Program goes infinite loop when semaphore is used (Deadlock) - c

Question

The program is being executed correctly without semaphores, but when I use semaphores for sync, the program goes into an infinite loop (deadlock)!. I can't find out why this is happening. After being executed the program get inputs from the user and goes to an infinite loop; *This is seeking tutor problem.
Here are my functions with semaphores ->
void* CoOrdinator(void* arg)
{
sem_wait(&co);
// sorting
std temp;
for (int i = 0; i < chair-1; i++)
{
for (int j = 0; j < chair-i-1; j++)
{
if (buffer[j].priority == buffer[j+1].priority)
{
continue;
}
else if(buffer[j].priority>buffer[j+1].priority)
{
temp = buffer[j];
buffer[j] = buffer[j+1];
buffer[j+1] = temp;
}
}
}
// End Sorting
printf("I'm a CoOrdinator\n");
for (int i = 0; i < chair; i++)
{
printf("Student - %d -",buffer[i].id);
printf("Priority - %d\n",buffer[i].priority);
}
sem_post(&tc);
}
// Teacher
void* Teacher(void* ox)
{
for (int i = 0; i < teachers; i++)
{
sem_wait(&tc);
pthread_mutex_lock(&mutex);
tcObj[i] = buffer[out];
out = (out+1)%chair;
printf("Student - %d is in classroom. Priority: %d\n",tcObj[i].id,tcObj[i].priority);
pthread_mutex_unlock(&mutex);
sem_post(&st);
}
// // Studing with Teacher
// for (int i = 0; i < teachers; i++)
// {
// printf("Stuying with Teacher - %d", i);
// printf("Student - %d -",tcObj[i].id);
// printf("Priority - %d\n",tcObj[i].priority);
// }
}
// Student
void* Student(void *px)
{
// we have to use semaphore
for (int i = 0; i < students; i++)
{
sem_wait(&st);
pthread_mutex_lock(&mutex);
buffer[in] = stObj[i];
in = (in+1)%chair;
pthread_mutex_unlock(&mutex);
sem_post(&co);
// if((int*)px == students)
// {
// break;
// }
}
}
and the rest of the program is
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include<semaphore.h>
// Global Variables
int teachers, students, chair, help;
// Student Structure
typedef struct Student
{
int id;
int priority;
}std;
// Objects Array
std *stObj, *tcObj, *buffer;
// Buffer
int in = 0, out = 0;
// Semaphores
sem_t st, tc, co;
pthread_mutex_t mutex;
main function
int main(int argc, char const *argv[])
{
// Getting Inputs
printf("Enter The Number Of Students: ");
scanf("%d",&students);
printf("Enter The Number Of Teachers: ");
scanf("%d",&teachers);
printf("Enter The Number Of Chair: ");
scanf("%d",&chair);
// Buffer Array
buffer = (std*)malloc(chair * sizeof(std));
stObj = (std*)malloc(students * sizeof(std));
tcObj = (std*)malloc(students * sizeof(std));
//Declaring Thread Variables
pthread_t tc_thread[teachers], st_thread[students], co_thread;
//Getting Priorities
for(int i=0; i<students; i++)
{
stObj[i].id = i+1;
printf("Enter The Priority of Student: ");
scanf("%d",&stObj[i].priority);
}
// Mutex & Semaphores
pthread_mutex_init(&mutex, NULL);
// sem_init(&empty,0,chair);
// sem_init(&full,0,0);
sem_init(&st, 0, 1);
sem_init(&tc, 0, 1);
sem_init(&co, 0, 1);
// Creating Threads
for (int i = 0; i < students; ++i)
{
pthread_create(&st_thread[i],NULL,Student,(void*)&stObj[i].id);
pthread_join(st_thread[i],NULL);
}
for (int i = 0; i < teachers; ++i)
{
pthread_create(&tc_thread[i],NULL,Teacher,(void*)i);
pthread_join(tc_thread[i],NULL);
}
pthread_create(&co_thread,NULL,CoOrdinator,NULL);
pthread_join(co_thread,NULL);
// Destroying Semaphores & Mutex
pthread_mutex_destroy(&mutex);
sem_destroy(&st);
sem_destroy(&tc);
sem_destroy(&co);
return 0;
}
is there any error?

Answer 1

The pthread_join is your problem. Once the first loop of student has run, it has consumed the st semaphore and there is nothing to replenish it.
for (int i = 0; i < students; ++i)
{
pthread_create(&st_thread[i],NULL,Student,(void*)&stObj[i].id);
pthread_join(st_thread[i],NULL);
}
Remove the pthread_join from these loops; and after all threads have been created, add loops to join them. So your main would look more like:
// Creating Threads
for (int i = 0; i < students; ++i)
{
pthread_create(&st_thread[i],NULL,Student,(void*)&stObj[i].id);
}
for (int i = 0; i < teachers; ++i)
{
pthread_create(&tc_thread[i],NULL,Teacher,(void*)i);
}
pthread_create(&co_thread,NULL,CoOrdinator,NULL);
pthread_join(co_thread,NULL);
for (int i = 0; i < students; ++i)
{
pthread_join(st_thread[i],NULL);
}
for (int i = 0; i < teachers; ++i)
{
pthread_join(tc_thread[i],NULL);
}
Also, unrelated to your immediate problem, this allocation: tcObj = (std*)malloc(students * sizeof(std)); should likely be tcObj = malloc(teachers * sizeof(*tcObj)); Note you had the wrong number; but also you should not cast malloc, and it is usually better to use sizeof(*var_you_are_assigning_malloc_to) since it is less fragile and self documenting.
Make these changes, and your program runs for a lot longer before hanging...