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thank you for taking a look at this question.

I am writing a simple misc char device with read and write operations. Please keep reading ...

The following code prints the required text in TEXT only once when i do

cat /dev/mydevice01

This is the code to the read function.

static ssize_t my_read(struct file *file, char __user *buff, size_t count, loff_t *ppos)
{
    char *print_str = TEXT;

    if (*ppos != 0)
        return 0;
    if ((count < TEXT_LENGTH) || (copy_to_user(buff, print_str, TLF_ID_LENGTH)))
        return -EINVAL;
    *ppos += count;
    return count;
}

TEXT is defined using #define and so is TEXT_LENGTH

I felt this code was a little too complicated and hence, I tried to modify it. This is the modified code, but that keeps printing the required text until I use a SIGINT (Ctrl+C) on the process:

static ssize_t my_read(struct file *file, char __user *buff, size_t count, loff_t *ppos)
{
        char *print_str = TLF_ID;

        if (copy_to_user(buff, print_str, TLF_ID_LENGTH))
                return -EINVAL;
        return count;
}

I am new to Linux kernel programming. I want to know what is going wrong with the code. And if possible, a single line of code that can implement the same functionality. Thanks in advance!

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11
  • What is it you cannot do, what is holding you back? Commented Jun 17, 2020 at 19:02
  • No, what I meant is I wanted to know why the modified code is printing the text repitatively. I basically want to simplify the code without affecting the result. Commented Jun 17, 2020 at 19:04
  • Is there a way to rewrite that function in a way that the whole function body is a single line of code? Other code in this program is just standard module_init(my_init) which in turn calls misc_register() for a struct miscdevice which has an fops .read attribute which ends up in this function. Commented Jun 17, 2020 at 19:24
  • 1
    Returning "count" means that "cat" never gets a read returning zero, so cat loops to read some more. You should see where *ppos is, and use that to see if it is greater than TEXT_LEN, and if so return 0. The original code is better than your modified code (at least it doesn't repeat indefinitely) but both are pretty bad. Commented Jun 17, 2020 at 20:22
  • 1
    And of course you need to update *ppos, the correct thing to do is to update it to the number of bytes written. Commented Jun 17, 2020 at 20:28

1 Answer 1

Reset to default
1

I figured it out myself, I'll leave the answer here to anyone else looking for something similar.

static ssize_t my_read(struct file *file, char __user *buff, size_t count, loff_t *ppos)
{
        char *print_str = TEXT;
        int len = TEXT_LENGTH;

        return simple_read_from_buffer(buff, count, ppos, print_str, len);
}

This works as it is supposed to. Just don't forget to include these modules.

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/miscdevice.h>
#include <linux/module.h>

Thank you all for the help :).

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6
  • simple_read_from_buffer doesn't do what you said in the comments you required it to do, in particular it allows partial reads so you can read from part way through the string and don't have to read all the string. Commented Jun 18, 2020 at 14:12
  • I might not have noticed that, but this seemed like a valid solution for a single line code to help me obtain TEXT when I say cat /dev/mydevice01. Thank you for the help Icarus :) Much appreciated. Commented Jun 18, 2020 at 14:15
  • dd bs=1 seek=2 count=3 if=/dev/mydevice01 according to your specification should give an I/O error of some kind. Commented Jun 18, 2020 at 23:49
  • No, it did not. Do you mind telling me why you expected an I/O error of some sort? Fri Jun 19 07:29:21 bkkarthik <2003> ~/Workspace/mydevice01 $ dd bs=1 seek=2 count=3 if=/dev/mydevice01 jbsdkj a243+0 records in 3+0 records out Please bear with me, I'm new to kernel programming. Commented Jun 19, 2020 at 14:00
  • The reason it should give an I/O error is because you told us that it is specified to do so! The original "complicated" code will do so, for example if you ask it to read less than the full length of the string. Usually people want what the simple_read_from_buffer provides, i.e. if you ask for only part of the string you get just part. You said that it had to produce "nothing less than the full string". so the only thing you can do if you are asked for part of the string is to return an error. Commented Jun 19, 2020 at 22:02

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