Dynamically increase array size in Struct "C"

Question

My question is a simple one...I have the following structs declared:

struct Address {
   int id;
   int set;
   char *name;
   char *email;
};

struct Database {
   struct Address rows[512];
};

struct Connection {
   FILE *file;
   struct Database *db;
};

Now having that clear, I initialize my "Database" inside my "Connection" with some dummy Addresses. I later take this database and save it into the file inside my "Connection" struct with:

void Database_write(struct Connection *conn){
   rewind(conn->file);

   int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
      if(rc != 1){
         die("Failed to write database.\n",conn);
      }

   rc = fflush(conn->file);
      if(rc == -1){
         die("Cannot flush database.\n",conn);
      }

Everything works great when I have a predetermined number of rows inside my "Database" struct for my Addresses i.e. 512. But, what if I want to make the number of rows dynamically? As in maybe as a param passed to a function? I have tried using the following...

struct Database {
   struct Address *rows;
};

And allocating space to this pointer with:

conn->db->rows = (struct Address*) malloc(sizeof(struct Address)*max_rows);

With max_rows being a param passed to a function...But, now the problem is that when I go and try to save this to the file inside my "Connection" struct I just save the pointer "struct Address *rows;" and not the data with the space allocated to it. Any suggestions as to how to save this allocated space or have a predetermined array inside a struct and then grow it dynamically?

Thanks in advance!

What does your newly updated write code look like that doesn't work? — Chris Turner
– Chris Turner, Commented May 9, 2017 at 10:50
You have to save the pointer? Were you just dumping the structure to file as bytes? That's a terrible idea. That file is only ever going to be properly read on a subset of machines. What you need is to write a logical representation of the data to file, not just dump bytes as they are in your process's memory. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented May 9, 2017 at 10:51
You'll have to write some serialization routine which parses the structs. — Lundin
– Lundin, Commented May 9, 2017 at 10:52
Do you NEED to cast malloc() to struct Address *? or you just do it because you saw it somewhere else? I am asking this because it would be a lot different if you MUST cast, it would be a whole different language. Also, conn->db->rows might dereference a NULL pointer if you are not careful. And what is die()?, it makes the code look like it's PHP, and it's PHP so, it's PHP. — Iharob Al Asimi
– Iharob Al Asimi, Commented May 9, 2017 at 10:52
All my structs are the same with the exception for my "Database" one which I changed to the one I mentioned using the pointer "struct Address *rows" and try to save this with the same "void Database_write" and when I try to open the file of course my data pointed to by my *rows pointer is not there :/ and my die() function is just a general routine to clean up any allocated memory — Sebastian Serrano
– Sebastian Serrano, Commented May 9, 2017 at 10:53

Gavin Shreeves · Accepted Answer · 2017-05-10 07:23:16Z

You are on the right track with malloc for creating a dynamic number of Addresses.

conn->db->rows = (struct Address*) malloc(sizeof(struct Address)*max_rows);

But then you have a problem writing them out to file in Database_write. This is because the dynamically-allocated structure no longer has the number of rows hardwired into it. You will have to change Database_write to

Pass in how many rows to write.
Adjust your fwrite line to write out all the rows.

You had:

void Database_write(struct Connection *conn)
{
    rewind(conn->file);

    int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
    if(rc != 1){
        die("Failed to write database.\n",conn);
    }
...

You now need something like:

void Database_write(struct Connection *conn, int num_rows)
{
    rewind(conn->file);

    int rc = fwrite(conn->db, sizeof(struct Database), num_rows, conn->file);
    if(rc != num_rows)
    {
        die("Failed to write database.\n",conn);
    }

...

You could also add the number of rows to your database struct to record how many rows are supposed to be in the file:

struct Database 
{
    int num_rows;
    struct Address *rows;
};

In which case you should fwrite the number of rows to file first, then write num_rows of struct Address.

You might also want to look up realloc for changing the number of rows on the fly. Hint - use with care, and pay close attention to the return value.

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