C programming Segmentation fault: 11 Thread issues - c

Question

Why does this error occur when I run my code?
error: RUN FINISHED; Segmentation fault: 11; real time: 3s; user: 0ms; system: 0m
I'm creating 10 threads where each thread is a ticket seller. There is a 10by10 array that holds the seats of the tickets. Depending on the type of the ticket-seller a person will be sold that specific seat.
Is the issue with the pthreads?
/*
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* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
/*
* File: ticketsellers.c
* Author: iantheflyinghawaiian
*
* Created on July 4, 2016, 11:27 AM
*/
#include <stdio.h>
#include <pthread.h>
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
// seller thread to serve one time slice (1 minute)
int theatre[10][10] ;
struct node
{
int info;
struct node *ptr;
}*front,*rear,*temp,*front1;
int count = 0;
/* Create an empty queue */
void create()
{
front = rear = NULL;
}
/* Returns queue size */
void queuesize()
{
printf("\n Queue size : %d", count);
}
/* Enqueing the queue */
void enq(int data)
{
if (rear == NULL)
{
rear = (struct node *)malloc(1*sizeof(struct node));
rear->ptr = NULL;
rear->info = data;
front = rear;
}
else
{
temp=(struct node *)malloc(1*sizeof(struct node));
rear->ptr = temp;
temp->info = data;
temp->ptr = NULL;
rear = temp;
}
count++;
}
/* Displaying the queue elements */
void display()
{
front1 = front;
if ((front1 == NULL) && (rear == NULL))
{
printf("Queue is empty");
return;
}
while (front1 != rear)
{
printf("%d ", front1->info);
front1 = front1->ptr;
}
if (front1 == rear)
printf("%d", front1->info);
}
/* Dequeing the queue */
void deq()
{
front1 = front;
if (front1 == NULL)
{
printf("\n Error: Trying to display elements from empty queue");
return;
}
else
if (front1->ptr != NULL)
{
front1 = front1->ptr;
printf("\n Dequed value : %d", front->info);
free(front);
front = front1;
}
else
{
printf("\n Dequed value : %d", front->info);
free(front);
front = NULL;
rear = NULL;
}
count--;
}
/* Returns the front element of queue */
int frontelement()
{
if ((front != NULL) && (rear != NULL))
return(front->info);
else
return 0;
}
/* Display if queue is empty or not */
void empty()
{
if ((front == NULL) && (rear == NULL))
printf("\n Queue empty");
else
printf("Queue not empty");
}
//Ticket Seller
void * sell(char *seller_type)
{
char seller_type1;
seller_type1 = *seller_type;
int i;
i = 0;
while (i == 0);
{
pthread_mutex_lock(&mutex);
pthread_cond_wait(&cond, &mutex);
pthread_mutex_unlock(&mutex);
// Serve any buyer available in this seller queue that is ready
// now to buy ticket till done with all relevant buyers in their queue
//………………
// Case statements for seller_types
switch(seller_type1)
{
case 'H' :
printf("Seller type: H\n");
i = 1;
break;
case 'M' :
printf("Seller type: M\n");
i = 1;
break;
case 'L' :
printf("Seller type: L\n");
i = 1;
break;
}
}
return NULL; // thread exits
}
void wakeup_all_seller_threads()
{
pthread_mutex_lock(&mutex);
pthread_cond_broadcast(&cond);
pthread_mutex_unlock(&mutex);
}
int main()
{
int i, N;
pthread_t tids[10];
printf("Enter N value of Customers: ");
scanf("%d", &N);
printf("Number of Customers: %d", N);
char seller_type;
// Create necessary data structures for the simulator.
// Create buyers list for each seller ticket queue based on the
// N value within an hour and have them in the seller queue.
// Create 10 threads representing the 10 sellers.
seller_type = 'H';
pthread_create(&tids[i], NULL, sell, &seller_type);
seller_type = 'M';
for (i = 1; i < 4; i++)
pthread_create(&tids[i], NULL, sell, &seller_type);
seller_type = 'L';
for (i = 4; i < 10; i++)
pthread_create(&tids[i], NULL, sell, &seller_type);
// wakeup all seller threads
wakeup_all_seller_threads();
// wait for all seller threads to exit
for (i = 0 ; i < 10 ; i++)
pthread_join(&tids[i], NULL);
// Printout simulation results
//…………
exit(0);
}

Answer 1

As already assumed by #2501: the culprit causing the segfault was the uninitialized variable i in main.
I took the liberty to write a minimal example for your pthread-creating, with the addition of some printf`s.
It compiles without warnings with
gcc -W -Wall threadtest.c -o threadtest -pthread
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
struct seller_type {
char st;
int tid;
};
void *sell(void *arg)
{
struct seller_type *seller_type1 = arg;
int i;
i = 0;
printf("thread #%d: seller_type1->st = %c\n", seller_type1->tid,
seller_type1->st);
// no semicolon after while()
while (i == 0) {
switch (seller_type1->st) {
case 'H':
printf("Seller type: H\n");
i = 1;
break;
case 'M':
printf("Seller type: M\n");
i = 1;
break;
case 'L':
printf("Seller type: L\n");
i = 1;
break;
}
}
printf("thread #%d: Work done\n", seller_type1->tid);
return NULL;
}
int main()
{
int i = 0;
struct seller_type *seller_type1;
pthread_t tids[10];
seller_type1 = calloc(10, sizeof(struct seller_type));
// All error handling ommitted! Yes, ALL!
seller_type1[0].st = 'H';
seller_type1[0].tid = 0;
pthread_create(&tids[0], NULL, sell, &seller_type1[0]);
for (i = 1; i < 4; i++) {
seller_type1[i].st = 'M';
seller_type1[i].tid = i;
pthread_create(&tids[i], NULL, sell, &seller_type1[i]);
}
for (i = 4; i < 10; i++) {
seller_type1[i].st = 'L';
seller_type1[i].tid = i;
pthread_create(&tids[i], NULL, sell, &seller_type1[i]);
}
puts("All threads created");
// wait for all seller threads to exit
for (i = 0; i < 10; i++) {
pthread_join(tids[i], NULL);
printf("Thread %d joined\n", i);
}
puts("All threads joined");
exit(EXIT_SUCCESS);
}
I hope you can build upon that.

Answer 2

You have a data race when you pass seller_type to the newly created thread, while at the same time modify the variable in the main.
The type of the function passed to pthread_create must be: void*(*)(void*), not the type you're using.
Function pthread_join requires a type pthread_t as the first argument, not a pointer to that type, which is what you're passing to it.
The variable i in the main isn't initialized, and is used to index the array tids, in the first pthread_create call.
All of those four problems cause undefined behavior by themselves. I suspect the last one causes the crash.