Question

I recently endeavoured to learn about multiple threading, and ran into the following unexpected – to me, at least – behaviour: printf just will not print more than a line at once when called in the very simple following code:

    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
    char buffer[2];

    void * thread_routine(void * args){
      pthread_mutex_lock(&mutex);
      pthread_cond_wait(&cond, &mutex);
      printf("test %s\n test\n", buffer);
      pthread_mutex_unlock(&mutex);
      return(NULL);
    }

    int main(void){
      pthread_t thread;
      pthread_create(&thread, NULL, thread_routine, NULL);
      sleep(1);
      buffer[0] = 'c';
      buffer[1] = '\0';
      pthread_mutex_lock(&mutex);
      pthread_cond_signal(&cond);
      pthread_mutex_unlock(&mutex);
      sleep(10);
      return(0);
    }

The output is

    test c

(wait for 10 seconds and)

    test prompt$]

What is wrong with this code? How come I can’t get printf to print two lines at once? Please note that blocking stdout with flockfile and unlocking with funlockfile does nothing to improve the situation.

Answer 1

If your program printed test prompt$] at the end as you say, this means that the version that you executed didn’t have the second newline in "test %s\n test\n".

Newlines are important, since this is when the output gets flushed to the screen. See Why does printf not flush after the call unless a newline is in the format string? for an explanation and recommendations.

Try re-compiling and re-running the exact code from your question, and I bet it’ll work as expected (it certainly does on my box).