pack_fopen_chunk(3alleg4)

💖 pack_fopen_chunk 💖

okay bestie, so this one's from the Allegro manual — the Allegro game programming library — and it's called pack_fopen_chunk. Strap in. 🎀

✨ NAME ✨

pack_fopen_chunk — opens a sub-chunk of a file. Allegro game programming library. 💅

🌸 SYNOPSIS 🌸

#include <allegro.h>

PACKFILE *pack_fopen_chunk(PACKFILE *f, int pack);

🦋 DESCRIPTION 🦋

So basically, this opens a sub-chunk of a file. Chunks are mainly the datafile code's whole thing, but honestly they can be lowkey useful for your own file routines too. 🫶

A chunk gives you a logical view of part of a file. It can be compressed all on its own as an individual entity, and it'll automatically insert and check length counts so you literally cannot read past the end of the chunk. Obsessed. ✨

The PACKFILE parameter is a file you already opened earlier, and pack is a boolean — if it's non-zero, compression gets turned on for the sub-chunk. 👑

Example time, bestie:

PACKFILE *output = pack_fopen("out.raw", "w!");
...
/* Create a sub-chunk with compression. */
output = pack_fopen_chunk(output, 1);
if (!output)
   abort_on_error("Error saving data!");
/* Write some data to the sub-chunk. */
...
/* Close the sub-chunk, recovering parent file. */
output = pack_fclose_chunk(output);

Now the technical tea 🍓: the data you write to the chunk gets prefixed with two length counts (32-bit, a.k.a. big-endian).

• For uncompressed chunks, both of those lengths are set to the size of the data in the chunk.
• For compressed chunks (the ones you made by setting the pack flag), the first length is the raw size of the chunk, and the second is the negative size of the uncompressed data. not me doing math on a Tuesday 😭

To read the chunk back, it's giving this:

PACKFILE *input = pack_fopen("out.raw", "rp");
...
input = pack_fopen_chunk(input, 1);
/* Read data from the sub-chunk and close it. */
...
input = pack_fclose_chunk(input);

This sequence reads the length counts that were created when the chunk got written, and automatically decompresses the contents if it was compressed. 💜 The length also gets used to stop you reading past the end of the chunk (Allegro will return EOF if you try), and to automatically skip past any unread chunk data when you call pack_fclose_chunk(). iconic behaviour honestly.

And yes, chunks can be nested inside each other — you just keep calling pack_fopen_chunk() over and over. 🌈 When you're writing a file, the compression status is inherited from the parent file, so you only need to set the pack flag if the parent isn't compressed but you want the chunk data packed.

⚠️ okay but listen: if the parent file is already open in packed mode, setting the pack flag means the data gets compressed twice — once as it's written to the chunk, and again as the chunk passes it on to the parent file. double compression, double the chaos 💀

💫 RETURN VALUE 💫

Returns a pointer to the sub-chunked PACKFILE, or NULL if something went wrong (eg. you're using a custom PACKFILE vtable). 🙈

🎀 SEE ALSO 🎀

• pack_fclose_chunk(3alleg4)
• pack_fopen(3alleg4)

and that's the whole page, bestie!! this was Allegro version 4.4.3, and yeah the title's still pack_fopen_chunk(3alleg4) because some things never change 💅✨ slay, go open some chunks, ily 🫶💖