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早期freebsd实现

#	@(#)README	5.2 (Berkeley) 3/1/93

For compiling:
	There are several flag options for compiling 'ed' into executable.

This table briefly describes the flags used in compilation:

	BSD | POSIX
		specifies over-riding behaviour in historical terms:
		BSD=> BSD behaviour when confilicting with P1003.2,
		POSIX=> as P1003.2 specified when conflicting with BSD.

	STDIO | DBI | MEMORY
		specifies which method is to be used for the buffer.
                Be sure to understand that there are trade-offs with
                any of the methods:
		STDIO=> standard I/O temp file,
		DBI=> BSD recno database file (db(3)),
		MEMORY=> ye olde silicon.

	One of each compile flag option _must_ be chosen to get a sensible
	compile.

One define not listed above, yet used as a flag, is RE_STARTEND. This
is used to automagically check if you are using the real BSD RE interfaces.
See regex(3) (4.4BSD) for a description of RE_STARTEND; an extension to
P1003.2 B.5. If you happen to be using another POSIX regex(3) this
automatically taken care of; the old RE interfaces are unsupported.

This implementation of `ed' is a superset of the POSIX 1003.2 description
for `ed' to remain compatible with earlier (BSD) versions of `ed'.
Hence, any BSD 'ed'isms that do not conflict with P1003.2 are available
to the user even under the POSIX flag compilation. Likewise, any POSIX
'ed'isms that do not conflict with historical BSD behaviour are available
to the user.

Refer to the man page ed(1) for information about using `ed'.

-Rodney