20.6.9.2 C API Prepared Statement Type Conversions

Prepared statements transmit data between the client and server using C language variables on the client side that correspond to SQL values on the server side. If there is a mismatch between the C variable type on the client side and the corresponding SQL value type on the server side, MySQL performs implicit type conversions in both directions.

MySQL knows the type code for the SQL value on the server side. The buffer_type value in the MYSQL_BIND structure indicates the type code of the C variable that holds the value on the client side. The two codes together tell MySQL what conversion must be performed, if any. Here are some examples:

Before binding variables to the MYSQL_BIND structures that are to be used for fetching column values, you can check the type codes for each column of the result set. This might be desirable if you want to determine which variable types would be best to use to avoid type conversions. To get the type codes, call mysql_stmt_result_metadata() after executing the prepared statement with mysql_stmt_execute(). The metadata provides access to the type codes for the result set as described in Section 20.6.11.22, “mysql_stmt_result_metadata()”, and Section 20.6.5, “C API Data Structures”.

To determine whether output string values in a result set returned from the server contain binary or nonbinary data, check whether the charsetnr value of the result set metadata is 63 (see Section 20.6.5, “C API Data Structures”). If so, the character set is binary, which indicates binary rather than nonbinary data. This enables you to distinguish BINARY from CHAR, VARBINARY from VARCHAR, and the BLOB types from the TEXT types.

If you cause the max_length member of the MYSQL_FIELD column metadata structures to be set (by calling mysql_stmt_attr_set()), be aware that the max_length values for the result set indicate the lengths of the longest string representation of the result values, not the lengths of the binary representation. That is, max_length does not necessarily correspond to the size of the buffers needed to fetch the values with the binary protocol used for prepared statements. Choose the size of the buffers according to the types of the variables into which you fetch the values. For example, a TINYINT column containing the value -128 might have a max_length value of 4. But the binary representation of any TINYINT value requires only 1 byte for storage, so you can supply a signed char variable in which to store the value and set is_unsigned to indicate that values are signed.