Memory Allocation C with pointers and structures

Question

I'm trying to write to a pointer by accessing it and allocating memory from other function. But after a few iterations on the while statement, core dumped is thrown. I get the data from a file that it's ensured to be in the correct format.

It fails when there are still names to read from the file (checked with the debugger)

Main

Category *category = NULL;
memoryAllocate(&category);

Memory Allocation

memoryAllocate(Category **category){

 int elements;
 char name[NAME];
 int i = 0;
 FILE *fPeces;

  [...]

    //Get the number of elements from the file
    fscanf(fPeces, "%d", &elements);
    *category = (Category *)malloc(sizeof(Category) * elements));
    if(*category == NULL){
        //ERROR
    }
    else{

        while(i < elements){

            fgets(name, NAME, fPeces);
            strcpy(category[i]->categoryName, name);
            i++;

        }
}

Structure

typedef struct categoria {
    char categoryName[NAME];
    int part_num;
    Parts *parts;
} Category;

FILE (first number is the number of categories and the rest are the names)

6
category 1
category 2
category 3
category 4
category 5
category 6

In the while loop condition don't check for feof, ckeck for the number of elements. — isrnick, May 31 '20 at 18:30
I have checked the number of elements and it does not work either. It fails when I still have elements int the file. For example it fails when i == 3 having elements == 6 — Eduardnol, May 31 '20 at 18:56

Carl Norum · Accepted Answer · 2020-05-31T19:08:26.853

1

You're derefencing the wrong thing in your while loop. This line:

strcpy(category[i]->categoryName, name);

Should be:

strcpy((*category)[i].categoryName, name);

Editorial notes:

This problem is why I recommend minimizing the use of nested pointers where possible - it gets confusing fast. Instead of using *category all over the place, allocate that array locally:
```
Category *myArray = malloc(elements * sizeof *myArray);
```
Do your work with it:
```
strcpy(myArray[i].name, name);
```
And then only when you're done, copy that pointer into the output parameter:
```
*category = myArray;
```
while(!feof(... is always wrong.
Don't cast the result of malloc.

edited May 31 '20 at 19:08

answered May 31 '20 at 19:03

Carl Norum

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After changing the nested pointers to a local allocation and copying the output parameter everything is working perfectly, thanks. – Eduardnol May 31 '20 at 19:12
Great - do you understand the problem with the code as you originally had it? – Carl Norum May 31 '20 at 19:15
Yes, it was really confusing the nested pointers and I had some trouble with it. Also with the wrong usage of feof() – Eduardnol May 31 '20 at 20:06

Memory Allocation C with pointers and structures

1 Answers1