Implement variable and data file attributes.

[pspp-builds.git] / doc / dev / system-file-format.texi

@node System File Format
@appendix System File Format

A system file encapsulates a set of cases and dictionary information
that describes how they may be interpreted.  This chapter describes
the format of a system file.

System files use three data types: 8-bit characters, 32-bit integers,
and 64-bit floating points, called here @code{char}, @code{int32}, and
@code{flt64}, respectively.  Data is not necessarily aligned on a word
or double-word boundary: the long variable name record (@pxref{Long
Variable Names Record}) and very long string records (@pxref{Very Long
String Record}) have arbitrary byte length and can therefore cause all
data coming after them in the file to be misaligned.

Integer data in system files may be big-endian or little-endian.  A
reader may detect the endianness of a system file by examining
@code{layout_code} in the file header record
(@pxref{layout_code,,@code{layout_code}}).

Floating-point data in system files may nominally be in IEEE 754, IBM,
or VAX formats.  A reader may detect the floating-point format in use
by examining @code{bias} in the file header record
(@pxref{bias,,@code{bias}}).

PSPP detects big-endian and little-endian integer formats in system
files and translates as necessary.  PSPP also detects the
floating-point format in use, as well as the endianness of IEEE 754
floating-point numbers, and translates as needed.  However, only IEEE
754 numbers with the same endianness as integer data in the same file
has actually been observed in system files, and it is likely that
other formats are obsolete or were never used.

The PSPP system-missing value is represented by the largest possible
negative number in the floating point format (@code{-DBL_MAX}).  Two
other values are important for use as missing values: @code{HIGHEST},
represented by the largest possible positive number (@code{DBL_MAX}),
and @code{LOWEST}, represented by the second-largest negative number
(in IEEE 754 format, @code{0xffeffffffffffffe}).

System files are divided into records, each of which begins with a
4-byte record type, usually regarded as an @code{int32}.

The records must appear in the following order:

@itemize @bullet
@item
File header record.

@item
Variable records.

@item
All pairs of value labels records and value label variables records,
if present.

@item
Document record, if present.

@item
Any of the following records, if present, in any order:

@itemize @minus
@item
Machine integer info record.

@item
Machine floating-point info record.

@item
Variable display parameter record.

@item
Long variable names record.

@item
Miscellaneous informational records.
@end itemize

@item
Dictionary termination record.

@item
Data record.
@end itemize

Each type of record is described separately below.

@menu
* File Header Record::
* Variable Record::
* Value Labels Records::
* Document Record::
* Machine Integer Info Record::
* Machine Floating-Point Info Record::
* Variable Display Parameter Record::
* Long Variable Names Record::
* Very Long String Record::
* Data File and Variable Attributes Records::
* Miscellaneous Informational Records::
* Dictionary Termination Record::
* Data Record::
@end menu

@node File Header Record
@section File Header Record

The file header is always the first record in the file.  It has the
following format:

@example
char                rec_type[4];
char                prod_name[60];
int32               layout_code;
int32               nominal_case_size;
int32               compressed;
int32               weight_index;
int32               ncases;
flt64               bias;
char                creation_date[9];
char                creation_time[8];
char                file_label[64];
char                padding[3];
@end example

@table @code
@item char rec_type[4];
Record type code, set to @samp{$FL2}.

@item char prod_name[60];
Product identification string.  This always begins with the characters
@samp{@@(#) SPSS DATA FILE}.  PSPP uses the remaining characters to
give its version and the operating system name; for example, @samp{GNU
pspp 0.1.4 - sparc-sun-solaris2.5.2}.  The string is truncated if it
would be longer than 60 characters; otherwise it is padded on the right
with spaces.

@anchor{layout_code}
@item int32 layout_code;
Normally set to 2, although a few system files have been spotted in
the wild with a value of 3 here.  PSPP use this value to determine the
file's integer endianness (@pxref{System File Format}).

@item int32 nominal_case_size;
Number of data elements per case.  This is the number of variables,
except that long string variables add extra data elements (one for every
8 characters after the first 8).  However, string variables do not
contribute to this value beyond the first 255 bytes.   Further, system
files written by some systems set this value to -1.  In general, it is
unsafe for systems reading system files to rely upon this value.

@item int32 compressed;
Set to 1 if the data in the file is compressed, 0 otherwise.

