SQLite

** in a Mem struct is returned by the MemType(Mem*) macro. The type is
** one of SQLITE3_NULL, SQLITE3_INTEGER, SQLITE3_REAL, SQLITE3_TEXT or
** SQLITE3_BLOB.
*/
struct Mem {
  i64 i;              /* Integer value */
  int n;              /* Number of characters in string value, including '\0' */
  int flags;          /* Some combination of MEM_Null, MEM_Str, MEM_Dyn, etc. */


  double r;           /* Real value */
  char *z;            /* String or BLOB value */
  char zShort[NBFS];  /* Space for short strings */
};
typedef struct Mem Mem;

/* One or more of the following flags are set to indicate the valid
** representations of the value stored in the Mem struct.
**
** If the MEM_Null flag is set, then the value is an SQL NULL value.
** No other flags may be set in this case.
**
** If the MEM_Str flag is set then Mem.z points at a string representation.
** Usually this is encoded in the same unicode encoding as the main
** database (see below for exceptions). If the MEM_Term flag is also
** set, then the string is nul terminated. The MEM_Int and MEM_Real 
** flags may coexist with the MEM_Str flag.
** 


*/
#define MEM_Null      0x0001   /* Value is NULL */
#define MEM_Str       0x0002   /* Value is a string */
#define MEM_Int       0x0004   /* Value is an integer */
#define MEM_Real      0x0008   /* Value is a real number */
#define MEM_Blob      0x0010   /* Value is a BLOB */

#define MEM_Term       0x0100   /* String rep is nul terminated */

/* Values with type NULL or BLOB can have only one representation. But
** values with a manifest type of REAL, INTEGER or STRING may have one
** or more representation cached in the Mem struct at any one time. The
** flags MEM_IntVal, MEM_RealVal and MEM_StrVal are true whenever the real,
** integer or string representation stored in a Mem struct is valid.
**
** When MEM_StrVal is set, then MEM_Term may also be set. This indicates
** that the string is terminated with a nul-terminator character.
*/
#define MEM_TypeInt    0x0020   /* Value type is an integer */
#define MEM_TypeReal   0x0040   /* Value type is real */
#define MEM_TypeStr    0x0080   /* Value type is string */


/* Whenever Mem contains a valid string or blob representation, one of
** the following flags must be set to determine the memory management
** policy for Mem.z

*/

#define MEM_Dyn       0x0200   /* Need to call sqliteFree() on Mem.z */
#define MEM_Static    0x0400   /* Mem.z points to a static string */
#define MEM_Ephem     0x0800   /* Mem.z points to an ephemeral string */
#define MEM_Short     0x1000   /* Mem.z points to Mem.zShort */

/* Internally, all strings manipulated by the  VDBE are encoded using the
** native encoding for the main database. Therefore the following three
** flags, which describe the text encoding of the string if the MEM_Str
** flag is true, are not generally valid for Mem* objects handled by the
** VDBE.
**
** When a user-defined function is called (see OP_Function), the Mem*
** objects that store the argument values for the function call are 
** passed to the user-defined function routine cast to sqlite3_value*.
** The user routine may then call sqlite3_value_text() or
** sqlite3_value_text16() to request a UTF-8 or UTF-16 string. If the
** string representation currently stored in Mem.z is not the requested
** encoding, then a translation occurs. To keep track of things, the
** MEM_Utf* flags are set correctly for the database encoding before a
** user-routine is called, and kept up to date if any translations occur
** thereafter.
** 
** When sqlite3_step() returns SQLITE3_ROW, indicating that a row of data
** is ready for processing by the caller, the data values are stored
** internally as Mem* objects. Before sqlite3_step() returns, the MEM_Utf*
** flags are set correctly for the database encoding. A translation may
** take place if the user requests a non-native encoding via
** sqlite3_column_text() or sqlite3_column_text16(). If this occurs, then
** the MEM_Utf* flags are updated accordingly.
*/
#define MEM_Utf8      0x2000   /* String uses UTF-8 encoding */
#define MEM_Utf16be   0x4000   /* String uses UTF-16 big-endian */
#define MEM_Utf16le   0x8000   /* String uses UTF-16 little-endian */

/* The following MEM_ value appears only in AggElem.aMem.s.flag fields.
** It indicates that the corresponding AggElem.aMem.z points to a
** aggregate function context that needs to be finalized.
*/
#define MEM_AggCtx    0x10000  /* Mem.z points to an agg function context */

/*
** The "context" argument for a installable function.  A pointer to an
** instance of this structure is the first argument to the routines used
** implement the SQL functions.
**
** There is a typedef for this structure in sqlite.h.  So all routines,