The connection information for a file data source is stored in a .dsn file. As a result, the connection string can be used repeatedly by a single user or shared among several users if they have the appropriate driver installed. The file contains a driver name (or another data source name in the case of an unshareable file data source) and optionally, a connection string that can be used by SQLDriverConnect. The Driver Manager builds the connection string for the call to SQLDriverConnect from the keywords in the .dsn file.
A file data source allows an application to specify connection options without having to build a connection string for use with SQLDriverConnect. The file data source usually is created by specifying the SAVEFILE keyword, which causes the Driver Manager to save the output connection string created by a call to SQLDriverConnect to the .dsn file. That connection string can be used repeatedly by calling SQLDriverConnect with the FILEDSN keyword. This streamlines the connection process and provides a persistent source of the connection string.
File data sources also can be created by calling SQLCreateDataSource in the installer DLL. Information can be written into the .dsn file by calling SQLWriteFileDSN, and read from the .dsn file by calling SQLReadFileDSN; both of these functions are also in the installer DLL. For information about the installer DLL, see Configuring Data Sources.
The keywords used for connection information are in the [ODBC] section of a .dsn file. The minimum information that a shareable .dsn file would have in the [ODBC] section is the DRIVER keyword:
DRIVER = SQL Server
The shareable .dsn file usually contains a connection string, as follows:
DRIVER = SQL Server UID = Larry DATABASE = MyDB
When the file data source is unshareable, the .dsn file contains only a DSN keyword. When the Driver Manager is sent the information in an unshareable file data source, it connects as necessary to the data source indicated by the DSN keyword. An unshareable .dsn file would contain the following keyword:
DSN = MyDataSource
The connection string used for a file data source is the union of the keywords specified in the .dsn file and the keywords specified in the connection string in the call to SQLDriverConnect. If any of the keywords in the .dsn file conflict with keywords in the connection string, the Driver Manager decides which keyword value should be used. For more information, see SQLDriverConnect.