Segmentation fault (core dumped) in C - While Using PTHREADS - c

Question

Hello guys,
I have a problem in my code and i do not know how to fix it(A Segmentation fault(core dumped))!
So my teacher wants me to write a program that creates N treads and makes them do some calculations.I have 3 global 2d arrays A,B,C (i have them as pointers because i don not know the size,the user gives it as argument).I try to allocate memory to them in the main function.
So the problem is i get a Segmentation fault when i try to create the treads in "pthread_create(&tid[id],NULL,add,(void *)(long) i);" :(.
I can't figure out why this is happening.I tried using the gdb command but the result was that the problem is in pthread_create.
However when i put in comment the arrays(A,B,C) and the malloc they are using is runs(but the final result is 0).
I am using a virtual box(with Ubuntu inside if that helps :D).
The following code is what i wrote so far:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
long int p,N,Total_Sum;
long int **A,**B,**C;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_barrier_t bar;
void * add(void *arg){
long int i,j,Local_Sum=0;
long int lines,start,end,id;
id = (long int)arg;
lines = N/p;
start = id*lines;
end = start+lines;
for(i=start;i<end;i++){
for(j=0;j<N;j++){
A[i][j] = 1;
B[i][j] = 1;
}
}
for(i=start;i<end;i++){
for(j=0;j<N;j++){
C[i][j] = A[i][j] * B[i][j];
Local_Sum += C[i][j];
printf("C[%ld][%ld] = %ld\n",i,j,C[i][j]);
}
}
pthread_mutex_lock(&mutex);
Total_Sum += Local_Sum;
pthread_mutex_unlock(&mutex);
pthread_barrier_wait(&bar);
pthread_exit(0);
}
int main(int argc, char *argv[]){
long int i,j,id;
pthread_t *tid;
if(argc!=3){
printf("Provide Number Of Threads And Size\n");
exit(1);
}
p = atoi(argv[1]);
tid = (pthread_t *) malloc(p*sizeof(pthread_t));
if(tid == NULL){
printf("Could Not Allocate Memory\n");
exit(1);
}
pthread_barrier_init(&bar,NULL,p);
N = atoi(argv[2]);
A = (long int**) malloc(N*sizeof(long int*));
B = (long int**) malloc(N*sizeof(long int*));
C = (long int**) malloc(N*sizeof(long int*));
for(i=0;i<N;i++){
A[i] = (long int*) malloc(N*sizeof(long int));
B[i] = (long int*) malloc(N*sizeof(long int));
C[i] = (long int*) malloc(N*sizeof(long int));
}
if((A==NULL) || (B == NULL) || (C == NULL)){
printf("Count Not Allocate Memory\n");
exit(1);
}
for(i=0;i<p;i++){
pthread_create(&tid[id],NULL,add,(void *)(long) i);
}
for(i=0;i<p;i++){
pthread_join(tid[id],NULL);
}
for(i=0;i<N;i++){
free(A[i]);
free(B[i]);
free(C[i]);
}
free(A);
free(B);
free(C);
printf("Final Result Is Equal To: %ld\n",Total_Sum);
return 0;
}
******I know it gets a little bit messy because of the mutex and the the barriers but ask me for further specifications :D.******
Thanks!!!!!!

Answer 1

I think the only problem are the indexes in the following lines:
for(i=0;i<p;i++){
pthread_create(&tid[id],NULL,add,(void *)(long) i);
}
for(i=0;i<p;i++){
pthread_join(tid[id],NULL);
}
id has only been declared, but never initialized! Maybe it's just a typo and you wanted to use i as index for the tid
The solutions should be:
for(i=0;i<p;i++){
pthread_create(&tid[i],NULL,add,(void *)(long) i);
}
for(i=0;i<p;i++){
pthread_join(tid[i],NULL);
}

Answer 2

The answer to the source of your core dump problem has already been addressed, but to address the other things you asked, or stated:
1st:
Regarding your statement: i have them as pointers because i don not know the size,the user gives it as argument.
Often times, in C, you can avoid using calloc/malloc in your code by using VLAs instead. Available in C99 and beyond. (see links)
VLA a
VLA b
2nd:
Regarding your statement: I know it gets a little bit messy...
Its really not that messy, but you could consider cleaning up the memory allocation/freeing steps by moving most of the work into a function:
long int **A,**B,**C;
int N;
...
//in main
N = atoi(argv[2]);
A = Create2D(N, N);
B = Create2D(N, N);
B = Create2D(N, N);
...
free2D(A, N);
free2D(B, N);
free2D(C, N);
long ** Create2D(int c, int r)
{
long **arr;
int y = 0;
arr = calloc(c, sizeof(long *));
for(y=0;y<c;y++)
{
arr[y] = calloc((2*y)+1, sizeof(long));
}
return arr;
}
void free2D(long **arr, int c)
{
int i;
if(!arr) return;
for(i=0;i<c;i++)
{
if(arr[i])
{
free(arr[i]);
arr[i] = NULL;
}
}
free(arr);
arr = NULL;
}
Side note:
There is nothing absolutely wrong with your memory statements as they are:
A = (long int**) malloc(N*sizeof(long int*));
However, although C++ requires it, there is no reason to cast the return of malloc ,calloc or realloc when using C. (See discussion here) The following is sufficient (in C):
A = malloc(N*sizeof(long int*));