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PROGRAM:

NAME

ncgen - From a CDL file generate a netCDF-3 file, a netCDF-4 file or a C program

SYNOPSIS

ncgen [-b] [-c] [-f] [-k format_name] [-format_code] [-l output language] [-n] [-o
netcdf_filename] [-x] [input_file]

DESCRIPTION

ncgen generates either a netCDF-3 (i.e. classic) binary .nc file, a netCDF-4 (i.e.
enhanced) binary .nc file or a file in some source language that when executed will
construct the corresponding binary .nc file. The input to ncgen is a description of a
netCDF file in a small language known as CDL (network Common Data form Language),
described below. Input is read from standard input if no input_file is specified. If no
options are specified in invoking ncgen, it merely checks the syntax of the input CDL
file, producing error messages for any violations of CDL syntax. Other options can be
used, for example, to create the corresponding netCDF file, or to generate a C program
that uses the netCDF C interface to create the netCDF file.

Note that this version of ncgen was originally called ncgen4. The older ncgen program has
been renamed to ncgen3.

ncgen may be used with the companion program ncdump to perform some simple operations on
netCDF files. For example, to rename a dimension in a netCDF file, use ncdump to get a
CDL version of the netCDF file, edit the CDL file to change the name of the dimensions,
and use ncgen to generate the corresponding netCDF file from the edited CDL file.

OPTIONS

-b Create a (binary) netCDF file. If the -o option is absent, a default file name
will be constructed from the basename of the CDL file, with any suffix replaced by
the `.nc' extension. If a file already exists with the specified name, it will be
overwritten.

-c Generate C source code that will create a netCDF file matching the netCDF
specification. The C source code is written to standard output; equivalent to -lc.

-f Generate FORTRAN 77 source code that will create a netCDF file matching the netCDF
specification. The source code is written to standard output; equivalent to -lf77.

-o netcdf_file
Name of the file to pass to calls to "nc_create()". If this option is specified it
implies (in the absence of any explicit -l flag) the "-b" option. This option is
necessary because netCDF files cannot be written directly to standard output, since
standard output is not seekable.

-k format_name

-format_code
The -k flag specifies the format of the file to be created and, by inference, the
data model accepted by ncgen (i.e. netcdf-3 (classic) versus netcdf-4 vs netcdf-5).
As a shortcut, a numeric format_code may be specified instead. The possible
format_name values for the -k option are:

'classic' or 'nc3' => netCDF classic format

'64-bit offset' or 'nc6' => netCDF 64-bit format

'64-bit data or 'nc5' => netCDF-5 (64-bit data) format

'netCDF-4' 0r 'nc4' => netCDF-4 format (enhanced data model)

'netCDF-4 classic model' or 'nc7' => netCDF-4 classic model format
Accepted format_number arguments, just shortcuts for format_names, are:

3 => netcdf classic format

5 => netcdf 5 format

6 => netCDF 64-bit format

4 => netCDF-4 format (enhanced data model)

7 => netCDF-4 classic model format
The numeric code "7" is used because "7=3+4", a mnemonic for the format that uses the
netCDF-3 data model for compatibility with the netCDF-4 storage format for performance.
Credit is due to NCO for use of these numeric codes instead of the old and confusing
format numbers.

Note: The old version format numbers '1', '2', '3', '4', equivalent to the format names
'nc3', 'nc6', 'nc4', or 'nc7' respectively, are also still accepted but deprecated, due to
easy confusion between format numbers and format names. Various old format name aliases
are also accepted but deprecated, e.g. 'hdf5', 'enhanced-nc3', etc. Also, note that -v is
accepted to mean the same thing as -k for backward compatibility.

-x Don't initialize data with fill values. This can speed up creation of large netCDF
files greatly, but later attempts to read unwritten data from the generated file
will not be easily detectable.

-l output_language
The -l flag specifies the output language to use when generating source code that
will create or define a netCDF file matching the netCDF specification. The output
is written to standard output. The currently supported languages have the
following flags.

c|C' => C language output.

f77|fortran77' => FORTRAN 77 language output
; note that currently only the classic model is supported.

j|java' => (experimental) Java language output
; targets the existing Unidata Java interface, which means that only
the classic model is supported.

