Can someone explain the C syntax in this block of linux device driver code?

Question

Specifically, is the (*llseek) term an object casting? Does this syntax < ( * < name > ) (...) > have a term that I can google search it to learn more.

Why doesn't "struct file *" have a variable name, and how can I access it (assuming I need to or allowed to access it). Similarly, the remaining parameters have no variable names "loff_t, int", why is that? Finally, is __user (in this example or __ in general) a Macro?

 struct file_operations {
        loff_t (*llseek) (struct file *, loff_t, int);
        ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);

Thanks!

Answer 1

Specifically, is the (*llseek) term an object casting?

Declarations in the form type (* name)(parameter information) declare name to be a pointer to a function returning type.

Does this syntax < ( * < name > ) (...) > have a term that I can google search it to learn more.

Not, it does not have a particular name although it can be described as a function pointer. To learn about these things, it is advisable to work through a C textbook rather than try to stab at individual topics. Or, if you have some C experience and have successfully completed some courses in computer science theory, you could read the C standard itself. The main part (discussing the base language, without the library or optional parts) is only 130 pages, so less than many textbooks.

Why doesn't "struct file *" have a variable name, and how can I access it (assuming I need to or allowed to access it).

This is a declaration of the type of a parameter in the function. It says the parameter is a pointer to a struct file. To call a function, you only need to know what the types of its parameters are, so that the arguments can be properly converted and passed to it. You do not need to have names for those parameters.

Similarly, the remaining parameters have no variable names "loff_t, int", why is that?

Same as above.

Finally, is __user (in this example or __ in general) a Macro?

It is probably some macro used to supply text for a compiler extension. It may provide attributes for the parameter being declared.