pack_fopen(3alleg4)

💖 pack_fopen — the Allegro manual, but make it cute 💖

okay bestie, so this is a page from the Allegro manual (you know, pack_fopen(3alleg4)), and it's literally about opening files. yes. opening files. but with ✨vibes✨. Allegro is a game programming library, so obviously it has opinions about how you read your data. sit down, grab a boba, we're doing this 🧋

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✨ NAME ✨

pack_fopen — opens a file according to mode. it's giving "Allegro game programming library," no cap. 🎀

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✨ SYNOPSIS ✨

#include <allegro.h>

PACKFILE *pack_fopen(const char *filename, const char *mode);

look at that signature. chef's kiss. 👑

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✨ DESCRIPTION ✨

so pack_fopen opens a file according to mode, and mode can contain any of these flags, periodt:

• r — open the file for reading. classic. the bread and butter. 🍞
• w — open the file for writing, and honestly? it overwrites any existing data. no hesitation. it's giving main character energy, it just takes what it wants 💅
• p — open the file in packed mode. data gets compressed as it's written to the file, and automatically uncompressed during read operations. lowkey magic ✨ BUT — and listen carefully bestie — files created in this mode will produce garbage if they're read without this flag being set. like, garbage garbage. 💀
• ! — open the file for writing in normal, unpacked mode, but adds the value F_NOPACK_MAGIC to the start of the file. why? so that you can later open it in packed mode and Allegro will automatically detect that the data doesn't need to be decompressed. it's a little note-to-self for future you. 🫶

and instead of these flag characters, you can use one of these constants as the mode parameter:

• F_READ
• F_WRITE
• F_READ_PACKED
• F_WRITE_PACKED
• F_WRITE_NOPACK

iconic. same energy, different font. 🦋

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🌈 the "magic" filenames 🌈

so the packfile functions also understand several "magic" filenames that are used for special purposes. these are:

• # — read data that has been appended to your executable file with the exedat utility, as if it was a regular independent disk file. sneaky. love that for us. 🕵️
• filename.dat#object_name — open a specific object from a datafile, and read from it as if it was a regular file. and bestie, you can treat nested datafiles exactly like a normal directory structure — for example you could open filename.dat#graphics/level1/mapdata. it's giving file system but make it ✨fancy✨
• #object_name — a combination of the above two, reading an object from a datafile that has been appended onto your executable. two tricks, one filename. obsessed. 🔥

with these special filenames, the contents of a datafile object or appended file can be read in an identical way to a normal disk file, so any of the file access functions in Allegro (eg. load_pcx() and set_config_file()) can be used to read from them. cute right?

⚠️ but ngl, you can't write to these special files. the fake file is read only. sorry bestie 😭

also — and this is important — you must save your datafile uncompressed or with per-object compression if you are planning on loading individual objects from it. otherwise there will be an excessive amount of seeking when it is read, and that's just not the vibe. it's giving lag. it's giving pain. 💀

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🎀 the object format tea 🎀

finally, be aware that the special Allegro object types aren't the same format as the files you import the data from. when you import data like bitmaps or samples into the grabber, they are converted into a special Allegro-specific format — but the # marker file syntax reads the objects as raw binary chunks.

this means that if, for example, you want to use load_pcx() to read an image from a datafile, you should import it as a binary block rather than as a BITMAP object. noted. written down. in my notes app. 📝

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🌸 example time 🌸

PACKFILE *input_file;

input_file = pack_fopen("scores.dat", "rp");
if (!input_file)
   abort_on_error("Couldn't read `scores.dat'!");

see how the mode is "rp"? reading + packed. and if it comes back empty, abort_on_error gets to do its dramatic little thing. 💅

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✨ RETURN VALUE ✨

on success, pack_fopen() returns a pointer to a PACKFILE structure 🎉 and on error it returns NULL and stores an error code in errno. so check that pointer, bestie, fr fr.

and lowkey important: an attempt to read a normal file in packed mode will cause errno to be set to EDOM. yes, EDOM. that's your sign that you asked for something the file simply cannot give. 🙈

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✨ SEE ALSO ✨

go read these next, they're all in the same friend group:

• pack_fclose(3alleg4)
• pack_fopen_chunk(3alleg4)
• packfile_password(3alleg4)
• pack_fread(3alleg4)
• pack_getc(3alleg4)
• file_select_ex(3alleg4)
• pack_fopen_vtable(3alleg4)
• expackf(3alleg4)

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okay that's the whole page, you ate it, you understood it, we're so proud 🫶 this was Allegro, version 4.4.3, and if you ever forget it, just remember: pack it up, read it out, check your errno, and slay 💖✨🌸 bye bestie!! 🦋