The file_operations Structure
The file_operations structure is defined in linux/fs.h, and holds pointers to functions defined by the driver that perform various operations on the device. Each field of the structure corresponds to the address of some function defined by the driver to handle a requested operation.The file_operations structure is defined in linux/fs.h, and holds pointers to functions defined by the driver that perform various operations on the device. Each field of the structure corresponds to the address of some function defined by the driver to handle a requested operation.The file_operations structure holds the address of the module’s function that performs that operation
The file structure
Each device is represented in the kernel by a file structure, which is defined in linux/fs.h. Be aware that a file is a kernel level structure and never appears in a user space program. It’s not the same thing as a FILE, which is defined by glibc and would never appear in a kernel space function. Also, its name is a bit misleading; it represents an abstract open `file’, not a file on a disk, which is represented by a structure named inode
Registering A Device
The major number tells you which driver handles which device file. The minor number is used only by the driver itself to differentiate which device it’s operating on, just in case the driver handles more than one device.Adding a driver to your system means registering it with the kernel.
Unregistering A Device
We can’t allow the kernel module to be rmmod’ed whenever root feels like it. If the device file is opened by a process and then we remove the kernel module, using the file would cause a call to the memory location where the appropriate function (read/write) used to be.If we’re lucky, no other code was loaded there, and we’ll get an ugly error message. If we’re unlucky, another kernel module was loaded into the same location, which means a jump into the middle of another function within the kernel. The results of this would be impossible to predict, but they can’t be very positive.
chardev
The next code sample creates a char driver named chardev. You can cat its device file (or open the file with a program) and the driver will put the number of times the device file has been read from into the file
Writing Modules for Multiple Kernel Versions
The system calls, which are the major interface the kernel shows to the processes, generally stay the same across versions. A new system call may be added, but usually the old ones will behave exactly like they used to. This is necessary for backward compatibility — a new kernel version is not supposed to break regular processes. In most cases, the device files will also remain the same. On the other hand, the internal interfaces within the kernel can and do change between versions.
