5.1. The /proc File System
In Linux, there is an additional mechanism for the kernel and kernel modules to send information to processes --- the /proc file system. Originally designed to allow easy access to information about processes (hence the name), it is now used by every bit of the kernel which has something interesting to report, such as /proc/modules which provides the list of modules and /proc/meminfo which stats memory usage statistics.
The method to use the proc file system is very similar to the one used with device drivers --- a structure is created with all the information needed for the /proc file, including pointers to any handler functions (in our case there is only one, the one called when somebody attempts to read from the /proc file). Then, init_module registers the structure with the kernel and cleanup_module unregisters it.
The reason we use proc_register_dynamic is because we don't want to determine the inode number used for our file in advance, but to allow the kernel to determine it to prevent clashes. Normal file systems are located on a disk, rather than just in memory (which is where /proc is), and in that case the inode number is a pointer to a disk location where the file's index-node (inode for short) is located. The inode contains information about the file, for example the file's permissions, together with a pointer to the disk location or locations where the file's data can be found.
Because we don't get called when the file is opened or closed, there's nowhere for us to put try_module_get and try_module_put in this module, and if the file is opened and then the module is removed, there's no way to avoid the consequences.
Here a simple example showing how to use a /proc file. This is the HelloWorld for the /proc filesystem. There are three parts: create the file /proc/helloworld in the function init_module, return a value (and a buffer) when the file /proc/helloworld is read in the callback function procfs_read, and delete the file /proc/helloworld in the function cleanup_module.
The /proc/helloworld is created when the module is loaded with the function create_proc_entry. The return value is a 'struct proc_dir_entry *', and it will be used to configure the file /proc/helloworld (for example, the owner of this file). A null return value means that the creation has failed.
Each time, everytime the file /proc/helloworld is read, the function procfs_read is called. Two parameters of this function are very important: the buffer (the first parameter) and the offset (the third one). The content of the buffer will be returned to the application which read it (for example the cat command). The offset is the current position in the file. If the return value of the function isn't null, then this function is called again. So be careful with this function, if it never returns zero, the read function is called endlessly.
% cat /proc/helloworld
Example 5-1. procfs1.c
In version 2.0, in version 2.2 this is done automatically if we set the inode to zero.