Question

I’m having some trouble with the following:

void BuildList(cs460hwp hw)
{

    FILE* fp;
    fp = fopen("HW2input.dat", "r");
    if(fp == NULL)
    {
        printf("Couldn't open the file.");
        return;
    }
    int numStudents;
    int i;
    bool success;
    char* dueDate = malloc(9*sizeof(char));
    char* course = malloc(7*sizeof(char));
    char* wsuid = malloc(9*sizeof(char));
    char* subDate = malloc(9*sizeof(char));
    double points1 = 0;
    double points2 = 0;
    cs460hwp stuInsert = NULL;
    fscanf(fp, "%d", &numStudents);
    fscanf(fp, "%s", dueDate);
    for(i = 0; i < numStudents; i++)
    {
        stuInsert = malloc(sizeof(cs460hwp));
        fscanf(fp, "%s %s %s %lf", course, wsuid, subDate, &points1);
        strcpy(stuInsert->course, course);
        strcpy(stuInsert->wsuid, wsuid);
        strcpy(stuInsert->subdate, subDate);
        stuInsert->points1 = points1;
        stuInsert->points2 = CalculatePoints(dueDate, subDate, points1);
        stuInsert->nextPtr = NULL;
        if(hw == NULL)
        {
            hw = stuInsert;
        } 
        else
        {
            stuInsert->nextPtr = hw;
            hw = stuInsert;
        }
    }
    free(course);
    free(wsuid);
    free(subDate);
    free(dueDate);
    PrintGrades(hw);
    fclose(fp);
}

struct hwpoints
{
    char course[7];
    char wsuid[9];
    char subdate[9];
    double points1;
    double points2;
    struct hwpoints *nextPtr;
};

typedef struct hwpoints *cs460hwp;

My goal here is to insert every entry to the top of the list. However, whenever I try to assign anything to nextPtr (such as in the else clause), it gets filled with garbage values. They’re mostly truncated versions of old data, which leads me to believe they’re being taken from the heap. I’ve been reading (a lot), but I’m having trouble finding advice on this particular problem.

nextPtr always becomes junk, and nextPtr->nextPtr causes a segfault. For every iteration of the loop. hw remains fine, but its pointer value never gets updated properly.

Even when I’ve attempted to move the memory allocation for the struct into a function, I’ve had the same (or similar) issues.

Can anyone point me in the right direction?

Answer 1

Two problems.

1) As pb2q mentioned, you are passing a pointer to a struct and trying to assign what the arg points to. That’s allowed by the compiler, but it doesn’t do anything for you outside the function. It might be OK in your case if:

void main()
{
   cs460hwp hw = NULL;
   BuildList(hw);
   return;
}

Is the whole of your function. I don’t know the assignment so you need to figure out if that’s acceptable to you or not.

2) The much bigger problem:

stuInsert = malloc(sizeof(cs460hwp)); 

Did you check what sizeof(cs460hwp) comes out to be? it’s 4. You’re allocating enough memory for the size of a pointer, not the size of your structure. I’m pretty sure this is not what you want to do and this is what is killing you. Just for kicks, replace it with malloc(100) and see if your problem goes away. If so you just need to figure out what size you really want. 😉