C- POSIX 线程基础

Question

I am learning the basics of POSIX threads. I want to create a program that prints "Hello World!" 10 times with a delay of a second between each printout. I've used a for loop to print it 10 times, but I am stuck on how to implement the time delay part.

This is my code so far:

#define MAX 10

void* helloFunc(void* tid)
{
printf("Hello World!\n", (int)(intptr_t)tid);
}

int main(int ac, char * argv)
{
pthread_t hej[MAX];
for (int i = 0; i < MAX; i++)
{
    pthread_create(&hej[i], NULL, helloFunc, (void*)(intptr_t)i);
    pthread_join(&hej[i], NULL);
}
pthread_exit(NULL);
return(0);
}

Thanks in advance!

Answer 1

There are two major problems with your code:

• First of all you must wait for the threads to finish. You do that by joining them with pthread_join . And for that to work you must save the pthread_t value from each and every thread (for example in an array).

  If you don't wait for the threads then the exit call will end the process, and that will also unexpectedly kill and end all threads in the process.

  For all threads to run in parallel you should wait in a separate loop after you have created them:

   pthread_t hej[MAX]; for (int i = 0; i < MAX; i++) { pthread_create(&hej[i], ...); } for (int i = 0; i < MAX; i++) { pthread_join(&hej[i], NULL); }

• The second problem is that you pass a pointer to i to the thread, so tid inside the thread functions will be all be the same (and a very large and weird value). To pass a value you must first cast it to intptr_t and then to void * :

   pthread_create(..., (void *) (intptr_t) i);

  And in the thread function you do the opposite casting:

   printf("Hello World %d!\\n", (int) (intptr_t) tid);

  Note that this is an exception to the rule that one should never pass values as pointers (or opposite).

---

Finally for the "delay" bit... On POSIX systems there are many ways to delay execution, or to sleep. The natural and simple solution would be to use sleep(1) which sleeps one second.

The problem is where do to this sleep(1) call. If you do it in the thread functions after the printf then all threads will race to print the message and then all will sleep at the same time.

If you do it in the loop where you create the threads, then the threads won't really run in parallel, but really in serial where one thread prints it message and exits, then the main thread will wait one second before creating the next thread. It makes the threads kind of useless.

As a possible third solution, use the value passed to the thread function to use as the sleep time, so the thread that is created first (when i == 0 ) will primt immediately, the second thread (when i == 1 ) will sleep one second. And so on, until the tenth thread is created and will sleep nine seconds before printing the message.

Could be done as:

void* helloFunc(void* tid)
{
    int value = (int) (intptr_t) tid;
    sleep(value);
    printf("Hello World %d!\n", value);

    // Must return a value, as the function is declared as such
    return NULL;
}