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public/blog/old3ds_romfs.md

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+---
+title: 3DS Programming - Using RomFS
+description: A guide to using RomFS on the 3DS. (Old)
+date: 2025-01-01
+tags: ['3ds', 'programming', 'c', 'devkitpro', 'old']
+previous: old3ds_helloworld.md
+next: old3ds_touchscreen.md
+---
+
+# Accessing ROM files with RomFS
+
+> [!warning]
+> This guide of mine is old - Like 3 years old at the time of posting. I prefer to keep it up, but I don't think it very highly - It should still work though.
+> I might make a new one in the future.
+> I'm doing some alterations at least to make it more readable.
+> It doesn't teach why things are done very well is my primary issue with it.
+
+RomFS allows us to access files stored on the ROM file, cartridge, or application.
+
+It can be used to read things such as graphics assets, audio data, and other data.
+
+## RomFS folder
+
+You may have noticed while building that there is a folder named 'romfs' in your project:
+
+```
+- Project
+  - build
+  - **romfs**
+  - source
+  - Makefile
+  - Project.3dsx
+```
+
+(If it is not already present, create it now.)
+
+This is where all the program files are stored. These files are mounted on 'romfs:/' when the app is running.
+
+If you have any sprites, they will appear in the 'gfx' directory inside of 'romfs:/', and are accessed accordingly.
+
+Create a text file inside of 'romfs', name it something like 'sample.txt' (or 'sample' if you have file extensions off):
+
+```
+- Project
+  - build
+  - **romfs**
+    - sample.txt
+  - source
+  - Makefile
+  - Project.3dsx
+```
+
+This file will appear as 'romfs:/sample.txt' in the program.
+
+## Using RomFS
+
+We will initialize RomFS, check if a file exists, and then deinitialize.
+
+### Initializing
+
+Initializing RomFS is trivial - It only takes one simple function to do it:
+
+```c
+// Initialize the console
+// ...
+
+// Initialize RomFS
+romfsInit();
+
+// Main loop
+// ...
+```
+
+### Using
+
+To access a file, we do basically the exact same thing we could do on a home computer in C or C++, just with a different drive.
+
+We will check if the file exists, first by trying to open it, and checking if it failed to open. This is what you would normally do in C:
+
+```c
+// Check if the file exists
+FILE* file = fopen("romfs:/sample.txt", "r");
+
+if (file == NULL) {
+    // File does not exist
+    puts("File does not exist.");
+} else {
+    // File exists
+    puts("File exists.");
+}
+
+// Close the file
+fclose(file);
+
+// Main loop
+```
+
+You can play with this by adding or removing sample.txt from the 'romfs' folder.
+
+### De-initializing
+
+Like initializing RomFS, it only takes another simple function to de-initialize:
+
+```c
+// Main loop
+// ...
+
+// De-initialize RomFS
+romfsExit();
+
+// Return...
+```
+
+And that is all you need to access files on the ROM.
+
+## Wrapping up
+
+Our code should now look like this:
+
+```c
+#include <stdio.h>
+#include <3ds.h>
+
+int main() {
+    // Initialize the console
+    gfxInitDefault();
+    consoleInit(GFX_BOTTOM, NULL);
+
+    // Initialize RomFS
+    romfsInit();
+
+    // Check if the file exists
+    FILE* file = fopen("romfs:/sample.txt", "r");
+
+    if (file == NULL) {
+        // File does not exist
+        puts("File does not exist.");
+    } else {
+        // File exists
+        puts("File exists.");
+    }
+
+    // Close the file
+    fclose(file);
+
+    // Main loop
+    while(aptMainLoop()) {
+        // Scan for input
+        hidScanInput();
+        u32 kDown = hidKeysDown();
+
+        // Close the program if START is pressed
+        if (kDown & KEY_START) break;
+    }
+
+    // De-initialize RomFS
+    romfsExit();
+
+    return 0;
+}
+```