Initial commit - Begun rewrite of website generator

pages/2013-12-13-Debug-Filesystem.md

@@ -0,0 +1,95 @@
+layout: post
+title: "The debug file system"
+subtitle: "It's still in the kernel!"
+tags: [osdev]
+
+This far in my vfs rewrite, I've made a [vfs mount tree](/blog/2013/12/Virtual-File-System2/) and written some [file operation syscalls](/blog/2013/12/VFS-syscalls/). Now it's time to actually use this, by writing a filesystem that can be mounted and then manipulated through the syscall interface.
+
+The filesystem I want to start with is what I call the `debug` file.
+
+It is a single file, which mounts to `/` for now (later `/dev/debug`)
+and cannot be read from. Writing to it will print the written text on
+the screen.
+
+Three functions are needed for newlib `fprintf` to be able to write to
+it: `stat`, `isatty` and `write`. Those are all rather simple, since
+they won't need to keep track of which file we're referencing.
+
+	uint32_t debug_stat(INODE node, struct stat *st)
+	{
+		memset(st, 0, sizeof(struct stat));
+		st->st_mode = S_IFCHR;
+		return 0;
+	}
+
+I don't care much about the stat for the debug file. Maybe I'll add some
+creation time or so later...
+
+	uint32_t debug_isatty(INODE node)
+	{
+		return 1;
+	}
+
+The debug output is a terminal.
+
+	uint32_t debug_write(INODE node, void *buffer, uint32_t size, uint32_t offset)
+	{
+		char *buf = calloc(size + 1, 1);
+		memcpy(buf, buffer, size);
+		kdbg_puts(buf);
+		free(buf);
+		return size;
+	}
+
+`kdbg_puts` is a function I wrote
+[a long time ago](/blog/2012/06/Kernel-Debug-Functions/) which prints a
+string to the screen. The first two lines are just to make sure that the
+data is null-terminated.
+
+With this, I can define a driver for the "debug device":
+
+	vfs_driver_t debug_driver = 
+	{
+		0, // open
+		0, // close
+		0, // read
+		debug_write, // write
+		0, // link
+		0, // unlink
+		debug_stat, // stat
+		debug_isatty, // isatty
+		0, // mkdir
+		0, // readdir
+		0 // finddir
+	};
+
+And then write a function to setup the device:
+
+	INODE debug_dev_init()
+	{
+		INODE node = calloc(1, sizeof(vfs_node_t));
+		strcpy(node->name, "debug");
+		node->d = &debug_driver;
+		node->type = FS_CHARDEV;
+		return node;
+	}
+
+Then, to activate it, all I need to do is add
+
+	vfs_init();
+	vfs_mount("/", debug_dev_init());
+
+in my kernel boot code. After that I can use the standard library
+functions:
+
+	FILE *dbg = fopen("/", "w");
+	fprintf(dbg, "Hello, world!\n");
+
+or even:
+
+	fopen("/", "r");
+	fopen("/", "w");
+	printf("Hello, world!\n");
+
+That's it for this time. Next time, we'll do some piping!
+