Why do different headers have the same name?

Question

I have several copies of, let's say, stddef.h on my system, one is in the path /usr/include/linux/stddef.h, and looks like this:

#ifndef _LINUX_STDDEF_H
#define _LINUX_STDDEF_H

#undef NULL
#if defined(__cplusplus)
#define NULL 0
#else
#define NULL ((void *)0)
#endif

#endif

Another one is in the path /usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5/include/, and that is the one which is used when I say #include <stddef.h>. That one's contents are a lot different from the first one, contains the definitions of ptrdiff_t, size_t etc.

My question is: As far as I know, the C/C++ standards require that the definition of size_t should be placed in stddef.h, however the first one doesn't follow that. That one clearly isn't the one mentioned by the C/C++ standards. If that file is there on for some other purpose, why are both of these files named as stddef.h, wouldn't it be safer/more clear if they had different names?

Answer 1

The linux kernel does not link with the c standard library, therefore - as a general rule - the standard include files cannot be used safely, so the linux kernel uses its own include files which are known not to rely on c library functions or data.

Any software that is to run in kernel space - such as kernel modules - should use the include/linux files and not the standard library ones.

Obviously the kernel include files only cover things that are likely to be needed in the kernel so are a very small subset of the standard c include files.