The Descriptor Handle

A descriptor handle is a pointer to a data storage area which contains a collection of metadata. The metadata describes either the parameters of an SQL statement, or the columns of a result data set, as seen by the application or driver.

ODBC 3.x recognizes four types of descriptors:

• Application Parameter Descriptor (APD) contains information about the application buffers bound to the parameters in an SQL statement, such as their addresses, lengths, and C data types.

• Implementation Parameter Descriptor (IPD) contains information about the parameters in an SQL statement, such as their SQL data types, lengths, and nullability.

• Application Row Descriptor (ARD) contains information about the application buffers bound to the columns in a result set, such as their addresses, lengths, and C data types.

• Implementation Row Descriptor (IRD) contains information about the columns in a result set, such as their SQL data types, lengths, and nullability.

Four descriptor handles (one for each type of descriptor described above) are automatically allocated when a statement handle is allocated. These descriptor handles remain associated with the statement handle which allocated them for the life of that handle. ODBC 3.x applications can allocate additional descriptor handles for a specific connection handle.

Most operations in ODBC can be performed without explicit use of descriptors by the application. However, descriptors provide a convenient shortcut for some operations. Several specific uses of descriptors will be explained in other sections of this manual.