I am trying to solve the Philosophers Dining Problem using semaphores.
The philosopher first picks up the left fork and then right fork and when finished eating puts them down.
I am implementing this using 5 threads one for each philosopher and 5 semaphores one for each chopstick.
Need to perform a deadlock check by the parent and break the deadlock if found.
When I just run the cycle of philosopher THINKING and EATINGthe program crashes with the error
The futex facility returned an unexpected error code.Aborted
.
I am not getting any information on how to debug this error.
The Thread of the Philosoper is given below
void *Philospoher_behaviour(void *param){
int id = *(int *)param; // assigns a ID to the Phil
int state; // hungry , thinking, eating
// first the philopher thinks
while(1) {
state = THINKING;
float time = (float)rand()/RAND_MAX;
printf("Philosopher %d starts THINKING for %f\n",id,time);
sleep(time);
// the phil goes hungrg
state = HUNGRY;
printf("Philosopher %d is now HUNGRY\n",id);
// first wait for left
sem_wait(&chopsticks[id]);
printf("Philosopher %d grabs chopstick %d to this LEFT\n",id,id);
// got left chopstick
sem_wait(&chopsticks[(id+1)%5]);
printf("Philosopher %d grabs chopstick %d to this RIGHT\n",id,(id+1)%5);
// got the right chopstick
state = EATING;
time = (float)rand()/RAND_MAX;
printf("Philosopher %d starts EATING for time %f\n",id,time);
sleep(time);
sem_post(&chopsticks[(id + 1)%5]);
printf("Philosopher %d releases chopstick %d to this RIGHT\n",id,(id+1)%5);
sem_post(&chopsticks[id]);
printf("Philosopher %d releases chopstick %d to this LEFT\n",id,(id));
state = THINKING;
time = (float)rand()/RAND_MAX;
sleep(time);
}
}
The Main program is as follows
sem_t chopsticks[5];// five chopsticks as a resource
pthread_t philosopher[5]; //five philosoophers
int main(){
srand(time(NULL));
for ( int i=0 ;i <5;i++){
sem_init(&chopsticks[i], 0, 1); // local to the threads with initial value of 1
}
// now create the indiviual threads
for(int i=0;i<5;i++){
if( pthread_create(&philosopher[i],NULL, Philospoher_behaviour ,&i) != 0) { // create thread one
printf("Cant create thread %d\n",i);
return 1;
}
else{
printf("Creadted Philosopher Number : %d\n",i);
}
}
for(int i=0;i<5;i++){
pthread_join(philosopher[i],NULL);
}
}
How to debug this error. I am also pasting the output of one run
Creadted Philosopher Number : 0
Philosopher 1 starts THINKING for 0.483853
Creadted Philosopher Number : 1
Philosopher 1 starts THINKING for 0.059081
Creadted Philosopher Number : 2
Philosopher 3 starts THINKING for 0.149168
Creadted Philosopher Number : 3
Philosopher 4 starts THINKING for 0.073436
Creadted Philosopher Number : 4
Philosopher 5 starts THINKING for 0.833351
Philosopher 5 is now HUNGRY
Philosopher 1 is now HUNGRY
Philosopher 5 grabs chopstick 5 to this LEFT
Philosopher 1 grabs chopstick 1 to this LEFT
Philosopher 1 grabs chopstick 2 to this RIGHT
Philosopher 1 starts EATING for time 0.147257
Philosopher 3 is now HUNGRY
Philosopher 3 grabs chopstick 3 to this LEFT
Philosopher 3 grabs chopstick 4 to this RIGHT
Philosopher 1 is now HUNGRY
Philosopher 3 starts EATING for time 0.572829
Philosopher 4 is now HUNGRY
Philosopher 1 releases chopstick 2 to this RIGHT
Philosopher 1 releases chopstick 1 to this LEFT
Philosopher 5 grabs chopstick 1 to this RIGHT
Philosopher 5 starts EATING for time 0.857843
Philosopher 3 releases chopstick 4 to this RIGHT
Philosopher 3 releases chopstick 3 to this LEFT
Philosopher 4 grabs chopstick 4 to this LEFT
Philosopher 4 grabs chopstick 0 to this RIGHT
Philosopher 4 starts EATING for time 0.783497
Philosopher 1 starts THINKING for 0.308573
Philosopher 5 releases chopstick 1 to this RIGHT
Philosopher 4 releases chopstick 0 to this RIGHT
Philosopher 4 releases chopstick 4 to this LEFT
Philosopher 1 grabs chopstick 1 to this LEFT
Philosopher 3 starts THINKING for 0.086635
Philosopher 1 grabs chopstick 2 to this RIGHT
Philosopher 1 starts EATING for time 0.015005
The futex facility returned an unexpected error code.Aborted
There also one issue as you can see there are no interactions by Philosopher 0 and 2 why is this happening.
Running it in the GDB I get this information
Thread 6 "part22" received signal SIGABRT, Aborted.
[Switching to Thread 0x7ffff57eb700 (LWP 12247)]
0x00007ffff7825428 in __GI_raise (sig=sig@entry=6)
at ../sysdeps/unix/sysv/linux/raise.c:54
54 ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
You're passing the address of your main()
loop variable i
to each thread:
pthread_create(&philosopher[i],NULL, Philospoher_behaviour ,&i)
By the time your thread executes
int id = *(int *)param;
the value in i
may have changed.