re_source.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: re_source.so,v 10.16 2003/10/18 19:15:55 bostic Exp $])

m4_ref_title(Access Methods,
    Flat-text backing files,
    @text backing files, am_conf/extentsize, am_conf/renumber)

m4_p([dnl
It is possible to back any Recno database (either fixed or variable
length) with a flat-text source file.  This provides fast read (and
potentially write) access to databases that are normally created and
stored as flat-text files.  The backing source file may be specified by
calling the m4_refT(dbh_set_re_source).])

m4_p([dnl
The backing source file will be read to initialize the database.  In the
case of variable length records, the records are assumed to be separated
as described for the m4_refT(dbh_set_re_delim).  For example, standard
UNIX byte stream files can be interpreted as a sequence of variable
length records separated by ASCII newline characters.  This is the
default.])

m4_p([dnl
When cached data would normally be written back to the underlying
database file (for example, when the m4_ref(dbh_close) or
m4_refT(dbh_sync)s are called), the in-memory copy of the database will
be written back to the backing source file.])

m4_p([dnl
The backing source file must already exist (but may be zero-length) when
m4_ref(dbh_open) is called.  By default, the backing source file is read
lazily, that is, records are not read from the backing source file until
they are requested by the application. If multiple processes (not
threads) are accessing a Recno database concurrently and either
inserting or deleting records, the backing source file must be read in
its entirety before more than a single process accesses the database,
and only that process should specify the backing source file as part of
the m4_ref(dbh_open) call.  This can be accomplished by calling the
m4_refT(dbh_set_flags) with the m4_ref(DB_SNAPSHOT) flag.])

m4_p([dnl
Reading and writing the backing source file cannot be transactionally
protected because it involves filesystem operations that are not part of
the m4_db transaction methodology. For this reason, if a temporary
database is used to hold the records (a NULL was specified as the file
argument to m4_ref(dbh_open)), m4_bold([it is possible to lose the
contents of the backing source file if the system crashes at the right
instant]). If a permanent file is used to hold the database (a filename
was specified as the file argument to m4_ref(dbh_open)), normal database
recovery on that file can be used to prevent information loss.  It is
still possible that the contents of the backing source file itself will
be corrupted or lost if the system crashes.])

m4_p([dnl
For all of the above reasons, the backing source file is generally used
to specify databases that are read-only for m4_db applications, and that
are either generated on the fly by software tools, or modified using a
different mechanism such as a text editor.])

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