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Exercise 1-3: Pseudo Character Driver (Single Device) - Testing

On the Host

1. Build the kernel module pcd.c against the host Linux kernel version

2. Insert the mode: sudo insmod pcd.ko

3. Test the pseudo character driver: echo "Hello, welcome!" > /dev/pcd $\to$$\to$ dmesg

   Output:

   xxxxxxxxxx
   8
   1
   [21343.359074] pcd_driver_init :Device number <major>:<minor> = 509:0
   2
   [21343.359135] pcd_driver_init :Module init was successful
   3
   [21401.491786] pcd_open :open was successful
   4
   [21401.491856] pcd_write :Write requested for 16 bytes
   5
   [21401.491863] pcd_write :Current file position = 0
   6
   [21401.491868] pcd_write :16 bytes successfully written
   7
   [21401.491871] pcd_write :Updated file position = 16
   8
   [21401.491901] pcd_release :close was successful
   

   echo command opens the pseudo character device by invoking the open() system call. (L5: pcd_open() gets called from the driver)

   L7: When a file is opened, the current file position is always 0.

   L10: echo invokes close() system call, and subsequently pcd_release() of the driver gets called.

4. Check the contents of the device (buffer): cat /dev/pcd

   Output:

   xxxxxxxxxx
   1
   1
   Hello, welcome!
   

5. Check the read activity: dmesg | tail

   xxxxxxxxxx
   10
   1
   [22741.144956] pcd_open :open was successful
   2
   [22741.144967] pcd_read :Read requested for 131072 bytes
   3
   [22741.144969] pcd_read :Current file position = 0
   4
   [22741.144971] pcd_read :512 bytes successfully read
   5
   [22741.144972] pcd_read :Updated file position = 512
   6
   [22741.144980] pcd_read :Read requested for 131072 bytes
   7
   [22741.144981] pcd_read :Current file position = 512
   8
   [22741.144982] pcd_read :0 bytes successfully read
   9
   [22741.144983] pcd_read :Updated file position = 512
   10
   [22741.144991] pcd_release :close was successful
   

   L2: Read request for 131072 bytes because that's how the cat program is implemented!

   L5: Updated file position is 512 bytes (i.e., DEV_MEM_SIZE) because that's the size of our buffer.

   L6: cat requests to read again

   L8: 0 bytes since no more to read

6. Attempt copying a large text file (> 512 Bytes) into /dev/pcd: cp file.txt /dev/pcd

   Output:

   xxxxxxxxxx
   1
   1
   cp: error writing '/dev/pcd': Cannot allocate memory
   

   And run dmesg to see what happened:

   xxxxxxxxxx
   10
   1
   [24039.217897] pcd_driver_init :Module init was successful
   2
   [24045.901773] pcd_open :open was successful
   3
   [24045.901788] pcd_write :Write requested for 1717 bytes
   4
   [24045.901790] pcd_write :Current file position = 0
   5
   [24045.901792] pcd_write :512 bytes successfully written
   6
   [24045.901793] pcd_write :Updated file position = 512
   7
   [24045.901795] pcd_write :Write requested for 1205 bytes
   8
   [24045.901796] pcd_write :Current file position = 512
   9
   [24045.901797] pcd_write :No more space left on the device
   10
   [24045.901973] pcd_release :release was successful
   

   L6: 512 bytes successfully written

   L7: The file position reached the end

   L8: cp command attempts to copy the rest of the contents (1717 - 512 = 1205 bytes)

   L9: Since the file position is already at the end, no more write can be done. (pcd_write() returns the error code -ENOMEM which triggers the error message cp: error writing '/dev/pcd': Cannot allocate memory.)