What does sizeof(int[1]) mean?

Question

I am new to the Linux kernel. I am reading the file ioctl.h, there I encountered a macro _IOC_TYPECHECK(t), which looks like this:

Answer 1

This is used to check the validity of the third parameter to the _IOR/_IOW/_IOWR macros, which is supposed to be a type. It checks that the parameter is actually a type (and not a variable or a number), and causes a compiler or linker error otherwise.

• If t is a type, then t[1] is the type "an array of 1 t". This type has the same size as t, and therefore sizeof(t) == sizeof(t[1]) is true.

• If t is a number, sizeof(t) will fail to compile.

• If t is a simple (non-array) variable, then t[1] will cause a compiler error.

• If t is an array variable, sizeof(t) == sizeof(t[1]) will be false, and a linker error will be caused (because __invalid_size_argument_for_IOC is not defined).

The expression sizeof(t) < (1 << _IOC_SIZEBITS) checks that the size of the type t does not exceed the maximum allowed for ioctl, and causes the same linker error otherwise.

There are still some invalid cases which will not be caught by this macro - for example, when t is a pointer to a pointer.