2.2.5.2.3 Data Type Dependent Data Streams

Some messages contain variable data types. The actual type of a given variable data type is dependent on the type of the data being sent within the message as defined in the TYPE_INFO rule.

For example, the RPCRequest message contains the TYPE_INFO and TYPE_VARBYTE rules. These two rules contain data of a type that is dependent on the actual type used in the value of the FIXEDLENTYPE or VARLENTYPE rules of the TYPE_INFO rule.

Data type-dependent data streams occur in three forms: integers, fixed and variable bytes, and partially length-prefixed bytes.

Integers

Data type-dependent integers can be either a BYTELEN, USHORTCHARBINLEN, or LONGLEN in length. This length is dependent on the TYPE_INFO associated with the message. If the data type (for example, FIXEDLENTYPE or VARLENTYPE rule of the TYPE_INFO rule) is of type SSVARIANTTYPE, TEXTTYPE, NTEXTTYPE, or IMAGETYPE, the integer length is LONGLEN. If the data type is BIGCHARTYPE, BIGVARCHARTYPE, NCHARTYPE, NVARCHARTYPE, BIGBINARYTYPE, or BIGVARBINARYTYPE, the integer length is USHORTCHARBINLEN. For all other data types, the integer length is BYTELEN.

 TYPE_VARLEN      =   BYTELEN
                      /
                      USHORTCHARBINLEN
                      /
                      LONGLEN

Fixed and Variable Bytes

The data type to be used in a data type-dependent byte stream is defined by the TYPE_INFO rule associated with the message.

For variable-length types, with the exception of PLP (see Partially Length-prefixed Bytes below), the TYPE_VARLEN value defines the length of the data to follow. As described above, the TYPE_INFO rule defines the type of TYPE_VARLEN (for example BYTELEN, USHORTCHARBINLEN, or LONGLEN).

For fixed-length types, the TYPE_VARLEN rule is not present. In these cases, the number of bytes to be read is determined by the TYPE_INFO rule . For example, if "INT2TYPE" is specified as the value for the FIXEDLENTYPE rule of the TYPE_INFO rule, 2 bytes are read because "INT2TYPE" is always 2 bytes in length. For more details, see Data Types Definitions.

The data following this can be a stream of bytes or a NULL value. The 2-byte CHARBIN_NULL rule is used for BIGCHARTYPE, BIGVARCHARTYPE, NCHARTYPE, NVARCHARTYPE, BIGBINARYTYPE, and BIGVARBINARYTYPE types, and the 4-byte CHARBIN_NULL rule is used for TEXTTYPE, NTEXTTYPE, and IMAGETYPE. The GEN_NULL rule applies to all other types aside from PLP:

 TYPE_VARBYTE = GEN_NULL / CHARBIN_NULL / PLP_BODY
                / ([TYPE_VARLEN] *BYTE)

Partially Length-prefixed Bytes

Unlike fixed or variable byte stream formats, Partially length-prefixed bytes (PARTLENTYPE), introduced in TDS 7.2, do not require the full data length to be specified before the actual data is streamed out. Thus, it is ideal for those applications where the data length is not known upfront (that is, xml serialization). A value sent as PLP can be either NULL, a length followed by chunks (as defined by PLP_CHUNK), or an unknown length token followed by chunks, which MUST end with a PLP_TERMINATOR. The rule below describes the stream format (for example, the format of a singleton PLP value):

 PLP_BODY=    PLP_NULL
                      /
                      ((ULONGLONGLEN / UNKNOWN_PLP_LEN)  
                      *PLP_CHUNK PLP_TERMINATOR)
  
 PLP_NULL         =   %xFFFFFFFFFFFFFFFF
  
 UNKNOWN_PLP_LEN  =   %xFFFFFFFFFFFFFFFE
  
 PLP_CHUNK        =   ULONGLEN 1*BYTE
  
 PLP_TERMINATOR   =   %x00000000

Notes

  • TYPE_INFO rule specifies a Partially Length-prefixed Data type (PARTLENTYPE, see 2.2.5.4.3).

  • In the UNKNOWN_PLP_LEN case, the data is represented as a series of zero or more chunks, each consisting of the length field followed by length bytes of data (see the PLP_CHUNK rule). The data is terminated by PLP_TERMINATOR (which is essentially a zero-length chunk).

  • In the actual data length case, the ULONGLONGLEN specifies the length of the data and is followed by any number of PLP_CHUNKs containing the data. The length of the data specified by ULONGLONGLEN is used as a hint for the receiver. The receiver SHOULD validate that the length value specified by ULONGLONGLEN matches the actual data length.

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