Segmentation fault in C multithreaded consumer-producer program - c

Question

I'm currently being introduced to the concept of threading programs, and have been given an assignment to simulate a stockmarket using threads and semaphores. Here is the code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <semaphore.h>
#include <pthread.h>
#define NUM_WRITERS 5
#define NUM_READERS 5
#define STOCKLIST_SIZE 10
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
sem_t stop_writers;
typedef struct
{ int readers;
int slots[STOCKLIST_SIZE];
} mem_structure;
int id[NUM_READERS + NUM_WRITERS];
mem_structure *stocklist;
pthread_t thr[NUM_WRITERS + NUM_READERS];
void init(){
sem_init(&stop_writers, 0, 1);
}
void cleanup(int signo) // clean up resources by pressing Ctrl-C
{ sem_destroy(&stop_writers);
printf("Closing...\n");
exit(0);
}
void write_stock(int n_writer)
{
int stock = (int)(rand()%STOCKLIST_SIZE);
int stock_value = 1 + (int)(100.0 * rand()/(RAND_MAX + 1.0));
stocklist->slots[stock] = stock_value;
fprintf(stderr, "Stock %d updated by BROKER %d to %d\n", stock, n_writer, stock_value);
}
void* writer(void* id){
int my_id = *((int*) id);
int i = my_id;
while (1)
{
pthread_mutex_lock(&mutex);
sem_wait(&stop_writers);
write_stock(i);
pthread_mutex_unlock(&mutex);
sem_post(&stop_writers);
sleep(1);
++i;
}
}
void read_stock(int pos, int n_reader){
fprintf(stderr, "Stock %d read by client %d = %d.\n", pos, n_reader, stocklist->slots[pos]);
}
void* reader(void* id){
int my_id = *((int*) id);
int i = my_id;
while (1)
{ sem_wait(&stop_writers);
read_stock((int)(rand()%STOCKLIST_SIZE), i);
sem_post(&stop_writers);
sleep(1 + (int) (3.0 * rand() / (RAND_MAX + 1.0)));
++i;
}
}
void monitor() // main process monitors the reception of Ctrl-C
{
struct sigaction act;
act.sa_handler = cleanup;
act.sa_flags = 0;
if ((sigemptyset(&act.sa_mask) == -1) ||
(sigaction(SIGINT, &act, NULL) == -1))
perror("Failed to set SIGINT to handle Ctrl-C");
while(1){
sleep(5);
printf("Still working...\n");
}
exit(0);
}
int main(void)
{ int i, j;
init();
for (i = 0; i < NUM_WRITERS; ++i){
id[i] = i;
pthread_create(&thr[i], NULL, writer, &id[i]);}
for (j = i; j < NUM_READERS; ++j){
id[j] = j;
pthread_create(&thr[j], NULL, reader, &id[j]);}
stocklist->readers = 0;
pthread_exit(NULL);
monitor();
return 0;
}
This is giving me a segmentation fault pretty much as soon as main() begins, with the creation of a thread... Since it's not code that I've created from root, I'm having trouble backtracing the error and fixing it - it would be great if there was someone who could take a look and maybe give me some hints
Thank you!
EDIT
Fixed the problem, thanks to your help :)
I initialized stocklist on init(),
void init(){
sem_init(&stop_writers, 0, 1);
stocklist = (mem_structure*)malloc(sizeof(mem_structure));
}
And changed the second cicle in main(),
for (j = i; j < NUM_READERS+NUM_WRITERS; ++j){
id[j] = j;
pthread_create(&thr[j], NULL, reader, &id[j]);}
Thanks

Answer 1

Here's what I did to track down the problem:
gcc -g program.c -lpthread
gdb a.out
run
Output:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7feeb70 (LWP 23060)]
0x08048834 in write_stock (n_writer=0) at program.c:44
44 stocklist->slots[stock] = stock_value;
Looks like the problem is here:
stocklist->slots[stock] = stock_value;