The Tcl API closely parallels the Berkeley DB programmatic interfaces. If you are already familiar with one of those interfaces, there will not be many surprises in the Tcl API.
Several pieces of Berkeley DB functionality are not available in the Tcl API. Any of the functions that require a user-provided function are not supported via the Tcl API. For example, there is no equivalent to the DB->set_dup_compare or the DB_ENV->set_errcall methods.
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