@item int32 weight_index;
If one of the variables in the data set is used as a weighting
variable, set to the dictionary index of that variable, plus 1
(@pxref{Dictionary Index}).  Otherwise, set to 0.

@item int32 ncases;
Set to the number of cases in the file if it is known, or -1 otherwise.

In the general case it is not possible to determine the number of cases
that will be output to a system file at the time that the header is
written.  The way that this is dealt with is by writing the entire
system file, including the header, then seeking back to the beginning of
the file and writing just the @code{ncases} field.  For `files' in which
this is not valid, the seek operation fails.  In this case,
@code{ncases} remains -1.

@anchor{bias}
@item flt64 bias;
Compression bias, ordinarily set to 100.  Only integers between
@code{1 - bias} and @code{251 - bias} can be compressed.

By assuming that its value is 100, PSPP uses @code{bias} to determine
the file's floating-point format and endianness (@pxref{System File
Format}).  If the compression bias is not 100, PSPP cannot auto-detect
the floating-point format and assumes that it is IEEE 754 format with
the same endianness as the system file's integers, which is correct
for all known system files.

@item char creation_date[9];
Date of creation of the system file, in @samp{dd mmm yy}
format, with the month as standard English abbreviations, using an
initial capital letter and following with lowercase.  If the date is not
available then this field is arbitrarily set to @samp{01 Jan 70}.

@item char creation_time[8];
Time of creation of the system file, in @samp{hh:mm:ss}
format and using 24-hour time.  If the time is not available then this
field is arbitrarily set to @samp{00:00:00}.

@item char file_label[64];
File label declared by the user, if any (@pxref{FILE LABEL,,,pspp,
PSPP Users Guide}).  Padded on the right with spaces.

@item char padding[3];
Ignored padding bytes to make the structure a multiple of 32 bits in
length.  Set to zeros.
@end table

@node Variable Record
@section Variable Record

There must be one variable record for each numeric variable and each
string variable with width 8 bytes or less.  String variables wider
than 8 bytes have one variable record for each 8 bytes, rounding up.
The first variable record for a long string specifies the variable's
correct dictionary information.  Subsequent variable records for a
long string are filled with dummy information: a type of -1, no
variable label or missing values, print and write formats that are
ignored, and an empty string as name.  A few system files have been
encountered that include a variable label on dummy variable records,
so readers should take care to parse dummy variable records in the
same way as other variable records.

@anchor{Dictionary Index}
The @dfn{dictionary index} of a variable is its offset in the set of
variable records, including dummy variable records for long string
variables.  The first variable record has a dictionary index of 0, the
second has a dictionary index of 1, and so on.

The system file format does not directly support string variables
wider than 255 bytes.  Such very long string variables are represented
by a number of narrower string variables.  @xref{Very Long String
Record}, for details.

@example
int32               rec_type;
int32               type;
int32               has_var_label;
int32               n_missing_values;
int32               print;
int32               write;
char                name[8];

/* @r{Present only if @code{has_var_label} is 1.} */
int32               label_len;
char                label[];

/* @r{Present only if @code{n_missing_values} is nonzero}. */
flt64               missing_values[];
@end example

@table @code
@item int32 rec_type;
Record type code.  Always set to 2.

@item int32 type;
Variable type code.  Set to 0 for a numeric variable.  For a short
string variable or the first part of a long string variable, this is set
to the width of the string.  For the second and subsequent parts of a
long string variable, set to -1, and the remaining fields in the
structure are ignored.

@item int32 has_var_label;
If this variable has a variable label, set to 1; otherwise, set to 0.

@item int32 n_missing_values;
If the variable has no missing values, set to 0.  If the variable has
one, two, or three discrete missing values, set to 1, 2, or 3,
respectively.  If the variable has a range for missing variables, set to
-2; if the variable has a range for missing variables plus a single
discrete value, set to -3.

@item int32 print;
Print format for this variable.  See below.

@item int32 write;
Write format for this variable.  See below.

@item char name[8];
Variable name.  The variable name must begin with a capital letter or
the at-sign (@samp{@@}).  Subsequent characters may also be digits, octothorpes
(@samp{#}), dollar signs (@samp{$}), underscores (@samp{_}), or full
stops (@samp{.}).  The variable name is padded on the right with spaces.