Choosing the output format

The choice of output format is determined by three flags.

-k flag.

_Format attribute (see below).

Occurrence of CDF-5 (64-bit data) or
netcdf-4 constructs in the input CDL." The term "netCDF-4 constructs" means
constructs from the enhanced data model, not just special performance-related
attributes such as
_ChunkSizes, _DeflateLevel, _Endianness, etc. The term "CDF-5 constructs" means
extended unsigned integer types allowed in the 64-bit data model.

Note that there is an ambiguity between the netCDF-4 case and the CDF-5 case is only an
unsigned type is seen in the input.

The rules are as follows, in order of application.

1. If either Fortran or Java output is specified, then -k flag value of 1 (classic
model) will be used. Conflicts with the use of enhanced constructs in the CDL will
report an error.

2. If both the -k flag and _Format attribute are specified, the _Format flag will be
ignored. If no -k flag is specified, and a _Format attribute value is specified,
then the -k flag value will be set to that of the _Format attribute. Otherwise the
-k flag is undefined.

3. If the -k option is defined and is consistent with the CDL, ncgen will output a
file in the requested form, else an error will be reported.

4. If the -k flag is undefined, and if there are CDF-5 constructs, only, in the CDL, a
-k flag value of 5 (64-bit data model) will be used. If there are true netCDF-4
constructs in the CDL, a -k flag value of 3 (enhanced model) will be used.

5. If special performance-related attributes are specified in the CDL, a -k flag value
of 4 (netCDF-4 classic model) will be used.

6. Otherwise ncgen will set the -k flag to 1 (classic model).

EXAMPLES

Check the syntax of the CDL file `foo.cdl':

ncgen foo.cdl

From the CDL file `foo.cdl', generate an equivalent binary netCDF file named `x.nc':

ncgen -o x.nc foo.cdl

From the CDL file `foo.cdl', generate a C program containing the netCDF function
invocations necessary to create an equivalent binary netCDF file named `x.nc':

ncgen -lc foo.cdl >x.c

USAGE

CDL Syntax Overview
Below is an example of CDL syntax, describing a netCDF file with several named dimensions
(lat, lon, and time), variables (Z, t, p, rh, lat, lon, time), variable attributes (units,
long_name, valid_range, _FillValue), and some data. CDL keywords are in boldface. (This
example is intended to illustrate the syntax; a real CDL file would have a more complete
set of attributes so that the data would be more completely self-describing.)
netcdf foo { // an example netCDF specification in CDL

types:
ubyte enum enum_t {Clear = 0, Cumulonimbus = 1, Stratus = 2};
opaque(11) opaque_t;
int(*) vlen_t;

dimensions:
lat = 10, lon = 5, time = unlimited ;

variables:
long lat(lat), lon(lon), time(time);
float Z(time,lat,lon), t(time,lat,lon);
double p(time,lat,lon);
long rh(time,lat,lon);

string country(time,lat,lon);
ubyte tag;

// variable attributes
lat:long_name = "latitude";
lat:units = "degrees_north";
lon:long_name = "longitude";
lon:units = "degrees_east";
time:units = "seconds since 1992-1-1 00:00:00";

// typed variable attributes
string Z:units = "geopotential meters";
float Z:valid_range = 0., 5000.;
double p:_FillValue = -9999.;
long rh:_FillValue = -1;
vlen_t :globalatt = {17, 18, 19};
data:
lat = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90;
lon = -140, -118, -96, -84, -52;
group: g {
types:
compound cmpd_t { vlen_t f1; enum_t f2;};
} // group g
group: h {
variables:
/g/cmpd_t compoundvar;
data:
compoundvar = { {3,4,5}, enum_t.Stratus } ;
} // group h
}

All CDL statements are terminated by a semicolon. Spaces, tabs, and newlines can be used
freely for readability. Comments may follow the characters `//' on any line.

A CDL description consists of five optional parts: types, dimensions, variables, data,
beginning with the keyword `types:', `dimensions:', `variables:', and `data:',
respectively. Note several things: (1) the keyword includes the trailing colon, so there
must not be any space before the colon character, and (2) the keywords are required to be
lower case.

