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+2
99-fakenmea.rules
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+#Rules file for the ebbchar device driver
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+KERNEL=="nmea", SUBSYSTEM=="nmea", MODE="0666"
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Makefile
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+obj-m+=fakenmea.o
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+
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+all:
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+	make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) modules
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+	$(CC) readnmea.c -o readnmea
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+clean:
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+	make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) clean
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+	rm readnmea
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fakenmea.c
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+/**
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+ * @file   fakenmea.c
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+ * @author Yanik Cawidrone
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+ * @date   1st July 2017
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+ * @version 0.1
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+ * @brief   A simple character device driver which send a preset GPS
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+ * coordinates when the device is read in userland.
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+ * Heavily inspired by Derek Molloy code
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+ * @see http://derekmolloy.ie/writing-a-linux-kernel-module-part-2-a-character-device/
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+ * @see https://github.com/derekmolloy/exploringBB.git
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+ */
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+
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+#include <linux/init.h>
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+#include <linux/module.h>
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+#include <linux/device.h>
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+#include <linux/kernel.h>
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+#include <linux/fs.h>
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+#include <asm/uaccess.h>
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+
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+#define  DEVICE_NAME "nmea"
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+#define  CLASS_NAME  "nmea"
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+MODULE_LICENSE("GPL");
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+MODULE_AUTHOR("Yanik Cawidrone");
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+MODULE_DESCRIPTION("A simple Linux FAKENMEA");
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+MODULE_VERSION("0.1");
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+#define MAX_BUFFER_SZ 256
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+
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+/* Default position */
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+static char *LATDIR="N";
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+static char *LONDIR="E";
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+static char *LAT="0116.87058";
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+static char *LON="10350.98809";
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+static char *ALT="110";
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+static char *SAT="06";
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+static int TZ=8;
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+
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+/* Module parameters */
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+module_param(ALT, charp, 0000);
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+MODULE_PARM_DESC(ALT, "Altitude (default: 110)");
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+module_param(SAT, charp, 0000);
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+MODULE_PARM_DESC(SAT, "Number of Satellite(s) (default: 06)");
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+module_param(LATDIR, charp, 0000);
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+MODULE_PARM_DESC(LATDIR, "Latitude Direction (default: N)");
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+module_param(LONDIR, charp, 0000);
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+MODULE_PARM_DESC(LONDIR, "Longitude Direction (default: E)");
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+module_param(LAT, charp, 0000);
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+MODULE_PARM_DESC(LAT, "Latitude (default:0116.87058)");
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+module_param(LON, charp, 0000);
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+MODULE_PARM_DESC(LON, "Longitude (default:10350.98809)");
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+module_param(TZ, int, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
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+MODULE_PARM_DESC(TZ, "Timezone drift (default:8 but currently unused)");
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+
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+
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+int checksum(const char *s) {
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+    int c = 0;
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+    while(*s)
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+        c ^= *s++;
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+    return c;
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+}
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+
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+static int    majorNumber;                  ///< Stores the device number -- determined automatically
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+static char   message[MAX_BUFFER_SZ] = {0};           ///< Memory for the string that is passed from userspace
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+static short  size_of_message=0;              ///< Used to remember the size of the string stored
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+static int    numberOpens = 0;              ///< Counts the number of times the device is opened
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+static struct class*  fakenmeaClass  = NULL; ///< The device-driver class struct pointer
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+static struct device* fakenmeaDevice = NULL; ///< The device-driver device struct pointer
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+
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+// The prototype functions for the character driver -- must come before the struct definition
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+static int     dev_open(struct inode *, struct file *);
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+static int     dev_release(struct inode *, struct file *);
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+static ssize_t dev_read(struct file *, char *, size_t, loff_t *);
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+static ssize_t dev_write(struct file *, const char *, size_t, loff_t *);
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+
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+static struct file_operations fops =
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+{
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+   .open = dev_open,
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+   .read = dev_read,
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+   .write = dev_write,
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+   .release = dev_release,
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+};
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+
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+static int __init fakenmea_init(void){
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+   printk(KERN_INFO "FNMEA: Initializing the FAKENMEA\n");
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+
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+   printk(KERN_INFO "FNMEA: Altitude %s\n",ALT);
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+   printk(KERN_INFO "FNMEA: Position %s%s %s%s\n",LAT,LATDIR, LON,LONDIR);
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+
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+   // Try to dynamically allocate a major number for the device -- more difficult but worth it
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+   majorNumber = register_chrdev(0, DEVICE_NAME, &fops);
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+   if (majorNumber<0){
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+      printk(KERN_ALERT "FNMEA: failed to register a major number\n");
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+      return majorNumber;
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+   }
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+   printk(KERN_INFO "FNMEA: registered correctly with major number %d\n", majorNumber);
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+
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+   // Register the device class
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+   fakenmeaClass = class_create(THIS_MODULE, CLASS_NAME);
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+   if (IS_ERR(fakenmeaClass)){                // Check for error and clean up if there is
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+      unregister_chrdev(majorNumber, DEVICE_NAME);
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+      printk(KERN_ALERT "FNMEA: Failed to register device class\n");
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+      return PTR_ERR(fakenmeaClass);          // Correct way to return an error on a pointer
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+   }
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+   printk(KERN_INFO "FNMEA: device class registered correctly\n");
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+
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+   // Register the device driver
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+   fakenmeaDevice = device_create(fakenmeaClass, NULL, MKDEV(majorNumber, 0), NULL, DEVICE_NAME);
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+   if (IS_ERR(fakenmeaDevice)){               // Clean up if there is an error
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+      class_destroy(fakenmeaClass);           // Repeated code but the alternative is goto statements
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+      unregister_chrdev(majorNumber, DEVICE_NAME);
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+      printk(KERN_ALERT "FNMEA: Failed to create the device\n");
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+      return PTR_ERR(fakenmeaDevice);
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+   }
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+   printk(KERN_INFO "FNMEA: device class created correctly\n"); // Made it! device was initialized
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+   return 0;
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+}
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+
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+/** @brief The LKM cleanup function
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+ *  Similar to the initialization function, it is static. The __exit macro notifies that if this
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+ *  code is used for a built-in driver (not a LKM) that this function is not required.
