Question

So my problem right now is that for some reason, I designate a pointer as NULL in the first function to be called, but then when I check it later, it’s not NULL anymore.

So I have a few structs, detailed here:

#include <stdio.h>
typedef struct filenode {
  struct filenode *next, *prev;
  const char *name;
} Filenode;

typedef struct foldernode {
  struct foldernode *next, *root, *subdir, *prev;
  Filenode *filenodes;
  const char *name;
} Folders;

typedef struct Filesystem {
  Folders *current;
} Filesystem;

And then I go to my actual file, where I initialize the Filesystem.

This is the initialization function:

void mkfs(Filesystem *files) {
  /* Initializes space for the filesystem itself */
  files = malloc(sizeof(*files));

  if(files == NULL) {
    printf("Memory allocation failed!\n");
    return;
  }
  /* Initializes space for the first root node */
  files->current = malloc(sizeof(files->current));

  if(files->current == NULL) {
    printf("Memory allocation failed!\n");
    return;
  }

  files->current->filenodes = NULL;
}

Now, when I go into the next function, mkdir, and I check if files->current->filenodes = NULL, and it is not NULL anymore. I’m extremely confused right now. And yes, the same *files variable is being passed in to every function.

Answer 1

From the comments, you say you can’t change the signature of mkfs(). I’m going to assume it’s called like this:

 Filesystem files;
 mkfs(&files);

If that’s the case, you don’t need to create space for the structure (since it’s created on the stack just before your mkfs() call), so you can remove this line:

  files = malloc(sizeof(*files));

Removing this line will fix the problem that you can’t see the changes to files after the function has returned. This was causing you trouble since C is always pass by value – the malloc was changing the value of the local copy of the files pointer, meaning that no further changes were seen outside your function.

---

You will also run into trouble with this line:

files->current = malloc(sizeof(files->current));

Since files->current is a pointer to a struct foldernode, the sizeof call only tells you the size of the pointer. You probably meant:

files->current = malloc(sizeof(struct foldernode));

or:

files->current = malloc(sizeof(Folders));