Question

I have a data structure in c that is an array of struct pointers:

struct trex *trex_arr[128];

struct trex{
    struct trex *next;
    char name[LEN];
    unsigned int id; 
    int groups[LEN];
    struct list *filenames; //linked list
    unsigned int fn_len;
};

I want to write the array to file. I understand that I need to write the object in the “next” pointer and each element in the linked list as well, but how do I do it so that I can read it back successfully into my original array (there is chaining in the array as well)?

Answer 1

Offhand, the simplest way to do it is to write the structure out sequentially so that:

[trex item #1]
[list item #1]
[list item #2]
[list item #3]
[trex item #2]
[list item #1]
[list item #2]
[list item #3]
[list item #4]
etc...

(Of course, this is just sort of the jist of it. These are binary writes and this is a visual only.)

I’m hoping that fn_len describes the number of items in the linked list struct list *filenames. If so, this is a snap.

On writing:

1. Write out the first structure in the trex linked list
2. Loop and write out the struct list *filenames from that first trex structure one at a time
3. Repeat #1 with the next item in the trex linked list until the list is exhausted.

On reading, remember: all of your pointers are going to be worthless initially. You have to stitch them together yourself. But the structures are arranged in the correct order in the file.

1. Read the first trex structure. You know its size, reading it isn’t a problem. Fix the next pointer to nothing.
2. Loop and read each of the struct list *filenames one at a time. You know how many they are because of fn_len in the structure you just read. Stitch together that linked list and attach it to the trex structure.
3. Repeat #1 with the next trex structure until EOF, join the structure onto the end of the linked list as appropriate.

And that should take care of it.