The variables: section may contain variable declarations and attribute assignments. All
sections may contain global attribute assignments.

In addition, after the data: section, the user may define a series of groups (see the
example above). Groups themselves can contain types, dimensions, variables, data, and
other (nested) groups.

The netCDF types: section declares the user defined types. These may be constructed using
any of the following types: enum, vlen, opaque, or compound.

A netCDF dimension is used to define the shape of one or more of the multidimensional
variables contained in the netCDF file. A netCDF dimension has a name and a size. A
dimension can have the unlimited size, which means a variable using this dimension can
grow to any length in that dimension.

A variable represents a multidimensional array of values of the same type. A variable has
a name, a data type, and a shape described by its list of dimensions. Each variable may
also have associated attributes (see below) as well as data values. The name, data type,
and shape of a variable are specified by its declaration in the variable section of a CDL
description. A variable may have the same name as a dimension; by convention such a
variable is one-dimensional and contains coordinates of the dimension it names.
Dimensions need not have corresponding variables.

A netCDF attribute contains information about a netCDF variable or about the whole netCDF
dataset. Attributes are used to specify such properties as units, special values, maximum
and minimum valid values, scaling factors, offsets, and parameters. Attribute information
is represented by single values or arrays of values. For example, "units" is an attribute
represented by a character array such as "celsius". An attribute has an associated
variable, a name, a data type, a length, and a value. In contrast to variables that are
intended for data, attributes are intended for metadata (data about data). Unlike
netCDF-3, attribute types can be any user defined type as well as the usual built-in
types.

In CDL, an attribute is designated by a a type, a variable, a ':', and then an attribute
name. The type is optional and if missing, it will be inferred from the values assigned
to the attribute. It is possible to assign global attributes not associated with any
variable to the netCDF as a whole by omitting the variable name in the attribute
declaration. Notice that there is a potential ambiguity in a specification such as
x : a = ...
In this situation, x could be either a type for a global attribute, or the variable name
for an attribute. Since there could both be a type named x and a variable named x, there
is an ambiguity. The rule is that in this situation, x will be interpreted as a type if
possible, and otherwise as a variable.

If not specified, the data type of an attribute in CDL is derived from the type of the
value(s) assigned to it. The length of an attribute is the number of data values assigned
to it, or the number of characters in the character string assigned to it. Multiple
values are assigned to non-character attributes by separating the values with commas. All
values assigned to an attribute must be of the same type.

The names for CDL dimensions, variables, attributes, types, and groups may contain any
non-control utf-8 character except the forward slash character (`/'). However, certain
characters must escaped if they are used in a name, where the escape character is the
backward slash `\'. In particular, if the leading character off the name is a digit
(0-9), then it must be preceded by the escape character. In addition, the characters `
!"#$%&()*,:;<=>?[]^`´{}|~\' must be escaped if they occur anywhere in a name. Note also
that attribute names that begin with an underscore (`_') are reserved for the use of
Unidata and should not be used in user defined attributes.

Note also that the words `variable', `dimension', `data', `group', and `types' are legal
CDL names, but be careful that there is a space between them and any following colon
character when used as a variable name. This is mostly an issue with attribute
declarations. For example, consider this.

netcdf ... {
...
variables:
int dimensions;
dimensions: attribute=0 ; // this will cause an error
dimensions : attribute=0 ; // this is ok.
...
}

The optional data: section of a CDL specification is where netCDF variables may be
initialized. The syntax of an initialization is simple: a variable name, an equals sign,
and a comma-delimited list of constants (possibly separated by spaces, tabs and newlines)
terminated with a semicolon. For multi-dimensional arrays, the last dimension varies
fastest. Thus row-order rather than column order is used for matrices. If fewer values
are supplied than are needed to fill a variable, it is extended with a type-dependent
`fill value', which can be overridden by supplying a value for a distinguished variable
attribute named `_FillValue'. The types of constants need not match the type declared for
a variable; coercions are done to convert integers to floating point, for example. The
constant `_' can be used to designate the fill value for a variable. If the type of the
variable is explicitly `string', then the special constant `NIL` can be used to represent
a nil string, which is not the same as a zero length string.

