c - `pthread_mutex_trylock` and `pthread_mutex_lock` behaviour -

this follow this question.

in code, when not using fflush(stdout) output not flushed screen when kept sleep(1).

#define s sleep(0) 

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void* xthread_fn(void* arg) { while(1)     {     s;     pthread_mutex_lock(&read_c_mutex);         if(!read_c)         {             pthread_mutex_unlock(&read_c_mutex);             printf(" x");         }         else         {             pthread_mutex_unlock(&read_c_mutex);             pthread_exit(null);         }     fflush(stdout); <---this 1 here     }  } 

but when keep sleep(0) there no need fflush(stdout), output updated on stdout. why so?

q1. why should presence of sleep(0) cause change in way output flushed?

if modify code follows (to track execution),

#define s sleep(1)  int read_c = 0; pthread_mutex_t read_c_mutex = pthread_mutex_initializer;  void* inputthread_fn(void* arg) { printf("%p input\n",pthread_self()); char inputchar; int = 0; while(1) {     s;     printf("\nchecking input");     scanf("%c",&inputchar);     if(inputchar=='c' || inputchar == 'c')     {      pthread_mutex_trylock(&read_c_mutex);      printf("%p has lock %d\n",pthread_self(),i);      read_c = 1;      pthread_mutex_unlock(&read_c_mutex);             printf("%p has unlocked %d\n",pthread_self(),i++);      printf("%p gone!\n",pthread_self());      fflush(stdout);      pthread_exit(null);     } } }  void* xthread_fn(void* arg) {     int = 0;     printf("%p x\n",pthread_self());     while(1)     {      s;      printf("x trying lock\n");      pthread_mutex_trylock(&read_c_mutex);      printf("%p has lock %d\n",pthread_self(),i);      if(!read_c)      {       pthread_mutex_unlock(&read_c_mutex);       printf("%p has unlocked %d\n",pthread_self(),i++);       printf("x\n");       fflush(stdout);      }       else      {       printf("%p gone!\n",pthread_self());          pthread_mutex_unlock(&read_c_mutex);       fflush(stdout);       pthread_exit(null);      }     } }  void* ythread_fn(void* arg) {  printf("%p y\n",pthread_self());  int = 0;  while(1)  {   s;   printf("y trying lock\n");   pthread_mutex_trylock(&read_c_mutex);   printf("%p has lock %d\n",pthread_self(),i);   if(!read_c)   {    pthread_mutex_unlock(&read_c_mutex);    printf("%p has unlocked %d\n",pthread_self(),i++);    printf("z\n");    fflush(stdout);   }   else   {     printf("%p gone!\n",pthread_self());     pthread_mutex_unlock(&read_c_mutex);     fflush(stdout);     pthread_exit(null);    }   } } 

a sample output

0xb6700b70 input 0xb6f01b70 y 0xb7702b70 x  checking inputy trying lock 0xb6f01b70 has lock 0 0xb6f01b70 has unlocked 0 z x trying lock 0xb7702b70 has lock 0 0xb7702b70 has unlocked 0 x y trying lock 0xb6f01b70 has lock 1 0xb6f01b70 has unlocked 1 z x trying lock 0xb7702b70 has lock 1 0xb7702b70 has unlocked 1 x y trying lock 0xb6f01b70 has lock 2 0xb6f01b70 has unlocked 2 z x trying lock 0xb7702b70 has lock 2 0xb7702b70 has unlocked 2 x y trying lock 0xb6f01b70 has lock 3 0xb6f01b70 has unlocked 3 z x trying lock 0xb7702b70 has lock 3 0xb7702b70 has unlocked 3 x y trying lock 0xb6f01b70 has lock 4 0xb6f01b70 has unlocked 4 z x trying lock 0xb7702b70 has lock 4 0xb7702b70 has unlocked 4 x c y trying lock 0xb6f01b70 has lock 5 0xb6f01b70 has unlocked 5 z x trying lock 0xb7702b70 has lock 5 0xb7702b70 has unlocked 5 x 0xb6700b70 has lock 0 0xb6700b70 has unlocked 0 0xb6700b70 gone! y trying lock 0xb6f01b70 has lock 6 0xb6f01b70 gone! x trying lock 0xb7702b70 has lock 6 0xb7702b70 gone! 

q2. have made use of pthread_mutex_trylock() because wanted code proceed in while loop till acquires lock check value of read_c. same appears achieved pthread_mutex_lock();. confuses me more. pthread_mutex_trylock(); too, output this? 1 x followed 1 z. won't there situation might x x z x (provided thread switched os , ythread tried lock , failed)?

answering "trylock" question.

pthread_mutex_trylock attempts lock mutex. if else has mutex locked, return error , keep running. since you're not checking return value might touch data protected mutex without holding mutex.

your code equivalent not having mutexes @ all.

pthread_mutex_trylock should used in situations special reason can't wait lock , fall different behavior when acquiring mutex has failed. calling without checking return value bug.

to correct, should check return value of pthread_mutex_lock too. can away not doing that. can never away not checking return value of trylock.