errno.h: Improve ENOSYS's comment

ENOSYS is the mechanism used by user code to detect whether the
running kernel implements a given system call.  It should not be
returned by anything except an unimplemented system call.

Unfortunately, it is rather frequently used in the kernel to
indicate that various new functions of existing system calls are not
implemented.  This should be discouraged.

Improve the comment in errno.h to help clarify ENOSYS's purpose.

Signed-off-by: Andy Lutomirski <>
1 file changed