File Format

This section and the following sections contain a detailed description of the IDL save file format. As described above, and IDL file consists of a stream of tagged records:

Format: SAVEFMT        (overall save file format)

Type	Name	Description
BYTE$\times$2	SIGNATURE	IDL SAVE file signature (=BYTE(['S','R']))
BYTE$\times$2	RECFMT	IDL SAVE record format (=['00'xb, '04'xb])
Record 1
Record 2
$\vdots$
Record $n$ -- END_MARKER

However, these records must appear in a certain order, which also depends on which version of IDL is being used. These records might be described as the save file ``preamble,'' which give important metadata about the information. For IDL 4 this preamble consists of only one record:

Format: PREAMBLE4        (preamble records for IDL 4 save files)

Type	Name	Description
TIMESTAMP	TIMESTAMP	Save file history information

The first record must be of type TIMESTAMP (see below for record types). Beyond that, save files produced by IDL 4 do not contain heap data. For IDL version 5 and beyond, the preamble is extended to include several more records after the TIMESTAMP record, which describes the file version information, such as which version of IDL was used to create the file:

Format: PREAMBLE5        (preamble records for IDL 5 save files)

Type	Name	Description
TIMESTAMP	TIMESTAMP	Save file history information
VERSION	VERSION	Save session IDL information
HEAP_HEADER	HEAP_HEADER	(optional) Heap index information
--	PROMOTE64	(optional IDL 5.4 only) if file has ULONG64 offsets
NOTICE	NOTICE	(optional) Disclaimer notice

The HEAP_HEADER record only appears if the save file contains heap data (i.e., pointers). In IDL 5.4, the PROMOTE64 record indicates the presence of ULONG64 record offsets for storing files larger than 4 gigabytes (IDL 5.4 only).