Introduction - If you have any usage issues, please Google them yourself
ssize_t (* read) (struct file* filp, char __ user* buffer, size_t size, loff_t* p) (the pointer parameter filp to read the target file pointer parameter buffer corresponding buffer placement information (i.e., the user-space memory address), the size parameter is the length of the information to be read, the parameter p is the position of the read offset relative to the beginning of the file, after the read-out information, this pointer will generally move, move the value to be read This function is used to take the length of the value of the information) to get the data from the device. causes the read system call to-EINVAL (" Invalid argument" ) failed a non-negative return value represents a successful read word in this position a null pointer section number (return value is a " signed size" type, is often the target platform for local integer type).