Primitive Data Types
char characters
byte 8-bit data
short 16-bit signed integers
int 32-bit signed integers
long (synonymous with int)
int64 64-bit signed integers
float IEEE single precision floating point (32 bits)
real (synonymous with float)
double IEEE double precision floating point (64 bits)
ubyte unsigned 8-bit data
ushort 16-bit unsigned integers
uint 32-bit unsigned integers
uint64 64-bit unsigned integers
string arbitrary length strings

CDL supports a superset of the primitive data types of C. The names for the primitive
data types are reserved words in CDL, so the names of variables, dimensions, and
attributes must not be primitive type names. In declarations, type names may be specified
in either upper or lower case.

Bytes are intended to hold a full eight bits of data, and the zero byte has no special
significance, as it mays for character data. ncgen converts byte declarations to char
declarations in the output C code and to the nonstandard BYTE declaration in output
Fortran code.

Shorts can hold values between -32768 and 32767. ncgen converts short declarations to
short declarations in the output C code and to the nonstandard INTEGER*2 declaration in
output Fortran code.

Ints can hold values between -2147483648 and 2147483647. ncgen converts int declarations
to int declarations in the output C code and to INTEGER declarations in output Fortran
code. long is accepted as a synonym for int in CDL declarations, but is deprecated since
there are now platforms with 64-bit representations for C longs.

Int64 can hold values between -9223372036854775808 and 9223372036854775807. ncgen
converts int64 declarations to longlong declarations in the output C code.

Floats can hold values between about -3.4+38 and 3.4+38. Their external representation is
as 32-bit IEEE normalized single-precision floating point numbers. ncgen converts float
declarations to float declarations in the output C code and to REAL declarations in output
Fortran code. real is accepted as a synonym for float in CDL declarations.

Doubles can hold values between about -1.7+308 and 1.7+308. Their external representation
is as 64-bit IEEE standard normalized double-precision floating point numbers. ncgen
converts double declarations to double declarations in the output C code and to DOUBLE
PRECISION declarations in output Fortran code.

The unsigned counterparts of the above integer types are mapped to the corresponding
unsigned C types. Their ranges are suitably modified to start at zero.

The technical interpretation of the char type is that it is an unsigned 8-bit value. The
encoding of the 256 possible values is unspecified by default. A variable of char type may
be marked with an "_Encoding" attribute to indicate the character set to be used: US-
ASCII, ISO-8859-1, etc. Note that specifying the encoding of UTF-8 is equivalent to
specifying US-ASCII This is because multi-byte UTF-8 characters cannot be stored in an
8-bit character. The only legal single byte UTF-8 values are by definition the 7-bit US-
ASCII encoding with the top bit set to zero.

Strings are assumed by default to be encoded using UTF-8. Note that this means that
multi-byte UTF-8 encodings may be present in the string, so it is possible that the number
of distinct UTF-8 characters in a string is smaller than the number of 8-bit bytes used to
store the string.

CDL Constants
Constants assigned to attributes or variables may be of any of the basic netCDF types.
The syntax for constants is similar to C syntax, except that type suffixes must be
appended to shorts and floats to distinguish them from longs and doubles.

A byte constant is represented by an integer constant with a `b' (or `B') appended. In
the old netCDF-2 API, byte constants could also be represented using single characters or
standard C character escape sequences such as `a' or `0. This is still supported for
backward compatibility, but deprecated to make the distinction clear between the numeric
byte type and the textual char type. Example byte constants include:
0b // a zero byte
-1b // -1 as an 8-bit byte
255b // also -1 as a signed 8-bit byte

short integer constants are intended for representing 16-bit signed quantities. The form
of a short constant is an integer constant with an `s' or `S' appended. If a short
constant begins with `0', it is interpreted as octal, except that if it begins with `0x',
it is interpreted as a hexadecimal constant. For example:
-2s // a short -2
0123s // octal
0x7ffs //hexadecimal

int integer constants are intended for representing 32-bit signed quantities. The form of
an int constant is an ordinary integer constant, although it is acceptable to optionally
append a single `l' or `L' (again, deprecated). Be careful, though, the L suffix is
interpreted as a 32 bit integer, and never as a 64 bit integer. This can be confusing
since the C long type can ambigously be either 32 bit or 64 bit.

