const char *sqlite3_db_filename(sqlite3 *db, const char *zDbName);
The sqlite3_db_filename(D,N) interface returns a pointer to the filename associated with database N of connection D. If there is no attached database N on the database connection D, or if database N is a temporary or in-memory database, then this function will return either a NULL pointer or an empty string.
The string value returned by this routine is owned and managed by the database connection. The value will be valid until the database N is DETACH-ed or until the database connection closes.
The filename returned by this function is the output of the xFullPathname method of the VFS. In other words, the filename will be an absolute pathname, even if the filename used to open the database originally was a URI or relative pathname.
If the filename pointer returned by this routine is not NULL, then it can be used as the filename input parameter to these routines:
See also lists of Objects, Constants, and Functions.
SQLite is in the Public Domain.
https://sqlite.org/c3ref/db_filename.html