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+ */
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+static void __exit fakenmea_exit(void){
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+   device_destroy(fakenmeaClass, MKDEV(majorNumber, 0));     // remove the device
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+   class_unregister(fakenmeaClass);                          // unregister the device class
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+   class_destroy(fakenmeaClass);                             // remove the device class
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+   unregister_chrdev(majorNumber, DEVICE_NAME);             // unregister the major number
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+   printk(KERN_INFO "FNMEA: Module removed\n");
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+}
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+
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+static int dev_open(struct inode *inodep, struct file *filep){
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+  numberOpens++;
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+  printk(KERN_INFO "FNMEA: Device has been opened %d time(s) %d\n", numberOpens,size_of_message);
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+  return 0;
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+}
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+
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+static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *offset){
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+  int error_count = 0;
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+  struct timeval ts;
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+  struct tm tm;
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+
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+  char mystring[MAX_BUFFER_SZ]="";
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+  char mystring_format[]="GPRMC,%02d%02d%02d.%d,V,%s,%s,%s,%s,0,0,%02d%02d%02d,,E";
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+  char mystring2[MAX_BUFFER_SZ]="";
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+  char mystring_format2[]="GPGGA,%02d%02d%02d.%d,%s,%s,%s,%s,0,%s,0,%s,M,0,M,,";
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+
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+  do_gettimeofday(&ts);
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+  time_to_tm(ts.tv_sec, TZ, &tm);
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+
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+  sprintf(mystring, mystring_format,
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+    tm.tm_hour,
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+    tm.tm_min,
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+    tm.tm_sec,
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+    ts.tv_usec/1000,
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+    LAT,
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+    LATDIR,
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+    LON,
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+    LONDIR,
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+    tm.tm_mday,
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+    tm.tm_mon+1,
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+    tm.tm_year-100);
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+  sprintf(message,"$%s*%02X\n", mystring, checksum(mystring));
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+  size_of_message = strlen(message);
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+
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+  sprintf(mystring2, mystring_format2,
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+    tm.tm_hour,
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+    tm.tm_min,
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+    tm.tm_sec,
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+    ts.tv_usec/1000,
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+    LAT,
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+    LATDIR,
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+    LON,
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+    LONDIR,
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+    SAT,
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+    ALT);
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+  sprintf(message+size_of_message,"$%s*%02X\n", mystring2, checksum(mystring2));
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+
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+  error_count = copy_to_user(buffer, message, strlen(message));
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+  if (error_count==0){            // if true then have success
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+    printk(KERN_INFO "FNMEA: snd %d\n", size_of_message);
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+    printk(KERN_INFO "FNMEA: snd %s", message);
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+    return (size_of_message=0);  // clear the position to the start and return 0
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+  }
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+  else {
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+    printk(KERN_INFO "FNMEA: Failed to send %d characters to the user\n", error_count);
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+    return -EFAULT;              // Failed -- return a bad address message (i.e. -14)
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+  }
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+}
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+
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+static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset){
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+  printk(KERN_INFO "FNMEA: recv %zu\n", len);
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+  printk(KERN_INFO "FNMEA: recv %.*s", (int)len, buffer);
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+  return len;
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+}
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+
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+static int dev_release(struct inode *inodep, struct file *filep){
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+   printk(KERN_INFO "FNMEA: Device successfully closed\n");
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+   return 0;
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+}
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+
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+module_init(fakenmea_init);
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+module_exit(fakenmea_exit);
+37
readnmea.c
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+/**
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+ * @file   readnmea.c
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+ * @author Yanik Cawidrone
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+ * @date   1st July 2017
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+ * @version 0.1
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+ * @brief  A Linux user space program that communicates with the fakenmea LKM.
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+ * @see http://derekmolloy.ie/writing-a-linux-kernel-module-part-2-a-character-device/
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+ * @see https://github.com/derekmolloy/exploringBB.git
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+*/
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+#include<stdio.h>
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+#include<stdlib.h>
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+#include<errno.h>
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+#include<fcntl.h>
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+#include<string.h>
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+#include<unistd.h>
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+#include <time.h>
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+
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+#define BUFFER_LENGTH 256
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+static char receive[BUFFER_LENGTH];
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+
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+int main(){
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+  int ret, fd;
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+  char stringToSend[BUFFER_LENGTH];
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+  fd = open("/dev/nmea", O_RDONLY);             // Open the device with read/write access
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+  if (fd < 0){
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+    perror("Failed to open the device...");
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+    return errno;
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+  }
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+  ret = read(fd, receive, BUFFER_LENGTH);        // Read the response from the LKM
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+  if (ret < 0){
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+    perror("Failed to read the message from the device.");
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+    return errno;
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+  }
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+  printf("%s", receive);
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+  close(fd);
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+  return 0;
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+}
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