attr_t attr;
void* test(void *a)
{
int i=*((int *)a);
printf("The thread %d has started.\n",i);
pthread_mutex_lock(&m);
sleep(1);
printf("The thread %d has finished.\n",i);
pthread_mutex_unlock(&m);
pthread_exit(NULL);
}
int main()
{
int i=0;
pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);
pthread_t thread[5];
for (i=0;i<5;i++)
pthread_create(&thread[i],&attr,test,&i);
for (i=0;i<5;i++)
pthread_join(thread[i],NULL);
return 0;
}
次のような値が得られるのはなぜですか。
The thread 0 has started.
The thread 0 has started.
The thread 5 has started.
The thread 5 has started.
The thread 0 has started.
The thread 0 has finished.
The thread 0 has finished.
The thread 5 has finished.
The thread 5 has finished.
The thread 0 has finished.
また
The thread 1 has started.
The thread 2 has started.
The thread 5 has started.
The thread 4 has started.
The thread 0 has started.
The thread 1 has finished.
The thread 2 has finished.
The thread 5 has finished.
The thread 4 has finished.
The thread 0 has finished.
あるいは:
The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.
など、私が得ると思ったとき:
The thread 0 has started.
The thread 1 has started.
The thread 2 has started.
The thread 3 has started.
The thread 4 has started.
The thread 0 has finished.
The thread 1 has finished.
The thread 2 has finished.
The thread 3 has finished.
The thread 4 has finished.
usleep(10)
後に入れたときにのみ、thread_create
「通常の」値が得られます。
Unix の Code::Blocks でこのコードをコンパイルして実行しました。