If an int constant begins with `0', it is interpreted as octal, except that if it begins
with `0x', it is interpreted as a hexadecimal constant (but see opaque constants below).
Examples of valid int constants include:
-2
1234567890L
0123 // octal
0x7ff // hexadecimal

int64 integer constants are intended for representing 64-bit signed quantities. The form
of an int64 constant is an integer constant with an `ll' or `LL' appended. If an int64
constant begins with `0', it is interpreted as octal, except that if it begins with `0x',
it is interpreted as a hexadecimal constant. For example:
-2ll // an unsigned -2
0123LL // octal
0x7ffLL //hexadecimal

Floating point constants of type float are appropriate for representing floating point
data with about seven significant digits of precision. The form of a float constant is
the same as a C floating point constant with an `f' or `F' appended. For example the
following are all acceptable float constants:
-2.0f
3.14159265358979f // will be truncated to less precision
1.f

Floating point constants of type double are appropriate for representing floating point
data with about sixteen significant digits of precision. The form of a double constant is
the same as a C floating point constant. An optional `d' or `D' may be appended. For
example the following are all acceptable double constants:
-2.0
3.141592653589793
1.0e-20
1.d

Unsigned integer constants can be created by appending the character 'U' or 'u' between
the constant and any trailing size specifier, or immediately at the end of the size
specifier. Thus one could say 10U, 100su, 100000ul, or 1000000llu, for example.

Single character constants may be enclosed in single quotes. If a sequence of one or more
characters is enclosed in double quotes, then its interpretation must be inferred from the
context. If the dataset is created using the netCDF classic model, then all such constants
are interpreted as a character array, so each character in the constant is interpreted as
if it were a single character. If the dataset is netCDF extended, then the constant may
be interpreted as for the classic model or as a true string (see below) depending on the
type of the attribute or variable into which the string is contained.

The interpretation of char constants is that those that are in the printable ASCII range
(' '..'~') are assumed to be encoded as the 1-byte subset ofUTF-8, which is equivalent to
US-ASCII. In all cases, the usual C string escape conventions are honored for values from
0 thru 127. Values greater than 127 are allowed, but their encoding is undefined. For
netCDF extended, the use of the char type is deprecated in favor of the string type.

Some character constant examples are as follows.
'a' // ASCII `a'
"a" // equivalent to 'a'
"Two\nlines\n" // a 10-character string with two embedded newlines
"a bell:\007" // a string containing an ASCII bell
Note that the netCDF character array "a" would fit in a one-element variable, since no
terminating NULL character is assumed. However, a zero byte in a character array is
interpreted as the end of the significant characters by the ncdump program, following the
C convention. Therefore, a NULL byte should not be embedded in a character string unless
at the end: use the byte data type instead for byte arrays that contain the zero byte.

String constants are, like character constants, represented using double quotes. This
represents a potential ambiguity since a multi-character string may also indicate a
dimensioned character value. Disambiguation usually occurs by context, but care should be
taken to specify thestring type to ensure the proper choice. String constants are assumed
to always be UTF-8 encoded. This specifically means that the string constant may actually
contain multi-byte UTF-8 characters. The special constant `NIL` can be used to represent
a nil string, which is not the same as a zero length string.

Opaque constants are represented as sequences of hexadecimal digits preceded by 0X or 0x:
0xaa34ffff, for example. These constants can still be used as integer constants and will
be either truncated or extended as necessary.

Compound Constant Expressions
In order to assign values to variables (or attributes) whose type is user-defined type,
the constant notation has been extended to include sequences of constants enclosed in
curly brackets (e.g. "{"..."}"). Such a constant is called a compound constant, and
compound constants can be nested.

Given a type "T(*) vlen_t", where T is some other arbitrary base type, constants for this
should be specified as follows.
vlen_t var[2] = {t11,t12,...t1N}, {t21,t22,...t2m};
The values tij, are assumed to be constants of type T.

Given a type "compound cmpd_t {T1 f1; T2 f2...Tn fn}", where the Ti are other arbitrary
base types, constants for this should be specified as follows.
cmpd_t var[2] = {t11,t12,...t1N}, {t21,t22,...t2n};
The values tij, are assumed to be constants of type Ti. If the fields are missing, then
they will be set using any specified or default fill value for the field's base type.

