Opening a database is done in two steps: first, a DB handle is
created using the Berkeley DB db_create interface, and then the
actual database is opened using the DB->open function.
The optional database name, not used in this example.
type
The type of database to open. This value will be one of the four access
methods Berkeley DB supports: DB_BTREE, DB_HASH, DB_QUEUE or DB_RECNO, or the
special value DB_UNKNOWN, which allows you to open an existing file
without knowing its type.
flags
Various flags that modify the behavior of DB->open. In our
simple case, the only interesting flag is DB_CREATE. This flag
behaves similarly to the IEEE/ANSI Std 1003.1 (POSIX) O_CREATE flag to the open system
call, causing Berkeley DB to create the underlying database if it does not
yet exist.
mode
The file mode of any underlying files that DB->open will create.
The mode behaves as does the IEEE/ANSI Std 1003.1 (POSIX) mode argument to the open
system call, and specifies file read, write and execute permissions.
Of course, only the read and write permissions are relevant to Berkeley DB.
Here's what the code to create the handle and then call DB->open
looks like:
If the call to db_create is successful, the variable dbp
will contain a database handle that will be used to configure and access
an underlying database.
As you see, the program opens a database named access.db. The
underlying database is a Btree. Because the DB_CREATE flag was
specified, the file will be created if it does not already exist. The
mode of any created files will be 0664 (i.e., readable and writeable by
the owner and the group, and readable by everyone else).
One additional function call is used in this code sample, DB->err.
This method works like the ANSI C printf interface. The second argument
is the error return from a Berkeley DB function, and the rest of the arguments
are a printf-style format string and argument list. The error message
associated with the error return will be appended to a message constructed
from the format string and other arguments. In the above code, if the
DB->open call were to fail, the message it would display would be
something like