More advanced character device with read, write, open and release

chardev/Makefile

@@ -0,0 +1,23 @@
+# This is a standard makefile for building a kernel module.
+# It looks a bit backwards compared to a normal makefile, but that's because
+# the kernel supplies its own sub-makefile that we use to build the module.
+# 
+# The M= option tells the kernel's makefile where to find the module's source
+# files, and the -C option sets the context directory for make.
+#
+# The obj-m variable tells the kernel's makefile what the module's object file
+# should be called. In this case, it's demo_module.o. This is how the sub-makefile
+# infers the name of the module's source file.
+# 
+# This build is highly sensitive to compiler version, and requires the system gcc
+# to be the same version as the gcc that built the current kernel.
+#
+# For more info, see: https://www.kernel.org/doc/html/latest/kbuild/modules.html
+
+obj-m = chardev.o
+
+all:
+	make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) modules
+
+clean:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

chardev/chardev.c

@@ -0,0 +1,113 @@
+#include "linux/fs.h"
+#include "linux/types.h"
+#include <linux/gpio/consumer.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+/*
+ * Last compiled with Linux 6.14.5-300.fc42.x86_64
+ */
+
+/* Meta Info */
+MODULE_DESCRIPTION("Chardev kernel module.");
+MODULE_VERSION("0.0.1");
+MODULE_AUTHOR("Imbus");
+MODULE_LICENSE("GPL");
+
+// Since were dynamically assigned a major device number, we will store it here
+static int major;
+static char text[64];
+
+// Regular forward decls
+static ssize_t my_read(struct file *f, char __user *u, size_t l, loff_t *o);
+static ssize_t my_write(struct file *, const char __user *, size_t, loff_t *);
+static int my_open(struct inode *, struct file *);
+static int my_release(struct inode *, struct file *);
+
+// See: https://elixir.bootlin.com/linux/v6.14.5/source/include/linux/fs.h#L2131
+static struct file_operations fops = {
+    .read = my_read, .write = my_write, .open = my_open, .release = my_release};
+
+static ssize_t my_write(struct file *filp, const char __user *user_buf,
+                        size_t len, loff_t *off) {
+  printk(KERN_INFO "hello_cdev - Write is called by %s...\n", current->comm);
+
+  int not_copied, delta,
+      to_copy = (len + *off) < sizeof(text) ? len : (sizeof(text) - *off);
+
+  if (*off >= sizeof(text))
+    return 0;
+
+  not_copied = copy_from_user(text, user_buf, to_copy);
+  delta = to_copy - not_copied;
+
+  if (not_copied)
+    pr_warn("hello_cdev - Could only copy %d bytes\n", delta);
+
+  *off = delta;
+
+  return delta;
+}
+
+static ssize_t my_read(struct file *filp, char __user *user_buf, size_t len,
+                       loff_t *off) {
+  printk(KERN_INFO "hello_cdev - Read is called\n");
+
+  int not_copied, delta,
+      to_copy = (len + *off) < sizeof(text) ? len : (sizeof(text) - *off);
+
+  if (*off >= sizeof(text))
+    return 0;
+
+  not_copied = copy_to_user(user_buf, text, to_copy);
+  delta = to_copy - not_copied;
+
+  if (not_copied)
+    pr_warn("hello_cdev - Could only copy %d bytes\n", delta);
+
+  *off = delta;
+
+  return delta;
+}
+
+static int my_open(struct inode *inode, struct file *filp) {
+  pr_info("hello-cdev - Major %d, Minor %d\n", imajor(inode), iminor(inode));
+
+  pr_info("hello-cdev - filp->f_pos: %lld\n", filp->f_pos);
+  pr_info("hello-cdev - filp->f_mode: 0x%x\n", filp->f_mode);
+  pr_info("hello-cdev - filp->f_flags: 0x%x\n", filp->f_flags);
+
+  return 0;
+}
+
+static int my_release(struct inode *inode, struct file *filp) {
+  pr_info("hello-cdev - Closed \n");
+  return 0;
+}
+
+/**
+ * @brief Called on load
+ */
+static int __init demo_mod_init(void) {
+  major = register_chrdev(0, "hello_cdev", &fops);
+
+  if (major < 0) {
+    printk(KERN_INFO "hello_cdev - Error registering chrdev\n");
+    return major;
+  }
+
+  printk(KERN_INFO "hello_cdev - Major nbr %d loaded successfully...", major);
+  return 0;
+}
+
+/**
+ * @brief Called on unload
+ */
+static void __exit demo_mod_exit(void) {
+  unregister_chrdev(major, "hello_cdev");
+  printk(KERN_INFO "hello_cdev - unloaded successfully...");
+}
+
+module_init(demo_mod_init);
+module_exit(demo_mod_exit);

chardev/test.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+int main(int argc, char **argv) {
+	int fd;
+
+	if (argc < 2) {
+		printf("I need the file to open as an argument!\n");
+		return 0;
+	}
+
+	fd = open(argv[1], O_RDONLY);
+
+	if (fd < 0) {
+		perror("open");
+		return fd;
+	}
+
+	close (fd);
+
+	fd = open(argv[1], O_RDWR | O_SYNC);
+
+	if (fd < 0) {
+		perror("open");
+		return fd;
+	}
+
+	close (fd);
+
+	fd = open(argv[1], O_WRONLY | O_NONBLOCK);
+
+	if (fd < 0) {
+		perror("open");
+		return fd;
+	}
+
+	close (fd);
+	return 0;
+}