5.2 C API Basic Data Structures

This section describes C API data structures other than those used for prepared statements, the asynchronous interface, or the replication stream interface. For information about those, see Section 6.2, “C API Prepared Statement Data Structures”, Section 7.2, “C API Asynchronous Interface Data Structures”, and Section 10.2, “C API Binary Log Data Structures”.

• MYSQL

  This structure represents the handler for one database connection. It is used for almost all MySQL functions. Do not try to make a copy of a MYSQL structure. There is no guarantee that such a copy will be usable.

• MYSQL_RES

  This structure represents the result of a query that returns rows (SELECT, SHOW, DESCRIBE, EXPLAIN). The information returned from a query is called the result set in the remainder of this section.

• MYSQL_ROW

  This is a type-safe representation of one row of data. It is currently implemented as an array of counted byte strings. (You cannot treat these as null-terminated strings if field values may contain binary data, because such values may contain null bytes internally.) Rows are obtained by calling mysql_fetch_row().

• MYSQL_FIELD

  This structure contains metadata: information about a field, such as the field's name, type, and size. Its members are described in more detail later in this section. You may obtain the MYSQL_FIELD structures for each field by calling mysql_fetch_field() repeatedly. Field values are not part of this structure; they are contained in a MYSQL_ROW structure.

• MYSQL_FIELD_OFFSET

  This is a type-safe representation of an offset into a MySQL field list. (Used by mysql_field_seek().) Offsets are field numbers within a row, beginning at zero.

• my_ulonglong

  A type used for 64-bit unsigned integers. The my_ulonglong type was used before MySQL 8.0.18. As of MySQL 8.0.18, use the uint64_t C type instead.

• my_bool

  A boolean type, for values that are true (nonzero) or false (zero). The my_bool type was used before MySQL 8.0. As of MySQL 8.0, use the bool or int C type instead.

  Note

  The change from my_bool to bool means that the mysql.h header file requires a C++ or C99 compiler to compile.

The MYSQL_FIELD structure contains the members described in the following list. The definitions apply primarily for columns of result sets such as those produced by SELECT statements. MYSQL_FIELD structures are also used to provide metadata for OUT and INOUT parameters returned from stored procedures executed using prepared CALL statements. For such parameters, some of the structure members have a meaning different from the meaning for column values.

Tip

To view the MYSQL_FIELD member values for result sets interactively, start the mysql client with the --column-type-info option, then execute some sample queries.

• char * name

  The name of the field, as a null-terminated string. If the field was given an alias with an AS clause, the value of name is the alias. For a procedure parameter, the parameter name.

• char * org_name

  The name of the field, as a null-terminated string. Aliases are ignored. For expressions, the value is an empty string. For a procedure parameter, the parameter name.

• char * table

  The name of the table containing this field, if it is not a calculated field. For calculated fields, the table value is an empty string. If the column is selected from a view, table names the view. If the table or view was given an alias with an AS clause, the value of table is the alias. For a UNION, the value is the empty string. For a procedure parameter, the procedure name.

• char * org_table

  The name of the table, as a null-terminated string. Aliases are ignored. If the column is selected from a view, org_table names the view. If the column is selected from a derived table, org_table names the base table. If a derived table wraps a view, org_table still names the base table. If the column is an expression, org_table is the empty string. For a UNION, the value is the empty string. For a procedure parameter, the value is the procedure name.

• char * db

  The name of the database that the field comes from, as a null-terminated string. If the field is a calculated field, db is an empty string. For a UNION, the value is the empty string. For a procedure parameter, the name of the database containing the procedure.

• char * catalog

  The catalog name. This value is always "def".

• char * def

  The default value of this field, as a null-terminated string. This is set only if you use mysql_list_fields().

• unsigned long length

  The width of the field. This corresponds to the display length, in bytes.

  The server determines the length value before it generates the result set, so this is the minimum length required for a data type capable of holding the largest possible value from the result column, without knowing in advance the actual values that will be produced by the query for the result set.

  For string columns, the length value varies on the connection character set. For example, if the character set is latin1, a single-byte character set, the length value for a SELECT 'abc' query is 3. If the character set is utf8mb4, a multibyte character set in which characters take up to 4 bytes, the length value is 12.

