The C data type of a buffer is specified by the application. In the case of a single variable, this occurs when the application allocates the variable. In the case of generic memory—that is, memory pointed to by a pointer of type void *—this occurs when the application casts the memory to a particular type. There are two ways in which the driver discovers this type:
Data buffer type argument. Buffers used to transfer parameter values and result set data, such as the
buffer bound with TargetValuePtr in SQLBindCol, usually have an associated type argument, such as the TargetType argument in SQLBindCol. In this argument, the application passes the C type identifier corresponding
to the type of the buffer. For example, in the following call to SQLBindCol, the value SQL_C_TYPE_DATE tells the driver that the Date buffer is an SQL_DATE_STRUCT.
SQL_DATE_STRUCT Date;
SQLINTEGER DateInd;
SQLBindCol(hstmt, 1, SQL_C_TYPE_DATE, &Date, 0, &DateInd);
For more information on type identifiers, see the “Data Types in ODBC” section, later in this chapter.
Predefined type. Buffers used to send and retrieve options or attributes, such as the buffer
pointed to by the InfoValuePtr argument in SQLGetInfo, have a fixed type that depends on the option specified. The driver assumes
that the data buffer is of this type; it is the application’s responsibility to
allocate a buffer of this type. For example, in the following call to SQLGetInfo, the driver assumes the buffer is a 32-bit integer because this is what the
SQL_STRING_FUNCTIONS option requires:
SQLUINTEGER StringFuncs;
SQLGetInfo(hdbc, SQL_STRING_FUNCTIONS, (SQLPOINTER) &StringFuncs, 0,
NULL);
The driver uses the C data type to interpret the data in the buffer.