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MySQL 5.0 Reference Manual :: 25 APIs and Libraries :: 25.2 MySQL C API :: 25.2.5 C API Prepared Statement Data types

25.2.5. C API Prepared Statement Data types

Prepared statements use several data structures:

  • To prepare a statement, pass the statement string to mysql_stmt_init(), which returns a pointer to a MYSQL_STMT data structure.

  • To provide input parameters for a prepared statement, set up MYSQL_BIND structures and pass them to mysql_stmt_bind_param(). To receive output column values, set up MYSQL_BIND structures and pass them to mysql_stmt_bind_result().

  • The MYSQL_TIME structure is used to transfer temporal data in both directions.

The following discussion describes the prepared statement data types in detail.

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  • MYSQL_STMT

    This structure represents a prepared statement. A statement is created by calling mysql_stmt_init(), which returns a statement handle (that is, a pointer to a MYSQL_STMT). The handle is used for all subsequent operations with the statement until you close it with mysql_stmt_close(), at which point the handle becomes invalid.

    The MYSQL_STMT structure has no members that are intended for application use. Also, you should not try to make a copy of a MYSQL_STMT structure. There is no guarantee that such a copy will be usable.

    Multiple statement handles can be associated with a single connection. The limit on the number of handles depends on the available system resources.

  • MYSQL_BIND

    This structure is used both for statement input (data values sent to the server) and output (result values returned from the server):

    To use a MYSQL_BIND structure, you should zero its contents to initialize it, and then set its members appropriately. For example, to declare and initialize an array of three MYSQL_BIND structures, use this code: 

    MYSQL_BIND bind[3];
memset(bind, 0, sizeof(bind));

    The MYSQL_BIND structure contains the following members for use by application programs. For several of the members, the manner of use depends on whether the structure is used for input or output.

    • enum enum_field_types buffer_type

      The type of the buffer. This member indicates the data type of the C language variable that you are binding to the statement parameter. The allowable buffer_type values are listed later in this section. For input, buffer_type indicates the type of the variable containing the value that you will send to the server. For output, it indicates the type of the variable into which you want a value received from the server to be stored.

    • void *buffer

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

      For input, buffer is a pointer to the variable in which a statement parameter's data value is stored. When you call mysql_stmt_execute(), MySQL takes the value that you have stored in the variable and uses it in place of the corresponding parameter marker in the statement.

      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 returns a column value and stores it in this variable. You can access the value when the call returns.

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      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 variables that have types similar to those of the corresponding SQL values. For numeric data types, buffer should point to a variable of the proper numeric C type. (For char or integer variables, you should also indicate whether the variable has the unsigned attribute by setting the is_unsigned member, described later in this list.) For character (non-binary) and binary string data types, buffer should point to a character buffer. For date and time data types, buffer should point to a MYSQL_TIME structure.

      See the notes about type conversions later in the section.

    • 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, length points to an unsigned long variable that indicates the actual length of the parameter value stored in *buffer; this is used by mysql_stmt_execute().

      For output value binding, the return value of mysql_stmt_fetch() determines the interpretation of the length:

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      • If mysql_stmt_fetch() returns 0, *length indicates the actual length of the parameter value.

      • If mysql_stmt_fetch() returns MYSQL_DATA_TRUNCATED, *length indicates the non-truncated length of the parameter value. In this case, the minimum of *length and buffer_length indicates the actual length of the value.

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

      If you need to be able to determine the length of a returned value before fetching it with mysql_stmt_fetch(), see Section 25.2.7.11, “mysql_stmt_fetch()”, for some strategies.

    • my_bool *is_null

      This member points to a my_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.

      The reason that is_null is not a boolean scalar but is instead a pointer to a boolean scalar is to provide flexibility in how you specify NULL values:

      • If your data values are always NULL, use MYSQL_TYPE_NULL as the buffer_type value when you bind the column. The other members do not matter.

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      • If your data values are always NOT NULL, set the other members appropriately for the variable you are binding, and set is_null = (my_bool*) 0.

