EINVAL is defined in the header file errno.h

     #define EINVAL  (_SIGN 22) /* invalid argument */

In header files kernel.h, vfs.h, and pm.h, the macro _SYSTEM is defined (with value 1). Then errno.h is included.

In errno.h, _SIGN is defined so that it can be included by a system process or by a user process:

     #ifdef _SYSTEM
     #   define _SIGN -
     #   define  OK    0
     #else
     #   define _SIGN
     #endif

So in a system process, EINVAL will be (-22)

In a user process, EINVAL will be (22)