2.2.9 Decoding

The decoding of a CDF determines how attribute entry and variable data values are passed to a calling application program from the CDF library. The default decoding when a CDF is initially opened is host decoding (the native encoding of the computer being used). When host decoding is in effect, all data values read by an application are immediately ready for manipulation and display. Almost all of your applications will simply use the default of host decoding and not be concerned with selecting a decoding. There are some situations, however, where selecting a different decoding will be advantageous. Some possibilities are as follows:

1. A client/server model where a number of CDFs are maintained on a server computer (in any of the supported encodings). Clients on different type computers could request data from a CDF on the server computer. The server computer would then select a decoding for the CDF based on the client's computer type and then read the data value(s). The value(s) could then be sent directly to the client computer by the server computer without a conversion being necessary by either the client or the server. The CDF library would perform the necessary conversions.
2. If data values were being read from a CDF and written in binary form to a file for use on a different type computer. The proper decoding could be selected for the CDF before any of the data values are read. No conversions would be necessary by the application program.

A CDF's decoding may be selected and reselected at any time after the CDF has been opened and as many times as necessary. A CDF's decoding is selected via the Internal Interface with the <SELECT_,CDF_DECODING_> operation. Also, a CDF's decoding does not affect the values that already exist in a CDF or any values subsequently written. A CDF's encoding determines how the values are written to the CDF file(s). Section 2.2.8 describes a CDF's encoding.

The supported decodings correspond to the supported encodings. They are as follows:

HOST_DECODING

  The data representation of the host computer.

NETWORK_DECODING

The External Data Representation (XDR).

VAX_DECODING

VAX and microVAX data representation. Double-precision floating-point values will be in Digital's D_FLOAT representation.

ALPHAVMSd_DECODING

DEC Alpha running OpenVMS data representation. Double-precision floating-point values will be in Digital's D_FLOAT representation.

ALPHAVMSg_DECODING

DEC Alpha running OpenVMS data representation. Double-precision floating-point values will be in Digital's G_FLOAT representation.

SUN_DECODING

Sun data representation.

SGi_DECODING

Silicon Graphics Iris and Power Series data representation.

DECSTATION_DECODING

DECstation data representation.

ALPHAOSF1_DECODING

DEC Alpha running OSF/1 data representation.

IBMRS_DECODING

IBM RS6000 series data representation.

HP_DECODING

HP 9000 series data representation.

IBMPC_DECODING

IBM PC data representation.

NeXT_DECODING

NeXT data representation.

MAC_DECODING

Macintosh data representation.

• Performance