The general set of rules for using braces are defined in the Specifying Datalists section
below.

Scoping Rules
With the addition of groups, the name space for defined objects is no longer flat.
References (names) of any type, dimension, or variable may be prefixed with the absolute
path specifying a specific declaration. Thus one might say
variables:
/g1/g2/t1 v1;
The type being referenced (t1) is the one within group g2, which in turn is nested in
group g1. The similarity of this notation to Unix file paths is deliberate, and one can
consider groups as a form of directory structure.

When name is not prefixed, then scope rules are applied to locate the specified
declaration. Currently, there are three rules: one for dimensions, one for types and
enumeration constants, and one for all others.

When an unprefixed name of a dimension is used (as in a variable declaration), ncgen first
looks in the immediately enclosing group for the dimension. If it is not found
there, then it looks in the group enclosing this group. This continues up the
group hierarchy until the dimension is found, or there are no more groups to
search.

2. When an unprefixed name of a type or an enumeration constant is used, ncgen searches
the group tree using a pre-order depth-first search. This essentially means that it
will find the matching declaration that precedes the reference textually in the cdl
file and that is "highest" in the group hierarchy.

3. For all other names, only the immediately enclosing group is searched.

One final note. Forward references are not allowed. This means that specifying, for
example, /g1/g2/t1 will fail if this reference occurs before g1 and/or g2 are defined.

Specifying Enumeration Constants
References to Enumeration constants (in data lists) can be ambiguous since the same
enumeration constant name can be defined in more than one enumeration. If a cdl file
specified an ambiguous constant, then ncgen will signal an error. Such constants can be
disambiguated in two ways.

1. Prefix the enumeration constant with the name of the enumeration separated by a
dot: enum.econst, for example.

2. If case one is not sufficient to disambiguate the enumeration constant, then one
must specify the precise enumeration type using a group path: /g1/g2/enum.econst,
for example.

Special Attributes
Special, virtual, attributes can be specified to provide performance-related information
about the file format and about variable properties. The file must be a netCDF-4 file for
these to take effect.

These special virtual attributes are not actually part of the file, they are merely a
convenient way to set miscellaneous properties of the data in CDL

The special attributes currently supported are as follows: `_Format', `_Fletcher32,
`_ChunkSizes', `_Endianness', `_DeflateLevel', `_Shuffle', and `_Storage'.

`_Format' is a global attribute specifying the netCDF format variant. Its value must be a
single string matching one of `classic', `64-bit offset', `64-bit data', `netCDF-4', or
`netCDF-4 classic model'.

The rest of the special attributes are all variable attributes. Essentially all of then
map to some corresponding `nc_def_var_XXX' function as defined in the netCDF-4 API. For
the attributes that are essentially boolean (_Fletcher32, _Shuffle, and _NOFILL), the
value true can be specified by using the strings `true' or `1', or by using the integer 1.
The value false expects either `false', `0', or the integer 0. The actions associated
with these attributes are as follows.

1. `_Fletcher32 sets the `fletcher32' property for a variable.

2. `_Endianness' is either `little' or `big', depending on how the variable is stored when
first written.

3. `_DeflateLevel' is an integer between 0 and 9 inclusive if compression has been
specified for the variable.

4. `_Shuffle' specifies if the the shuffle filter should be used.

5. `_Storage' is `contiguous' or `chunked'.

6. `_ChunkSizes' is a list of chunk sizes for each dimension of the variable

Note that attributes such as "add_offset" or "scale_factor" have no special meaning to
ncgen. These attributes are currently conventions, handled above the library layer by
other utility packages, for example NCO.

Specifying Datalists
Specifying datalists for variables in the `data:` section can be somewhat complicated.
There are some rules that must be followed to ensure that datalists are parsed correctly
by ncgen.

First, the top level is automatically assumed to be a list of items, so it should not be
inside {...}. That means that if the variable is a scalar, there will be a single top-
level element and if the variable is an array, there will be N top-level elements. For
each element of the top level list, the following rules should be applied.

1. Instances of UNLIMITED dimensions (other than the first dimension) must be surrounded
by {...} in order to specify the size.

