Writing data
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## Writing data

The write call back function allows a userland process to write data to the kernel, so it has some kind of control over the kernel. The write function should have the following format:

`int write_func(` | struct file* | file,
| const char* | buffer,
| unsigned long | count,
| void* | data`)`;

The write function should read `count` bytes at maximum from the `buffer`. Note that the `buffer` doesn't live in the kernel's memory space, so it should first be copied to kernel space with `copy_from_user`. The `file` parameter is usually ignored. the section called “A single call back for many files” shows how to use the `data` parameter.

Again, Chapter 5, Example shows how to use this call back function.

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