@item int32 label_len;
This field is present only if @code{has_var_label} is set to 1.  It is
set to the length, in characters, of the variable label, which must be a
number between 0 and 120.

@item char label[];
This field is present only if @code{has_var_label} is set to 1.  It has
length @code{label_len}, rounded up to the nearest multiple of 32 bits.
The first @code{label_len} characters are the variable's variable label.

@item flt64 missing_values[];
This field is present only if @code{n_missing_values} is not 0.  It has
the same number of elements as the absolute value of
@code{n_missing_values}.  For discrete missing values, each element
represents one missing value.  When a range is present, the first
element denotes the minimum value in the range, and the second element
denotes the maximum value in the range.  When a range plus a value are
present, the third element denotes the additional discrete missing
value.  HIGHEST and LOWEST are indicated as described in the chapter
introduction.
@end table

The @code{print} and @code{write} members of sysfile_variable are output
formats coded into @code{int32} types.  The least-significant byte
of the @code{int32} represents the number of decimal places, and the
next two bytes in order of increasing significance represent field width
and format type, respectively.  The most-significant byte is not
used and should be set to zero.

Format types are defined as follows:

@quotation
@multitable {Value} {@code{DATETIME}}
@headitem Value
@tab Meaning
@item 0
@tab Not used.
@item 1
@tab @code{A}
@item 2
@tab @code{AHEX}
@item 3
@tab @code{COMMA}
@item 4
@tab @code{DOLLAR}
@item 5
@tab @code{F}
@item 6
@tab @code{IB}
@item 7
@tab @code{PIBHEX}
@item 8
@tab @code{P}
@item 9
@tab @code{PIB}
@item 10
@tab @code{PK}
@item 11
@tab @code{RB}
@item 12
@tab @code{RBHEX}
@item 13
@tab Not used.
@item 14
@tab Not used.
@item 15
@tab @code{Z}
@item 16
@tab @code{N}
@item 17
@tab @code{E}
@item 18
@tab Not used.
@item 19
@tab Not used.
@item 20
@tab @code{DATE}
@item 21
@tab @code{TIME}
@item 22
@tab @code{DATETIME}
@item 23
@tab @code{ADATE}
@item 24
@tab @code{JDATE}
@item 25
@tab @code{DTIME}
@item 26
@tab @code{WKDAY}
@item 27
@tab @code{MONTH}
@item 28
@tab @code{MOYR}
@item 29
@tab @code{QYR}
@item 30
@tab @code{WKYR}
@item 31
@tab @code{PCT}
@item 32
@tab @code{DOT}
@item 33
@tab @code{CCA}
@item 34
@tab @code{CCB}
@item 35
@tab @code{CCC}
@item 36
@tab @code{CCD}
@item 37
@tab @code{CCE}
@item 38
@tab @code{EDATE}
@item 39
@tab @code{SDATE}
@end multitable
@end quotation

@node Value Labels Records
@section Value Labels Records

The value label record has the following format:

@example
int32               rec_type;
int32               label_count;

/* @r{Repeated @code{label_cnt} times}. */
char                value[8];
char                label_len;
char                label[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 3.

@item int32 label_count;
Number of value labels present in this record.
@end table

The remaining fields are repeated @code{count} times.  Each
repetition specifies one value label.

@table @code
@item char value[8];
A numeric value or a short string value padded as necessary to 8 bytes
in length.  Its type and width cannot be determined until the
following value label variables record (see below) is read.

@item char label_len;
The label's length, in bytes.

@item char label[];
@code{label_len} bytes of the actual label, followed by up to 7 bytes
of padding to bring @code{label} and @code{label_len} together to a
multiple of 8 bytes in length.
@end table

The value label record is always immediately followed by a value label
variables record with the following format:

@example
int32               rec_type;
int32               var_count;
int32               vars[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 4.

@item int32 var_count;
Number of variables that the associated value labels from the value
label record are to be applied.

@item int32 vars[];
A list of dictionary indexes of variables to which to apply the value
labels (@pxref{Dictionary Index}).  There are @code{var_count}
elements.

String variables wider than 8 bytes may not have value labels.
@end table

@node Document Record
@section Document Record

The document record, if present, has the following format:

@example
int32               rec_type;
int32               n_lines;
char                lines[][80];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 6.

