Populating a Blob Descriptor

Go Up to Working with Blob Data

There are four possible ways to populate a Blob descriptor, as follows:

• Call isc_blob_default_desc2(). This stores default values into the descriptor fields. The default subtype is 1 (TEXT), segment size is 80 bytes, and charset is the default charset for your process.
• Call isc_blob_lookup_desc2(). This accesses the database system metadata tables to look up and copy information for the specified Blob column into the descriptor fields.
• Call isc_blob_set_desc2(). This initializes the descriptor from parameters you call it with, rather than accessing the database metadata.
• Set the descriptor fields directly.

The following example calls isc_blob_lookup_desc2() to look up the current subtype and character set information for a Blob column named PROJ_DESC in a table named PROJECT. It stores the information into the source descriptor, ­from_desc.

isc_blob_lookup_desc2 (
status_vector,
&db_handle; /* Set by previous isc_attach_database() call. */
&tr_handle, /* Set by previous isc_start_transaction() call. */
"PROJECT", /* Table name. */
"PROJ_DESC", /* Column name. */
&from_desc, /* Blob descriptor filled in by this function call. */
&global ) /* Global column name, returned by this function. */

For more information about the usage of Blob descriptors in applications that request data filtering, and for further examples of populating Blob descriptors, see Writing an Application that Requests Filtering.