28.7.9 C API Prepared Statement Data Structures

For input, use MYSQL_BIND structures with mysql_stmt_bind_param() to bind parameter data values to buffers for use by mysql_stmt_execute().

For output, use MYSQL_BIND structures with mysql_stmt_bind_result() to bind buffers to result set columns, for use in fetching rows with mysql_stmt_fetch().

enum enum_field_types buffer_type

The type of the buffer. This member indicates the data type of the C language variable bound to a statement parameter or result set column. For input, buffer_type indicates the type of the variable containing the value to be sent to the server. For output, it indicates the type of the variable into which a value received from the server should be stored. For permissible buffer_type values, see Section 28.7.9.1, “C API Prepared Statement Type Codes”.

void *buffer

A pointer to the buffer to be used for data transfer. This is the address of a C language variable.

For input, buffer is a pointer to the variable in which you store the data value for a statement parameter. When you call mysql_stmt_execute(), MySQL use the value stored in the variable in place of the corresponding parameter marker in the statement (specified with ? in the statement string).

For output, buffer is a pointer to the variable in which to return a result set column value. When you call mysql_stmt_fetch(), MySQL stores a column value from the current row of the result set in this variable. You can access the value when the call returns.

To minimize the need for MySQL to perform type conversions between C language values on the client side and SQL values on the server side, use C variables that have types similar to those of the corresponding SQL values:

For guidelines about mapping between C types and SQL types and notes about type conversions, see Section 28.7.9.1, “C API Prepared Statement Type Codes”, and Section 28.7.9.2, “C API Prepared Statement Type Conversions”.

unsigned long buffer_length

The actual size of *buffer in bytes. This indicates the maximum amount of data that can be stored in the buffer. For character and binary C data, the buffer_length value specifies the length of *buffer when used with mysql_stmt_bind_param() to specify input values, or the maximum number of output data bytes that can be fetched into the buffer when used with mysql_stmt_bind_result().

unsigned long *length

A pointer to an unsigned long variable that indicates the actual number of bytes of data stored in *buffer. length is used for character or binary C data.

For input parameter data binding, set *length to indicate the actual length of the parameter value stored in *buffer. This is used by mysql_stmt_execute().

For output value binding, MySQL sets *length when you call mysql_stmt_fetch(). The mysql_stmt_fetch() return value determines how to interpret the length:

length is ignored for numeric and temporal data types because the buffer_type value determines the length of the data value.

If you must determine the length of a returned value before fetching it, see Section 28.7.11.11, “mysql_stmt_fetch()”, for some strategies.

bool *is_null

This member points to a bool variable that is true if a value is NULL, false if it is not NULL. For input, set *is_null to true to indicate that you are passing a NULL value as a statement parameter.

is_null is a pointer to a boolean scalar, not a boolean scalar, to provide flexibility in how you specify NULL values:

For output, when you fetch a row, MySQL sets the value pointed to by is_null to true or false according to whether the result set column value returned from the statement is or is not NULL.

bool is_unsigned

This member applies for C variables with data types that can be unsigned (char, short int, int, long long int). Set is_unsigned to true if the variable pointed to by buffer is unsigned and false otherwise. For example, if you bind a signed char variable to buffer, specify a type code of MYSQL_TYPE_TINY and set is_unsigned to false. If you bind an unsigned char instead, the type code is the same but is_unsigned should be true. (For char, it is not defined whether it is signed or unsigned, so it is best to be explicit about signedness by using signed char or unsigned char.)

is_unsigned applies only to the C language variable on the client side. It indicates nothing about the signedness of the corresponding SQL value on the server side. For example, if you use an int variable to supply a value for a BIGINT UNSIGNED column, is_unsigned should be false because int is a signed type. If you use an unsigned int variable to supply a value for a BIGINT column, is_unsigned should be true because unsigned int is an unsigned type. MySQL performs the proper conversion between signed and unsigned values in both directions, although a warning occurs if truncation results.

bool *error

For output, set this member to point to a bool variable to have truncation information for the parameter stored there after a row fetching operation. When truncation reporting is enabled, mysql_stmt_fetch() returns MYSQL_DATA_TRUNCATED and *error is true in the MYSQL_BIND structures for parameters in which truncation occurred. Truncation indicates loss of sign or significant digits, or that a string was too long to fit in a column. Truncation reporting is enabled by default, but can be controlled by calling mysql_options() with the MYSQL_REPORT_DATA_TRUNCATION option.