@item int32 n_lines;
Number of lines of documents present.

@item char lines[][80];
Document lines.  The number of elements is defined by @code{n_lines}.
Lines shorter than 80 characters are padded on the right with spaces.
@end table

@node Machine Integer Info Record
@section Machine Integer Info Record

The integer info record, if present, has the following format:

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Data.} */
int32               version_major;
int32               version_minor;
int32               version_revision;
int32               machine_code;
int32               floating_point_rep;
int32               compression_code;
int32               endianness;
int32               character_code;
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 3.

@item int32 size;
Size of each piece of data in the data part, in bytes.  Always set to 4.

@item int32 count;
Number of pieces of data in the data part.  Always set to 8.

@item int32 version_major;
PSPP major version number.  In version @var{x}.@var{y}.@var{z}, this
is @var{x}.

@item int32 version_minor;
PSPP minor version number.  In version @var{x}.@var{y}.@var{z}, this
is @var{y}.

@item int32 version_revision;
PSPP version revision number.  In version @var{x}.@var{y}.@var{z},
this is @var{z}.

@item int32 machine_code;
Machine code.  PSPP always set this field to value to -1, but other
values may appear.

@item int32 floating_point_rep;
Floating point representation code.  For IEEE 754 systems this is 1.
IBM 370 sets this to 2, and DEC VAX E to 3.

@item int32 compression_code;
Compression code.  Always set to 1.

@item int32 endianness;
Machine endianness.  1 indicates big-endian, 2 indicates little-endian.

@item int32 character_code;
Character code.  1 indicates EBCDIC, 2 indicates 7-bit ASCII, 3
indicates 8-bit ASCII, 4 indicates DEC Kanji.
Windows code page numbers are also valid.
@end table

@node Machine Floating-Point Info Record
@section Machine Floating-Point Info Record

The floating-point info record, if present, has the following format:

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Data.} */
flt64               sysmis;
flt64               highest;
flt64               lowest;
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 4.

@item int32 size;
Size of each piece of data in the data part, in bytes.  Always set to 8.

@item int32 count;
Number of pieces of data in the data part.  Always set to 3.

@item flt64 sysmis;
The system missing value.

@item flt64 highest;
The value used for HIGHEST in missing values.

@item flt64 lowest;
The value used for LOWEST in missing values.
@end table

@node Variable Display Parameter Record
@section Variable Display Parameter Record

The variable display parameter record, if present, has the following
format:

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Repeated @code{count} times}. */
int32               measure;
int32               width;           /* @r{Not always present.} */
int32               alignment;
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 11.

@item int32 size;
The size of @code{int32}.  Always set to 4.

@item int32 count;
The number of sets of variable display parameters (ordinarily the
number of variables in the dictionary), times 2 or 3.
@end table

The remaining members are repeated @code{count} times, in the same
order as the variable records.  No element corresponds to variable
records that continue long string variables.  The meanings of these
members are as follows:

@table @code
@item int32 measure;
The measurement type of the variable:
@table @asis
@item 1
Nominal Scale
@item 2
Ordinal Scale
@item 3
Continuous Scale
@end table

SPSS 14 sometimes writes a @code{measure} of 0.  PSPP interprets this
as nominal scale.

@item int32 width;
The width of the display column for the variable in characters.

This field is present if @var{count} is 3 times the number of
variables in the dictionary.  It is omitted if @var{count} is 2 times
the number of variables.

@item int32 alignment;
The alignment of the variable for display purposes:

@table @asis
@item 0
Left aligned
@item 1
Right aligned
@item 2
Centre aligned
@end table
@end table

@node Long Variable Names Record
@section Long Variable Names Record

If present, the long variable names record has the following format:

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Exactly @code{count} bytes of data.} */
char                var_name_pairs[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 13.

@item int32 size;
The size of each element in the @code{var_name_pairs} member. Always set to 1.

@item int32 count;
The total number of bytes in @code{var_name_pairs}.

