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ncgen(1)                       UNIDATA UTILITIES                      ncgen(1)


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


       ncgen [-format_code] [-1|3|4|5|6|7] [-b] [-c] [-d] [-D debuglevel] [-f]
              [-h]  [-H]  [-k format_name] [-l b|c|f77|java] [-L loglevel] [-M
              name] [-n] [-N datasetname] [-o netcdf_filename] [-P] [-x]


       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 lan-
       guage 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 speci-
       fied.  If no options are specified in invoking ncgen, it merely  checks
       the syntax of the input CDL file, producing error messages for any vio-
       lations of CDL syntax.  Other options can be used, for example, to cre-
       ate 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 old-
       er 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.


              Alternate method to specify the format.

                     3 => netcdf classic format

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

                     5 => netcdf 5 format

                     6 => netCDF 64-bit format

                     7 => netCDF-4 classic model format (3+4 == 7)
       See the -k flag.

       -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  over-

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

       -d     Same as -D1.

       -D debuglevel
              Set the level of debug output.

       -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.

       -h     Output help information.

       -H     Output the header only; ignore the data section.

       -k format_name

              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

                     'netCDF-4  classic  model'  or  'nc7' => netCDF-4 classic
                     model format
       Accepted   format_code  numeric  arguments,  just  shortcuts  for  for-
       mat_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  ac-
       cepted  but  deprecated,  e.g. 'hdf5', 'enhanced-nc3', etc.  Also, note
       that -v is accepted to mean the same thing as -k for backward  compati-

       -l b|c|f77|java
              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

                     c|C' => C language output.

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

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

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  con-
              structs" 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 val-
              ue 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, nc-
              gen 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).

       -L loglevel

       -M name
              Specify the name for the main function for C, F77, or Java.


       -N datasetname

       -o netcdf_file
              Name of the file to pass to calls to "nc_create()".  If this op-
              tion  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 stan-
              dard output is not seekable.

       -P     Use NC_DISKLESS mode to create the file totally in memory before
              persisting it to disk.

       -W maxwholevarsize
              Set wholevarsizem where if total number of elements is less than
              maxwholevarsize  then  updata  a   variable   using   a   single
              nc_put_var.  Requires  that the variable has no unlimited dimen-

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


       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 `':

              ncgen -o 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 `':

              ncgen -lc foo.cdl >x.c


   CDL Syntax Overview
       Below is an example of CDL syntax, describing a netCDF file with sever-
       al  named dimensions (lat, lon, and time), variables (Z, t, p, rh, lat,
       lon, time), variable attributes (units, long_name, valid_range,  _Fill-
       Value), 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
              netcdf foo {  // an example netCDF specification in CDL

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

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

                   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};
                   lat   = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90;
                   lon   = -140, -118, -96, -84, -52;
              group: g {
                  compound cmpd_t { vlen_t f1; enum_t f2;};
              } // group g
              group: h {
                   /g/cmpd_t  compoundvar;
                      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 be-
       fore the colon character, and (2) the keywords are required to be lower

       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

       A netCDF dimension is used to define the shape of one or  more  of  the
       multidimensional  variables contained in the netCDF file.  A netCDF di-
       mension 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  coordi-
       nates  of the dimension it names.  Dimensions need not have correspond-
       ing 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, at-
       tributes 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 decla-
       ration.   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 `variables',  `dimensions',  `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 exam-
       ple, consider this.

               netcdf ... {
                  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 vari-
       ables 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  `_Fill-
       Value'.   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

       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 con-
       verts 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 non-
       standard INTEGER*2 declaration in output Fortran code.

