Can you explain this C definition from efilib.h?

Question

I'm learning UEFI programming and I notice that many of the EFI header files show chunks with this syntax I can't understand:

typedef
EFI_STATUS
(EFIAPI *EFI_TEXT_STRING) (
    IN EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *This,
    IN CHAR16 *String
);

Could you please explain what are the arguments of that typedef in this case?
How come those IN's are legal syntax? Is it a compiler specific feature?

Possible duplicate of [Understanding typedefs for function pointers in C](http://stackoverflow.com/questions/1591361/understanding-typedefs-for-function-pointers-in-c) — Ivan Aksamentov - Drop, Aug 23 '16 at 09:16
you can first go through macros and see how they are effectively used. This will help you understand this little faster. — Sridhar Nagarajan, Aug 23 '16 at 09:18
`IN` is most likely the macro, which is most likely expands to `const` — Ivan Aksamentov - Drop, Aug 23 '16 at 09:18
looking at [this source code](http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Devices/PC/ipxe/src/include/ipxe/efi/Base.h) `IN` and `OUT` are empty defines used as "comment" of struct members — LPs, Aug 23 '16 at 09:20
@pmg I tried both gcc -E and a "raw" cpp, but 'IN' is still there.Since the two question refer to the same block and, as far as I knew when I wrote the question, could be related, I don't see how posting the same code in two different question could be of any use. Is this the reason for the downvote? — pr0gma, Aug 23 '16 at 10:37
@LPs Yes I did, and I've finally found the preprocessor directives which define IN, OUT and OPTIONAL (they happen to be in efidef.h). — pr0gma, Aug 23 '16 at 12:26

score 1 · Accepted Answer · answered Aug 23 '16 at 20:09

It is a simple typdef for a function pointer. Type EFI_TEXT_STRING is a pointer to a function that takes a pointer to EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL as its first argument and a pointer to CHAR16 as its second argument. The function returns type EFI_STATUS.

Can you explain this C definition from efilib.h?

1 Answers1