      • In all other cases, set the other members appriopriately, and set is_null to the address of a my_bool variable. Set that variable's value to true or false appropriately between executions to indicate whether data values are NULL or NOT NULL, respectively.

      For output, the value pointed to by is_null is set to true after you fetch a row if the result set column value returned from the statement is NULL.

    • my_bool is_unsigned

      This member is used 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.

    • my_bool *error

      For output, set this member to point to a my_bool variable to have truncation information for the parameter stored there after a row fetching operation. (Truncation reporting is enabled by default, but can be controlled by calling mysql_options() with the MYSQL_REPORT_DATA_TRUNCATION option.) 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. The error member was added in MySQL 5.0.3.

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  • MYSQL_TIME

    This structure is used to send and receive DATE, TIME, DATETIME, and TIMESTAMP data directly to and from the server. Set the buffer_type member of a MYSQL_BIND structure to one of the temporal types (MYSQL_TYPE_TIME, MYSQL_TYPE_DATE, MYSQL_TYPE_DATETIME, MYSQL_TYPE_TIMESTAMP), and set the buffer member to point to a MYSQL_TIME structure.

    The MYSQL_TIME structure contains the members listed in the following table.

    MemberDescription
    unsigned int yearThe year
    unsigned int monthThe month of the year
    unsigned int dayThe day of the month
    unsigned int hourThe hour of the day
    unsigned int minuteThe minute of the hour
    unsigned int secondThe second of the minute
    my_bool negA boolean flag to indicate whether the time is negative
    unsigned long second_partThe fractional part of the second in microseconds; currently unused

    Only those parts of a MYSQL_TIME structure that apply to a given type of temporal value are used. The year, month, and day elements are used for DATE, DATETIME, and TIMESTAMP values. The hour, minute, and second elements are used for TIME, DATETIME, and TIMESTAMP values. See Section 25.2.10, “C API Handling of Date and Time Values”.

The following table shows the allowable values that may be specified in the buffer_type member of MYSQL_BIND structures for input values. The value should be chosen according to the data type of the C language variable that you are binding. If the variable is unsigned, you should also set the is_unsigned member to true. The table shows the C variable types that you can use, the corresponding type codes, and the SQL data types for which the supplied value can be used without conversion.

Input Variable C Typebuffer_type ValueSQL Type of Destination Value
signed charMYSQL_TYPE_TINYTINYINT
short intMYSQL_TYPE_SHORTSMALLINT
intMYSQL_TYPE_LONGINT
long long intMYSQL_TYPE_LONGLONGBIGINT
floatMYSQL_TYPE_FLOATFLOAT
doubleMYSQL_TYPE_DOUBLEDOUBLE
MYSQL_TIMEMYSQL_TYPE_TIMETIME
MYSQL_TIMEMYSQL_TYPE_DATEDATE
MYSQL_TIMEMYSQL_TYPE_DATETIMEDATETIME
MYSQL_TIMEMYSQL_TYPE_TIMESTAMPTIMESTAMP
char[]MYSQL_TYPE_STRING (for non-binary data)TEXT, CHAR, VARCHAR
char[]MYSQL_TYPE_BLOB (for binary data)BLOB, BINARY, VARBINARY
 MYSQL_TYPE_NULLNULL

The use of MYSQL_TYPE_NULL is described earlier in connection with the is_null member.

The following table shows the allowable values that may be specified in the buffer_type member of MYSQL_BIND structures for output values. The value should be chosen according to the data type of the C language variable that you are binding. If the variable is unsigned, you should also set the is_unsigned member to true. The table shows the SQL types of received values, the corresponding type code that such values have in result set metadata, and the recommended C language data types to bind to the MYSQL_BIND structure to receive the SQL values without conversion.

If there is a mismatch between the C variable type on the client side and the corresponding SQL value on the server side, MySQL performs implicit type conversions in both directions.