       Ints  can  hold  values between -2147483648 and 2147483647.  ncgen con-
       verts 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 exter-
       nal representation is as 32-bit IEEE normalized single-precision float-
       ing point numbers.  ncgen converts float declarations to float declara-
       tions 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 ex-
       ternal representation is as 64-bit IEEE standard normalized double-pre-
       cision  floating  point numbers.  ncgen converts double declarations to
       double declarations in the output C code and to DOUBLE PRECISION decla-
       rations 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" at-
       tribute 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 spec-
       ifying 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

       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

   CDL Constants
       Constants  assigned to attributes or variables may be of any of the ba-
       sic netCDF types.  The syntax for constants is similar to C syntax, ex-
       cept  that  type suffixes must be appended to shorts and floats to dis-
       tinguish 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  se-
       quences  such  as `a' or `0.  This is still supported for backward com-
       patibility, but deprecated to make the distinction  clear  between  the
       numeric  byte  type  and the textual char type.  Example byte constants
               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 in-
       terpreted  as  octal,  except that if it begins with `0x', it is inter-
       preted as a hexadecimal constant.  For example:
              -2s  // a short -2
              0123s     // octal
              0x7ffs  //hexadecimal

       int integer constants are intended for representing 32-bit signed quan-
       tities.   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  con-
       stant  (but  see  opaque  constants below).  Examples of valid int con-
       stants include:
              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 inter-
       preted 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 accept-
       able float constants:
              3.14159265358979f   // will be truncated to less precision

       Floating point constants of type double are appropriate for  represent-
       ing floating point data with about sixteen significant digits of preci-
       sion.  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:

       Unsigned integer constants can be created by  appending  the  character
       'U' or 'u' between the constant and any trailing size specifier, or im-
       mediately 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 se-
       quence 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 in-
       terpreted  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 signif-
       icant characters by the ncdump program,  following  the  C  convention.
       Therefore, a NULL byte should not be embedded in a character string un-
       less 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 dou-
       ble quotes. This represents a potential ambiguity since a multi-charac-
       ter string may also indicate a dimensioned character value. Disambigua-
       tion usually occurs by context, but care should  be  taken  to  specify
       thestring  type  to ensure the proper choice.  String constants are as-
       sumed 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 ex-
       tended 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
           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
               /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 di-
       rectory structure.

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

       When an unprefixed name of a dimension is used (as in a variable decla-
              ration), 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 hi-
              erarchy  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

       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

       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 us-
              ing 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 proper-
       ties.  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', `_Shuf-
       fle', 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.  Essen-
       tially  all of then map to some corresponding `nc_def_var_XXX' function
       as defined in the netCDF-4 API.  For the attributes that are essential-
       ly  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 compres-
          sion has been specified for the variable.

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

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

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

       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

   Specifying Datalists
       Specifying datalists for variables in the `data:` section can be  some-
       what  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  can-
       not   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

       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  en-
       closed in braces.

   Specifying Character Datalists
       Specifying datalists for variables of type char also has some complica-
       tions. 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 warn-

       There are three other cases of note.

       1. If there is only a single, unlimited dimension, then all of the con-
          stants  are concatenated and fill characters are added to the end of
          the resulting string to make its length be that of the unlimited di-
          mension.  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 de-
       limited 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  in-
       stance 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 di-
       mensions  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
            datalist: var={{"1","t","w","o"," "," "},{" "," "," "," "," "," "}}, {{"t","h","r","e","e"," "},{"f","o","u","r","x","y"}};


       The programs generated by ncgen when using the -c flag use  initializa-
       tion  statements  to  store data in variables, and will fail to produce
       compilable programs if you try to use them for  large  datasets,  since
       the  resulting  statements may exceed the line length or number of con-
       tinuation statements permitted by the compiler.

       The CDL syntax makes it easy to assign what  looks  like  an  array  of
       variable-length  strings to a netCDF variable, but the strings may sim-
       ply be concatenated into a single array of characters.  Specific use of
       the string type specifier may solve the problem