• unsigned long max_length

  The maximum width of the field for the result set (the length in bytes of the longest field value for the rows actually in the result set). If you use mysql_store_result() or mysql_list_fields(), this contains the maximum length for the field. If you use mysql_use_result(), the value of this variable is zero.

  The value of max_length is the length of the string representation of the values in the result set. For example, if you retrieve a FLOAT column and the “widest” value is -12.345, max_length is 7 (the length of '-12.345').

  If you are using prepared statements, max_length is not set by default because for the binary protocol the lengths of the values depend on the types of the values in the result set. (See Section 6.2, “C API Prepared Statement Data Structures”.) If you want the max_length values anyway, enable the STMT_ATTR_UPDATE_MAX_LENGTH option with mysql_stmt_attr_set() and the lengths will be set when you call mysql_stmt_store_result(). (See Section 6.4.3, “mysql_stmt_attr_set()”, and Section 6.4.29, “mysql_stmt_store_result()”.)

• unsigned int name_length

  The length of name.

• unsigned int org_name_length

  The length of org_name.

• unsigned int table_length

  The length of table.

• unsigned int org_table_length

  The length of org_table.

• unsigned int db_length

  The length of db.

• unsigned int catalog_length

  The length of catalog.

• unsigned int def_length

  The length of def.

• unsigned int flags

  Bit-flags that describe the field. The flags value may have zero or more of the bits set that are shown in the following table.

  Flag Value	Flag Description
  NOT_NULL_FLAG	Field cannot be NULL
  PRI_KEY_FLAG	Field is part of a primary key
  UNIQUE_KEY_FLAG	Field is part of a unique key
  MULTIPLE_KEY_FLAG	Field is part of a nonunique key
  UNSIGNED_FLAG	Field has the UNSIGNED attribute
  ZEROFILL_FLAG	Field has the ZEROFILL attribute
  BINARY_FLAG	Field has the BINARY attribute
  AUTO_INCREMENT_FLAG	Field has the AUTO_INCREMENT attribute
  ENUM_FLAG	Field is an ENUM
  SET_FLAG	Field is a SET
  BLOB_FLAG	Field is a BLOB or TEXT (deprecated)
  TIMESTAMP_FLAG	Field is a TIMESTAMP (deprecated)
  NUM_FLAG	Field is numeric; see additional notes following table
  NO_DEFAULT_VALUE_FLAG	Field has no default value; see additional notes following table

  Some of these flags indicate data type information and are superseded by or used in conjunction with the MYSQL_TYPE_xxx value in the field->type member described later:

  • To check for BLOB or TIMESTAMP values, check whether type is MYSQL_TYPE_BLOB or MYSQL_TYPE_TIMESTAMP. (The BLOB_FLAG and TIMESTAMP_FLAG flags are unneeded.)

  • ENUM and SET values are returned as strings. For these, check that the type value is MYSQL_TYPE_STRING and that the ENUM_FLAG or SET_FLAG flag is set in the flags value.

  NUM_FLAG indicates that a column is numeric. This includes columns with a type of MYSQL_TYPE_DECIMAL, MYSQL_TYPE_NEWDECIMAL, MYSQL_TYPE_TINY, MYSQL_TYPE_SHORT, MYSQL_TYPE_LONG, MYSQL_TYPE_FLOAT, MYSQL_TYPE_DOUBLE, MYSQL_TYPE_NULL, MYSQL_TYPE_LONGLONG, MYSQL_TYPE_INT24, and MYSQL_TYPE_YEAR.

  NO_DEFAULT_VALUE_FLAG indicates that a column has no DEFAULT clause in its definition. This does not apply to NULL columns (because such columns have a default of NULL), or to AUTO_INCREMENT columns (which have an implied default value).

  The following example illustrates a typical use of the flags value:

  if (field->flags & NOT_NULL_FLAG)
      printf("Field cannot be null\n");

  You may use the convenience macros shown in the following table to determine the boolean status of the flags value.