@item char var_name_pairs[];
A list of @var{key}--@var{value} tuples, where @var{key} is the name
of a variable, and @var{value} is its long variable name.
The @var{key} field is at most 8 bytes long and must match the
name of a variable which appears in the variable record (@pxref{Variable
Record}).
The @var{value} field is at most 64 bytes long.
The @var{key} and @var{value} fields are separated by a @samp{=} byte.
Each tuple is separated by a byte whose value is 09.  There is no
trailing separator following the last tuple.
The total length is @code{count} bytes.
@end table

@node Very Long String Record
@section Very Long String Record

Old versions of SPSS limited string variables to a width of 255 bytes.
For backward compatibility with these older versions, the system file
format represents a string longer than 255 bytes, called a @dfn{very
long string}, as a collection of strings no longer than 255 bytes
each.  The strings concatenated to make a very long string are called
its @dfn{segments}; for consistency, variables other than very long
strings are considered to have a single segment.

A very long string with a width of @var{w} has @var{n} =
(@var{w} + 251) / 252 segments, that is, one segment for every
252 bytes of width, rounding up.  It would be logical, then, for each
of the segments except the last to have a width of 252 and the last
segment to have the remainder, but this is not the case.  In fact,
each segment except the last has a width of 255 bytes.  The last
segment has width @var{w} - (@var{n} - 1) * 252; some versions
of SPSS make it slightly wider, but not wide enough to make the last
segment require another 8 bytes of data.

Data is packed tightly into segments of a very long string, 255 bytes
per segment.  Because 255 bytes of segment data are allocated for
every 252 bytes of the very long string's width (approximately), some
unused space is left over at the end of the allocated segments.  Data
in unused space is ignored.

Example: Consider a very long string of width 20,000.  Such a very
long string has 20,000 / 252 = 80 (rounding up) segments.  The first
79 segments have width 255; the last segment has width 20,000 - 79 *
252 = 92 or slightly wider (up to 96 bytes, the next multiple of 8).
The very long string's data is actually stored in the 19,890 bytes in
the first 78 segments, plus the first 110 bytes of the 79th segment
(19,890 + 110 = 20,000).  The remaining 145 bytes of the 79th segment
and all 92 bytes of the 80th segment are unused.

The very long string record explains how to stitch together segments
to obtain very long string data.  For each of the very long string
variables in the dictionary, it specifies the name of its first
segment's variable and the very long string variable's actual width.
The remaining segments immediately follow the named variable in the
system file's dictionary.

The very long string record, which is present only if the system file
contains very long string variables, has the following format:

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Exactly @code{count} bytes of data.} */
char                string_lengths[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 14.

@item int32 size;
The size of each element in the @code{string_lengths} member. Always set to 1.

@item int32 count;
The total number of bytes in @code{string_lengths}.

@item char string_lengths[];
A list of @var{key}--@var{value} tuples, where @var{key} is the name
of a variable, and @var{value} is its length.
The @var{key} field is at most 8 bytes long and must match the
name of a variable which appears in the variable record (@pxref{Variable
Record}).
The @var{value} field is exactly 5 bytes long. It is a zero-padded,
ASCII-encoded string that is the length of the variable.
The @var{key} and @var{value} fields are separated by a @samp{=} byte.
Tuples are delimited by a two-byte sequence @{00, 09@}.
After the last tuple, there may be a single byte 00, or @{00, 09@}.
The total length is @code{count} bytes.
@end table

@node Data File and Variable Attributes Records
@section Data File and Variable Attributes Records

The data file and variable attributes records represent custom
attributes for the system file or for individual variables in the
system file, as defined on the DATAFILE ATTRIBUTE (@pxref{DATAFILE
ATTRIBUTE,,,pspp, PSPP Users Guide}) and VARIABLE ATTRIBUTE commands
(@pxref{VARIABLE ATTRIBUTE,,,pspp, PSPP Users Guide}), respectively.

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Exactly @code{count} bytes of data.} */
char                attributes[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  Always set to 17 for a data file attribute record or
to 18 for a variable attributes record.

@item int32 size;
The size of each element in the @code{attributes} member. Always set to 1.

@item int32 count;
The total number of bytes in @code{attributes}.

@item char attributes[];
The attributes, in a text-based format.

In record type 17, this field contains a single attribute set.  An
attribute set is a sequence of one or more attributes concatenated
together.  Each attribute consists of a name, which has the same
syntax as a variable name, followed by, inside parentheses, a sequence
of one or more values.  Each value consists of a string enclosed in
single quotes (@code{'}) followed by a line feed (byte 0x0a).  A value
may contain single quote characters, which are not themselves escaped
or quoted or required to be present in pairs.  There is no apparent
way to embed a line feed in a value.  There is no distinction between
an attribute with a single value and an attribute array with one
element.

