What does sizeof(int[1]) mean?
c, linux-kernel, macros, sizeof
Solution
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.
Problem
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: ``` #define _IOC_TYPECHECK(t) \ ((sizeof(t) == sizeof(t[1]) && \ sizeof(t) < (1 << _IOC_SIZEBITS)) ? \ sizeof(t) : __invalid_size_argument_for_IOC) ``` Can you explain me this code? In this code, what does `sizeof(t[1])` mean?