[英]Synchronization problem with pthreads using pthread_join
我有以下循環,
for (i = 1; i < nDiag; ++i)
{
elem = findelemnum();
taskarr[3]= elem;
if(threadnum > elem) limit = elem;
else limit=threadnum;
if (i <= lenA)
{
si = i;
sj = 1;
}
else
{
si = lenA;
sj = i - lenA + 1;
}
taskarr[0] = si, taskarr[1] = sj;
for (j = 0; j < limit; ++j)
{
taskarr[2] = j;
wakeup = 0;
pthread_create(&threads[j], NULL, mytask, taskarr);
while(!wakeup){
}
}
for (int j=0; j < limit ;j++){
pthread_join(threads[j],NULL);
}
}
我想同步我的線程,以便所有線程都先完成內部循環,然后再開始一個新的循環(i ++)。 因此,我使用了pthread_join函數。 雖然有一個新的for i循環在其先前的結尾之前開始。 我做錯了什么?
如果要等待特定情況發生,請使用函數類pthread_cond_*
。
如果只想等待線程執行操作,則刪除while(!wakeup){}
。 這行得通,因為
int pthread_join(pthread_t線程,void ** value_ptr);
pthread_join()函數會暫停調用線程的執行,直到目標線程終止為止,除非目標線程已經終止。
因此,在以下代碼段中,您首先創建了限制線程,它們立即開始運行。 然后,您等待每個線程結束,即暫停調用線程,直到限制線程終止。
for (j = 0; j < limit; ++j)
{
taskarr[2] = j;
wakeup = 0;
pthread_create(&threads[j], NULL, mytask, taskarr);
}
for (int j=0; j < limit ;j++){
pthread_join(threads[j],NULL);
}
這是一個小演示
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#define NTHREAD 4
#define ITERATIONS 5
typedef struct _pcontext
{
int id;
int iteration;
}pcontext_t;
void* work(void* arg)
{
pcontext_t* ctx = (pcontext_t*)arg;
sleep(2);
printf("Thread ID: %d\n", ctx->id);
printf("Iteration nr: %d\n", ctx->iteration);
printf("Done.\n\n");
return NULL;
}
int main()
{
pthread_t thread[NTHREAD];
pcontext_t ctx[NTHREAD];
int err;
int i,j;
for(j = 0; j < ITERATIONS; j++)
{
for(i = 0; i < NTHREAD; i++)
{
ctx[i].id = i;
ctx[i].iteration = j;
err = pthread_create(&thread[i], NULL, work, (void*)(&(ctx[i])));
if (err)
{
printf("An error occured: %d", err);
return err;
}
}
printf("Waiting for the threads to end...\n");
for(i = 0; i < NTHREAD; i++)
{
pthread_join(thread[i], (void**)NULL);
}
printf("Threads ended.\n");
printf("Iteration %d ended.\n\n", j);
}
return 0;
}
輸出:
gcc -Wall -Wextra -pedantic -pthread -o main main.c
Waiting for the threads to end...
Thread ID: 1
Iteration nr: 0
Done.
Thread ID: 0
Iteration nr: 0
Done.
Thread ID: 2
Iteration nr: 0
Done.
Thread ID: 3
Iteration nr: 0
Done.
Threads ended.
Iteration 0 ended.
Waiting for the threads to end...
Thread ID: 0
Iteration nr: 1
Done.
Thread ID: 3
Iteration nr: 1
Done.
Thread ID: 2
Iteration nr: 1
Done.
Thread ID: 1
Iteration nr: 1
Done.
Threads ended.
Iteration 1 ended.
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