CODEWITHSUNDEEP
clinux-kernelkernelkernel-module
Develobeer
Develobeer

Reputation: 435

copy_to_user: treating more data

In my kernel module I have the following read function:

static ssize_t sample_read(struct file *filp, char *buffer, size_t length, loff_t * offset) //read function here means to manage the communication with the button
{
    int ret = 1;
    int c;

    c = gpio_get_value( BTN );

    copy_to_user( buffer, &c, 1 ); //Buffer is the stack where to place the data read by function. copy_to_user copies the buffer on the user space. Here the reading is very simple. But if I would like to transfer more data?

    printk( KERN_INFO "%s: %s read %d from BTN\n", module_name, __func__, c );

    return( ret );
}

Here I copy to user space the value of c (that is the value of gpio) by buffer.

In case where for example I need to copy to user space more data using the copy_to_user function?

For example if I would like to copy to user space also a value int x = gpio_get_value( BTN_2 )?

Upvotes: 0

Views: 1020

Answers (1)

Santosh A
Santosh A

Reputation: 5351

Firstly, in your code for sample_read() change

copy_to_user( buffer, &c, 1 );

to 

copy_to_user( buffer, &c, sizeof(c));

Because the third argument should be number of bytes to copy, which should be size of data which you intend to copy to user space.

Next, to copy more data: use a struct. For example

typedef struct data {
    int x;
    int c;
} data_t;

data_t val;
val.x = gpio_get_value(BTN_2);
val.c = gpio_get_value(BTN);

copy_to_user( buffer, &val, sizeof(data_t));

Upvotes: 2

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