2. Compound instances must be embedded in {...}

3. Non-scalar fields of compound instances must be embedded in {...}.

4. Instances of vlens must be surrounded by {...} in order to specify the size.

Datalists associated with attributes are implicitly a vector (i.e., a list) of values of
the type of the attribute and the above rules must apply with that in mind.

7. No other use of braces is allowed.

Note that one consequence of these rules is that arrays of values cannot have subarrays
within braces. Consider, for example, int var(d1)(d2)...(dn), where none of d2...dn are
unlimited. A datalist for this variable must be a single list of integers, where the
number of integers is no more than D=d1*d2*...dn values; note that the list can be less
than D, in which case fill values will be used to pad the list.

Rule 6 about attribute datalist has the following consequence. If the type of the
attribute is a compound (or vlen) type, and if the number of entries in the list is one,
then the compound instances must be enclosed in braces.

Specifying Character Datalists
Specifying datalists for variables of type char also has some complications. consider, for
example
dimensions: u=UNLIMITED; d1=1; d2=2; d3=3;
d4=4; d5=5; u2=UNLIMITED;
variables: char var(d4,d5);
datalist: var="1", "two", "three";

We have twenty elements of var to fill (d5 X d4) and we have three strings of length 1, 3,
5. How do we assign the characters in the strings to the twenty elements?

This is challenging because it is desirable to mimic the original ncgen (ncgen3). The
core algorithm is notionally as follows.

1. Assume we have a set of dimensions D1..Dn, where D1 may optionally be an Unlimited
dimension. It is assumed that the sizes of the Di are all known (including unlimited
dimensions).

2. Given a sequence of string or character constants C1..Cm, our goal is to construct a
single string whose length is the cross product of D1 thru Dn. Note that for purposes
of this algorithm, character constants are treated as strings of size 1.

3. Construct Dx = cross product of D1 thru D(n-1).

4. For each constant Ci, add fill characters as needed so that its length is a multiple of
Dn.

5. Concatenate the modified C1..Cm to produce string S.

6. Add fill characters to S to make its length be a multiple of Dn.

8. If S is longer than the Dx * Dn, then truncate and generate a warning.

There are three other cases of note.

1. If there is only a single, unlimited dimension, then all of the constants are
concatenated and fill characers are added to the end of the resulting string to make
its length be that of the unlimited dimension. If the length is larger than the
unlimited dimension, then it is truncated with a warning.

2. For the case of character typed vlen, "char(*) vlen_t" for example. we simply
concatenate all the constants with no filling at all.

3. For the case of a character typed attribute, we simply concatenate all the constants.

In netcdf-4, dimensions other than the first can be unlimited. Of course by the rules
above, the interior unlimited instances must be delimited by {...}. For example.
variables: char var(u,u2);
datalist: var={"1", "two"}, {"three"};
In this case u will have the effective length of two. Within each instance of u2, the
rules above will apply, leading to this.
datalist: var={"1","t","w","o"}, {"t","h","r","e","e"};
The effective size of u2 will be the max of the two instance lengths (five in this case)
and the shorter will be padded to produce this.
datalist: var={"1","t","w","o","\0"}, {"t","h","r","e","e"};

Consider an even more complicated case.
variables: char var(u,u2,u3);
datalist: var={{"1", "two"}}, {{"three"},{"four","xy"}};
In this case u again will have the effective length of two. The u2 dimensions will have a
size = max(1,2) = 2; Within each instance of u2, the rules above will apply, leading to
this.
datalist: var={{"1","t","w","o"}}, {{"t","h","r","e","e"},{"f","o","u","r","x","y"}};
The effective size of u3 will be the max of the two instance lengths (six in this case)
and the shorter ones will be padded to produce this.
datalist: var={{"1","t","w","o"," "," "}}, {{"t","h","r","e","e"," "},{"f","o","u","r","x","y"}};
Note however that the first instance of u2 is less than the max length of u2, so we need
to add a filler for another instance of u2, producing this.
datalist: var={{"1","t","w","o"," "," "},{" "," "," "," "," "," "}}, {{"t","h","r","e","e"," "},{"f","o","u","r","x","y"}};

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