segmentation fault in multithread program - c

Question

I'm trying to write a program that uses 3 threads with a shared memory. the shared memory is an array with 101 values. the first value shared memory[0](initialized to 0) is status value which determines which operation should take place. the three threads do
The first one should fill the shared memory array with 100 random values. and set the status value to 1.
The second should print the product of the 100 random values (from index 1 to 100). and set the status value to 2.
The third should print the average of the 100 random variables. and set the status value to 0. so that thread one fill the shared memory with different random variables.
this is my code
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>
unsigned int product=0;
float avg=0;
int* shared_memory;
int status=0;
void productAllThread();
void averageAllThread();
void *parentProcess();
void *prodAll();
void *avgAll();
void initializeArray();
int main(int argc, const char * argv[])
{
time_t t;
key_t key = 9876;
// Create shared memory area
int shm_id = shmget(key, sizeof(int)*101, IPC_CREAT | 0666);
// initialize the random variable
srand((unsigned) time(&t));
// Create shared memory
shared_memory=shmat(shm_id, NULL, 0);
//create threads
pthread_t tid1, tid2, tid3;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_create(&tid1, &attr, parentProcess, NULL);
pthread_create(&tid2, &attr, prodAll, NULL);
pthread_create(&tid3, &attr, avgAll, NULL);
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
pthread_join(tid3, NULL);
return 0;
}
void initializeArray() {
shared_memory[0]=0;
status=shared_memory[0];
int i= 0;
printf("Initial Array:{");
for(i=1; i<100; i++)
{
shared_memory[i]=rand()% 50;
printf("%d,", shared_memory[i]);
}
printf("}\n");
}
void *parentProcess()
{
while(1)
{
status=shared_memory[0];
if(status==0) {
// initialize array
initializeArray();
shared_memory[0]=1;
} else {
sleep(10);
}
}
}
void averageAllThread() {
while(1) {
status=shared_memory[0];
if(status==2)
{
avgAll();
wait(NULL);
printf("Avg:%.2f\n", avg);
shared_memory[0]=0;
} else {
sleep(5);
}
}
}
void productAllThread() {
while(1){
status=shared_memory[10];
if (status==1)
{
prodAll();
wait(NULL);
printf("Sum:%d\n",product);
shared_memory[0]=2;
} else {
sleep(5);
}
}
}
void *prodAll()
{
while(1){
int i=1;
product=0;
for(i=1; i<100; i++)
{
product=product+shared_memory[i];
}
}
}
void *avgAll()
{
while(1){
int i=0;
avg=0;
for(i=1; i<100; i++)
{
avg=avg+shared_memory[i];
}
avg=avg/100;
}
}
when I run it in the terminal, it gives me this error
"Segmentation fault: 11"
what might cause this type of errors? If this error is fixed will the program work fine to do the job I want it to do?

Answer 1

I found a few problems in your program:
You are calling the wrong functions to start your threads:
pthread_create(&tid1, &attr, parentProcess, NULL);
pthread_create(&tid2, &attr, prodAll, NULL);
pthread_create(&tid3, &attr, avgAll, NULL);
Should be:
pthread_create(&tid1, &attr, parentProcess, NULL);
pthread_create(&tid2, &attr, productAllThread, NULL);
pthread_create(&tid3, &attr, averageAllThread, NULL);
You have a few calls to wait() like this:
wait(NULL);
You should remove all of them.
The while loops in avgAll() and prodAll() should be removed since there are already while loops in the callers of those functions.
The call to srand() should be made from parentProcess() otherwise it might not affect the rand() calls in that thread.