Segmentation Fault with pointers arrays

Question

I have a Segmentation fault according to my debugger but I don't know why. Here is the code involved. Did I miss something ?

typedef struct Process Process;
struct Process {
    unsigned int m_id;
    unsigned int m_size;
    int m_startAddr;
};

---

    unsigned int** memory = calloc(MEMORY_SIZE, sizeof(unsigned int*));
    Process** processList = calloc(MEMORY_SIZE, sizeof(Process*));
    unsigned int itr;
    unsigned int nb_process = 0;
    int previous_process = -1;



    for(itr = 0; itr < MEMORY_SIZE; ++itr)
    {
      if(memory[itr] != NULL)
      {  
        previous_process = *memory[itr]; // works but risk of integer overflow
        processList[nb_process]->m_id = *memory[itr]; // segfault
        processList[nb_process]->m_size = 1;         
        processList[nb_process]->m_startAddr = itr;

        nb_process++;
     }

    }
 }

EDIT : I tried to make the following changes :

Process* processList = calloc(MEMORY_SIZE,sizeof(Process));
unsigned int** memory = calloc(MEMORY_SIZE, sizeof(unsigned int*));
unsigned int nb_process = 0;
int previous_process = -1;
unsigned int itr;
Process temp;


for(itr = 0; itr < MEMORY_SIZE; ++itr)
    {
        /* if memory unit is not occupied */
        if(memory[itr] != NULL)
        {
            /* if process is not known yet */
            if(*memory[itr] != previous_process)
            {
                previous_process = *memory[itr];
                printf("previous_process %u \n", previous_process);
                temp.m_id = *memory[itr];
                temp.m_size = 1;
                temp.m_startAddr = itr;

                processList[nb_process] = temp;

                nb_process++;

            }
            /* if the process is already known */
            else
            {
                printf("size %u \n", processList[nb_process].m_size);
                processList[nb_process].m_size++;
            }
        }
    }

The output of previous_process is correct. However, the output of size got a problem. First, it has always two values below what it should (14 instead of 16, 30 instead of 32; etc...) at the end of the for loop. Worse, the count start at 0, while it should start at 1, since i initialize temp.m_size with one before copying it into processList. So the copy doesn't work... Why ? Should i use memcpy ?

Answer 1

You do this:

unsigned int** memory = calloc(MEMORY_SIZE, sizeof(unsigned int*));
*memory[0]

You allocated space for an array of pointers, but then you dereference those pointers. But they are all zero (which on most systems is NULL, but in any case is not a valid pointer because it came from nowhere). That can't work.

Answer 2

In addition to John's answer above you are also allocating an array of pointers

Process** processList = calloc(MEMORY_SIZE, sizeof(Process*));
unsigned int nb_process = 0;

You must ensure that each of that array of pointers is initialized with something valid before de-referencing them as they are.