In record type 18, this field contains a sequence of one or more
variable attribute sets.  If more than one variable attribute set is
present, each one after the first is delimited from the previous by
@code{/}.  Each variable attribute set consists of a variable name,
followed by @code{:}, followed by an attribute set with the same
syntax as on record type 17.

The total length is @code{count} bytes.
@end table

@subheading Example

A system file produced with the following VARIABLE ATTRIBUTE commands
in effect:

@example
VARIABLE ATTRIBUTE VARIABLES=dummy ATTRIBUTE=fred[1]('23') fred[2]('34').
VARIABLE ATTRIBUTE VARIABLES=dummy ATTRIBUTE=bert('123').
@end example

@noindent
will contain a variable attribute record with the following contents:

@example
00000000  07 00 00 00 12 00 00 00  01 00 00 00 22 00 00 00  |............"...|
00000010  64 75 6d 6d 79 3a 66 72  65 64 28 27 32 33 27 0a  |dummy:fred('23'.|
00000020  27 33 34 27 0a 29 62 65  72 74 28 27 31 32 33 27  |'34'.)bert('123'|
00000030  0a 29                                             |.)              |
@end example

@node Miscellaneous Informational Records
@section Miscellaneous Informational Records

Some specific types of miscellaneous informational records are
documented here, but others are known to exist.  PSPP ignores unknown
miscellaneous informational records when reading system files.

@example
/* @r{Header.} */
int32               rec_type;
int32               subtype;
int32               size;
int32               count;

/* @r{Exactly @code{size * count} bytes of data.} */
char                data[];
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 7.

@item int32 subtype;
Record subtype.  May take any value.  According to Aapi
H@"am@"al@"ainen, value 5 indicates a set of grouped variables and 6
indicates date info (probably related to USE).

@item int32 size;
Size of each piece of data in the data part.  Should have the value 1,
4, or 8, for @code{char}, @code{int32}, and @code{flt64} format data,
respectively.

@item int32 count;
Number of pieces of data in the data part.

@item char data[];
Arbitrary data.  There must be @code{size} times @code{count} bytes of
data.
@end table

@node Dictionary Termination Record
@section Dictionary Termination Record

The dictionary termination record separates all other records from the
data records.

@example
int32               rec_type;
int32               filler;
@end example

@table @code
@item int32 rec_type;
Record type.  Always set to 999.

@item int32 filler;
Ignored padding.  Should be set to 0.
@end table

@node Data Record
@section Data Record

Data records must follow all other records in the system file.  There must
be at least one data record in every system file.

The format of data records varies depending on whether the data is
compressed.  Regardless, the data is arranged in a series of 8-byte
elements.

When data is not compressed,
each element corresponds to
the variable declared in the respective variable record (@pxref{Variable
Record}).  Numeric values are given in @code{flt64} format; string
values are literal characters string, padded on the right when
necessary to fill out 8-byte units.

Compressed data is arranged in the following manner: the first 8 bytes
in the data section is divided into a series of 1-byte command
codes.  These codes have meanings as described below:

@table @asis
@item 0
Ignored.  If the program writing the system file accumulates compressed
data in blocks of fixed length, 0 bytes can be used to pad out extra
bytes remaining at the end of a fixed-size block.

@item 1 through 251
A number with
value @var{code} - @var{bias}, where
@var{code} is the value of the compression code and @var{bias} is the
variable @code{bias} from the file header.  For example,
code 105 with bias 100.0 (the normal value) indicates a numeric variable
of value 5.

@item 252
End of file.  This code may or may not appear at the end of the data
stream.  PSPP always outputs this code but its use is not required.

@item 253
A numeric or string value that is not
compressible.  The value is stored in the 8 bytes following the
current block of command bytes.  If this value appears twice in a block
of command bytes, then it indicates the second group of 8 bytes following the
command bytes, and so on.

@item 254
An 8-byte string value that is all spaces.

@item 255
The system-missing value.
@end table

When the end of the an 8-byte group of command bytes is reached, any
blocks of non-compressible values indicated by code 253 are skipped,
and the next element of command bytes is read and interpreted, until
the end of the file or a code with value 252 is reached.
@setfilename ignored