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SQL Type of Received Valuebuffer_type ValueOutput Variable C Type
TINYINTMYSQL_TYPE_TINYsigned char
SMALLINTMYSQL_TYPE_SHORTshort int
MEDIUMINTMYSQL_TYPE_INT24int
INTMYSQL_TYPE_LONGint
BIGINTMYSQL_TYPE_LONGLONGlong long int
FLOATMYSQL_TYPE_FLOATfloat
DOUBLEMYSQL_TYPE_DOUBLEdouble
DECIMALMYSQL_TYPE_NEWDECIMALchar[]
YEARMYSQL_TYPE_SHORTshort int
TIMEMYSQL_TYPE_TIMEMYSQL_TIME
DATEMYSQL_TYPE_DATEMYSQL_TIME
DATETIMEMYSQL_TYPE_DATETIMEMYSQL_TIME
TIMESTAMPMYSQL_TYPE_TIMESTAMPMYSQL_TIME
CHAR, BINARYMYSQL_TYPE_STRINGchar[]
VARCHAR, VARBINARYMYSQL_TYPE_VAR_STRINGchar[]
TINYBLOB, TINYTEXTMYSQL_TYPE_TINY_BLOBchar[]
BLOB, TEXTMYSQL_TYPE_BLOBchar[]
MEDIUMBLOB, MEDIUMTEXTMYSQL_TYPE_MEDIUM_BLOBchar[]
LONGBLOB, LONGTEXTMYSQL_TYPE_LONG_BLOBchar[]
BITMYSQL_TYPE_BITchar[]

MySQL knows the type code for the SQL value on the server side. The buffer_type value indicates the MySQL 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:

  • If you use MYSQL_TYPE_LONG with an int variable to pass an integer value to the server that is to be stored into a FLOAT column, MySQL converts the value to floating-point format before storing it.

  • If you fetch a SQL MEDIUMINT column value, but specify a buffer_type value of MYSQL_TYPE_LONGLONG and use a C variable of type long long int as the destination buffer, MySQL will convert the MEDIUMINT value (which requires less than 8 bytes) for storage into the long long int (an 8-byte variable).

  • If you fetch a numeric column with a value of 255 into a char[4] character array and specify a buffer_type value of MYSQL_TYPE_STRING, the resulting value in the array will be a 4-byte string containing '255\0'.

  • DECIMAL values are returned as strings, which is why the corresponding C type is char[]. DECIMAL values returned by the server correspond to the string representation of the original server-side value. For example, 12.345 is returned to the client as '12.345'. If you specify MYSQL_TYPE_NEWDECIMAL and bind a string buffer to the MYSQL_BIND structure, mysql_stmt_fetch() stores the value in the buffer without conversion. If instead you specify a numeric variable and type code, mysql_stmt_fetch() converts the string-format DECIMAL value to numeric form.

  • For the MYSQL_TYPE_BIT type code, BIT values are returned into a string buffer (thus, the corresponding C type is char[] here, too). The value represents a bit string that requires interpretation on the client side. To return the value as a type that is easier to deal with, you can cause the value to be cast to integer using either of the following types of expressions: 

    SELECT bit_col + 0 FROM t
SELECT CAST(bit_col AS UNSIGNED) FROM t

    To retrieve the value, bind an integer variable large enough to hold the value and specify the appropriate corresponding integer type code.

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 25.2.7.22, “mysql_stmt_result_metadata()”, and Section 25.2.1, “C API Data Types”.

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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. The size of the buffers should be chosen according to the types of the variables into which you fetch the values.

For input character (non-binary) string data (indicated by MYSQL_TYPE_STRING), the value is assumed to be in the character set indicated by the character_set_client system variable. If the value is stored into a column with a different character set, the appropriate conversion to that character set occurs. For input binary string data (indicated by MYSQL_TYPE_BLOB), the value is treated as having the binary character set; that is, it is treated as a byte string and no conversion occurs.

To determine whether output string values in a result set returned from the server contain binary or non-binary data, check whether the charsetnr value of the result set metadata is 63 (see Section 25.2.1, “C API Data Types”). If so, the character set is binary, which indicates binary rather than non-binary data. This enables you to distinguish BINARY from CHAR, VARBINARY from VARCHAR, and the BLOB types from the TEXT types.


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