I'd say that those constant values are Linux specific, not Ubuntu specific.

In your C file, you're getting fcntl.h from /usr/include/fcntl.h which contains:

/* Get the definitions of O_*, F_*, FD_*: all the

   numbers and flag bits for `open', `fcntl', et al.  */

#include <bits/fcntl.h>

In /usr/include/<your_arch>/bits/fcntl.h you can see the following code:

/* Include generic Linux declarations.  */

#include <bits/fcntl-linux.h>

Finally this /usr/include/<your_arch>/bits/fcntl-linux.h file defines those values as follow:

#ifndef F_RDLCK

/* For posix fcntl() and `l_type' field of a `struct flock' for lockf().  */

# define F_RDLCK        0   /* Read lock.  */

# define F_WRLCK        1   /* Write lock.  */

# define F_UNLCK        2   /* Remove lock.  */

#endif

To confirm that it's not Ubuntu specific, you can check the definitions in the libc source, they are the same.