Question

I am having a problem with my linked list. I’m pretty sure it is my pointers being off, or I didn’t pass a pointer in the correct way as I am new to c. Structs are also new to me, and c++ is the language I am used to and there are more differences than I was aware of. I could do this program in c++ in no time but anyway here is my code.

void add_process(struct process new_process, struct process *head, struct process *current){

    new_process.next = NULL;

    if(head == NULL){
        head = &new_process;
        current = head;
        head->next = NULL;
    }
    else if(new_process.timeNeeded < head->timeNeeded){
        temp = head->next;
        head = &new_process;
        new_process.next = temp;
    }
    else{
        current = head;
        while(new_process.timeNeeded > current->timeNeeded){
            temp = current;
            current = current->next;
        }
        temp->next = &new_process;
        new_process.next = current;
    }
}

I am reading values from a file into the process, the only one I am currently using is timeNeeded which is an int. And I am trying to order the list by shortest timeNeeded first.

int main(){
    FILE *readfile;
    readfile = fopen("data.txt","r");


    head = NULL;
    current = NULL;

    while(fscanf(readfile, "%s %i %i %i", 
        &new_process.processName, &new_process.arrivalTime, 
            &new_process.timeNeeded, &new_process.priority) != EOF)  {

                add_process(new_process, head, current);
      }
    current = head;

    while(current->next != NULL){
        printf("%s %i %i %i\n", new_process.processName, new_process.arrivalTime, new_process.timeNeeded, new_process.priority);
        current = current->next;
    }

    return 0;
}

The program crashes at the print which is not the problem. The first problem is that it my program enters the if(head==NULL) loop everytime and inserts there. So head is probably never being changed but I am not sure how to fix that, I’m pretty sure its a double pointer but not positive. And I am also am sure there are other problems so if you could point me into the correct direction, and if I’m doing anything completely wrong let me know.

EDIT: OK so after adding the pointer to head I get an error at head->next = NULL saying “expression must have pointer-to-class type.” Tried adding a * before head but didn’t seem to help. Anyone know how to fix it?

Answer 1

Your add_process function here:

void add_process(struct process new_process, 
                 struct process *head, 
                 struct process *current)

Takes whatever pointer you pass into it by-value. That means after your call in the while loop here:

while(fscanf(readfile, "%s %i %i %i", 
    &new_process.processName, &new_process.arrivalTime, 
    &new_process.timeNeeded, &new_process.priority) != EOF)  
{

        add_process(new_process, head, current);
}

head will still be NULL because it never got changed. To have it actually change head pointer and not some other pointer modify your add_process to take a double level pointer:

void add_process(struct process new_process, 
                 struct process **head, 
                 struct process *current)

Another problem with your above code is that the new_process argument is taken by value as well. So this is a temporary copy of whatever process you passed in. Once add_process returns, new_process goes out of scope. Now that means you have a dangling pointer in your linked list pointing to invalid memory.

To fix this you should use malloc to dynamically allocate the memory and then make a copy of new_process. Then have your linked list point to the malloc‘ed process. Objects created on the heap with malloc will persist until it’s freed.

Here’s a quick example to give you an idea:

typedef struct process Process;
void add_process(Process new_process, Process **head, Process *current)
{
    Process *new_proc_copy = (Process *)malloc( sizeof(Process) );
    // now copy over the stuff from 
    // new_process over to this one
    memcpy((char *)new_proc_copy, (char *)new_proc, sizeof(Process));

    if(*head == NULL)
    {
        *head = new_process_copy;
        current = *head;
        (*head)->next = NULL;
    }
    else if(new_process.timeNeeded < head->timeNeeded)
    {
        // handle this case
    }
    else
    {
        // handle rest of your stuff
    }
}

Don’t forget to free the malloc’ed memory when done. This is best done in your process cleanup function — the destructor equivalent in C++ only you have to call it manually.