What's the purpose of 'rectype' in '__STRUCT_KFIFO_COMMON' ? How should I make sence of this macro?

Question

This field is in linux-4.16.12\include\linux\kfifo.h :

#define __STRUCT_KFIFO_COMMON(datatype, recsize, ptrtype) \
    union { \
        struct __kfifo  kfifo; \
        datatype    *type; \
        const datatype  *const_type; \
        char        (*rectype)[recsize];\
        ptrtype     *ptr; \
        ptrtype const   *ptr_const; \
    }

There are less information about this union. So What's the purpose of rectype and recsize ?

MARK: Could somebody provide reference material about this union ? thank you.

Scheff's Cat · Answer 1 · 2018-11-30T07:06:23.013

That's “meta-programming” in C – sorry, I'm joking.

The #define __STRUCT_KFIFO_COMMON(datatype, recsize, ptrtype) is a macro to define a union.

It could be used for any kind of declaration, e.g.

typedef __STRUCT_KFIFO_COMMON(int, 64, int*) IntFiFo;

to define a union type (alias) or

static __STRUCT_KFIFO_COMMON(int, 64, int*) intFiFo;

to declare a static variable.

I must admit that I don't know anything about Linux Kernel programming – that's just what I read from this code (and how it would make sense).

Concerning

char        (*rectype)[recsize];

rectype is a pointer to a char[recsize].

Why the parentheses (())?: They are necessary because if missing this would be another (not intended) type:

char        *rectype[recsize];

is an array of char* with recsize elements.

So, pointer to array vs. array of pointers, hence, the parentheses.

recsize is one of the macro parameters. IMHO, it must be exclusively used with a constant integral value. Otherwise, it's probably an error. (VLA in a union? No, that doesn't work.)

Last a hint for how to decode confusing C types:

Try cdecl: C gibberish &lrarr; English if in doubt. With recsize it didn't work but replacing recsize by a number it provided:

char (*rectype)[10];

    declare rectype as pointer to array 10 of char

After having sent this answer, I googled c pointer to array site:stackoverflow.com out of curiosity and found

SO: C pointer to array/array of pointers disambiguation

as first hit which might be interesting for further reading.

What's the purpose of 'rectype' in '__STRUCT_KFIFO_COMMON' ? How should I make sence of this macro?

1 Answers1