Chapter 3. Communicating with userland

Chapter 3. Communicating with userland
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Reading data
Writing data
A single call back for many files

Instead of reading (or writing) information directly from kernel memory, procfs works with call back functions for files: functions that are called when a specific file is being read or written. Such functions have to be initialised after the procfs file is created by setting the read_proc and/or write_proc fields in the struct proc_dir_entry* that the function create_proc_entry returned:

struct proc_dir_entry* entry;

entry->read_proc = read_proc_foo;
entry->write_proc = write_proc_foo;

If you only want to use a the read_proc, the function create_proc_read_entry described in the section called “Convenience functions” may be used to create and initialise the procfs entry in one single call.

Reading data

The read function is a call back function that allows userland processes to read data from the kernel. The read function should have the following format:

`int read_func(`	char*	`buffer`,
	char**	`start`,
	off_t	`off`,
	int	`count`,
	int*	`peof`,
	void*	`data)`;

The read function should write its information into the buffer, which will be exactly PAGE_SIZE bytes long.

The parameter peof should be used to signal that the end of the file has been reached by writing 1 to the memory location peof points to.

The data parameter can be used to create a single call back function for several files, see the section called “A single call back for many files”.

The rest of the parameters and the return value are described by a comment in fs/proc/generic.c as follows:

You have three ways to return data:
Leave *start = NULL. (This is the default.) Put the data of the requested offset at that offset within the buffer. Return the number (n) of bytes there are from the beginning of the buffer up to the last byte of data. If the number of supplied bytes (= n - offset) is greater than zero and you didn't signal eof and the reader is prepared to take more data you will be called again with the requested offset advanced by the number of bytes absorbed. This interface is useful for files no larger than the buffer.
Set *start to an unsigned long value less than the buffer address but greater than zero. Put the data of the requested offset at the beginning of the buffer. Return the number of bytes of data placed there. If this number is greater than zero and you didn't signal eof and the reader is prepared to take more data you will be called again with the requested offset advanced by *start. This interface is useful when you have a large file consisting of a series of blocks which you want to count and return as wholes. (Hack by [email protected])
Set *start to an address within the buffer. Put the data of the requested offset at *start. Return the number of bytes of data placed there. If this number is greater than zero and you didn't signal eof and the reader is prepared to take more data you will be called again with the requested offset advanced by the number of bytes absorbed.

Chapter 5, Example shows how to use a read call back function.