  Flag Status	Description
  IS_NOT_NULL(flags)	True if this field is defined as NOT NULL
  IS_PRI_KEY(flags)	True if this field is a primary key
  IS_BLOB(flags)	True if this field is a BLOB or TEXT (deprecated; test field->type instead)

• unsigned int decimals

  The number of decimals for numeric fields, and the fractional seconds precision for temporal fields.

• unsigned int charsetnr

  An ID number that indicates the character set/collation pair for the field.

  Normally, character values in result sets are converted to the character set indicated by the character_set_results system variable. In this case, charsetnr corresponds to the character set indicated by that variable. Character set conversion can be suppressed by setting character_set_results to NULL. In this case, charsetnr corresponds to the character set of the original table column or expression. See also Connection Character Sets and Collations.

  To distinguish between binary and nonbinary data for string data types, check whether the charsetnr value is 63. 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.

  charsetnr values are the same as those displayed in the Id column of the SHOW COLLATION statement or the ID column of the INFORMATION_SCHEMA COLLATIONS table. You can use those information sources to see which character set and collation specific charsetnr values indicate:

  mysql> SHOW COLLATION WHERE Id = 63;
  +-----------+---------+----+---------+----------+---------+
  | Collation | Charset | Id | Default | Compiled | Sortlen |
  +-----------+---------+----+---------+----------+---------+
  | binary    | binary  | 63 | Yes     | Yes      |       1 |
  +-----------+---------+----+---------+----------+---------+
  
  mysql> SELECT COLLATION_NAME, CHARACTER_SET_NAME
         FROM INFORMATION_SCHEMA.COLLATIONS WHERE ID = 33;
  +-----------------+--------------------+
  | COLLATION_NAME  | CHARACTER_SET_NAME |
  +-----------------+--------------------+
  | utf8_general_ci | utf8               |
  +-----------------+--------------------+

• enum enum_field_types type

  The type of the field. The type value may be one of the MYSQL_TYPE_ symbols shown in the following table.

  Type Value	Type Description
  MYSQL_TYPE_TINY	TINYINT field
  MYSQL_TYPE_SHORT	SMALLINT field
  MYSQL_TYPE_LONG	INTEGER field
  MYSQL_TYPE_INT24	MEDIUMINT field
  MYSQL_TYPE_LONGLONG	BIGINT field
  MYSQL_TYPE_DECIMAL	DECIMAL or NUMERIC field
  MYSQL_TYPE_NEWDECIMAL	Precision math DECIMAL or NUMERIC
  MYSQL_TYPE_FLOAT	FLOAT field
  MYSQL_TYPE_DOUBLE	DOUBLE or REAL field
  MYSQL_TYPE_BIT	BIT field
  MYSQL_TYPE_TIMESTAMP	TIMESTAMP field
  MYSQL_TYPE_DATE	DATE field
  MYSQL_TYPE_TIME	TIME field
  MYSQL_TYPE_DATETIME	DATETIME field
  MYSQL_TYPE_YEAR	YEAR field
  MYSQL_TYPE_STRING	CHAR or BINARY field
  MYSQL_TYPE_VAR_STRING	VARCHAR or VARBINARY field
  MYSQL_TYPE_BLOB	BLOB or TEXT field (use max_length to determine the maximum length)
  MYSQL_TYPE_SET	SET field
  MYSQL_TYPE_ENUM	ENUM field
  MYSQL_TYPE_GEOMETRY	Spatial field
  MYSQL_TYPE_NULL	NULL-type field

  The MYSQL_TYPE_TIME2, MYSQL_TYPE_DATETIME2, and MYSQL_TYPE_TIMESTAMP2) type codes are used only on the server side. Clients see the MYSQL_TYPE_TIME, MYSQL_TYPE_DATETIME, and MYSQL_TYPE_TIMESTAMP codes.

  You can use the IS_NUM() macro to test whether a field has a numeric type. Pass the type value to IS_NUM() and it evaluates to TRUE if the field is numeric:

  if (IS_NUM(field->type))
      printf("Field is numeric\n");

  ENUM and SET values are returned as strings. For these, check that the type value is MYSQL_TYPE_STRING and that the ENUM_FLAG or SET_FLAG flag is set in the flags value.