CDL Grammar

       The file ncgen.y is the definitive grammar for CDL, but a stripped down
       version is included here for completeness.
              ncdesc: NETCDF

              datasetid: DATASETID

              rootgroup: '{'


                   | subgrouplist namedgroup

              namedgroup: GROUP ident '{'

              typesection:    /* empty */
                              | TYPES
                        | TYPES typedecls

                   | typedecls type_or_attr_decl

              typename: ident ;

                   | attrdecl ';'

                     enumdecl optsemicolon
                   | compounddecl optsemicolon
                   | vlendecl optsemicolon
                   | opaquedecl optsemicolon

                   | ';'

              enumdecl: primtype ENUM typename ;

              enumidlist:   enumid
                       | enumidlist ',' enumid

              enumid: ident '=' constint ;

              opaquedecl: OPAQUE '(' INT_CONST ')' typename ;

              vlendecl: typeref '(' '*' ')' typename ;

              compounddecl: COMPOUND typename '{' fields '}' ;

              fields:   field ';'
                   | fields field ';'

              field: typeref fieldlist ;

              primtype:         CHAR_K
                              | BYTE_K
                              | SHORT_K
                              | INT_K
                              | FLOAT_K
                              | DOUBLE_K
                              | UBYTE_K
                              | USHORT_K
                              | UINT_K
                              | INT64_K
                              | UINT64_K

              dimsection:     /* empty */
                              | DIMENSIONS
                        | DIMENSIONS dimdecls

              dimdecls:       dim_or_attr_decl ';'
                              | dimdecls dim_or_attr_decl ';'

              dim_or_attr_decl: dimdeclist  | attrdecl  ;

              dimdeclist:     dimdecl
                              | dimdeclist ',' dimdecl

                     dimd '=' UINT_CONST
                   | dimd '=' INT_CONST
                      | dimd '=' DOUBLE_CONST
                      | dimd '=' NC_UNLIMITED_K

              dimd:           ident ;

              vasection:      /* empty */
                              | VARIABLES
                              | VARIABLES vadecls

              vadecls:        vadecl_or_attr ';'
                              | vadecls vadecl_or_attr ';'

              vadecl_or_attr: vardecl  | attrdecl  ;

              vardecl:        typeref varlist ;

              varlist:      varspec
                          | varlist ',' varspec

              varspec:        ident dimspec ;

              dimspec:        /* empty */
                              | '(' dimlist ')'

              dimlist:        dimref
                              | dimlist ',' dimref

              dimref: path ;

                   | fieldlist ',' fieldspec

              fieldspec: ident fielddimspec ;

              fielddimspec:     /* empty */
                              | '(' fielddimlist ')'

                   | fielddimlist ',' fielddim

                   | INT_CONST

              /* Use this when referencing defined objects */
              varref: type_var_ref ;

              typeref: type_var_ref       ;

                   | primtype

              /* Use this for all attribute decls */
              /* Watch out; this is left recursive */
              attrdecllist: /*empty*/  | attrdecl ';' attrdecllist  ;

                     ':' ident '=' datalist
                   | typeref type_var_ref ':' ident '=' datalist
                   | type_var_ref ':' ident '=' datalist
                   | type_var_ref ':' _FILLVALUE '=' datalist
                   | typeref type_var_ref ':' _FILLVALUE '=' datalist
                   | type_var_ref ':' _STORAGE '=' conststring
                   | type_var_ref ':' _CHUNKSIZES '=' intlist
                   | type_var_ref ':' _FLETCHER32 '=' constbool
                   | type_var_ref ':' _DEFLATELEVEL '=' constint
                   | type_var_ref ':' _SHUFFLE '=' constbool
                   | type_var_ref ':' _ENDIANNESS '=' conststring
                   | type_var_ref ':' _NOFILL '=' constbool
                   | ':' _FORMAT '=' conststring

                   | PATH

              datasection:    /* empty */
                              | DATA
                              | DATA datadecls

                     datadecl ';'
                   | datadecls datadecl ';'

              datadecl: varref '=' datalist ;
                   | datalist1


              /* Must have at least 1 element */
                   | datalist ',' dataitem

                   | '{' datalist '}'

                   | OPAQUESTRING
                   | FILLMARKER
                   | NIL
                   | econstref
                   | function

              econstref: path ;

              function: ident '(' arglist ')' ;

                   | arglist ',' simpleconstant

                     CHAR_CONST /* never used apparently*/
                   | BYTE_CONST
                   | SHORT_CONST
                   | INT_CONST
                   | INT64_CONST
                   | UBYTE_CONST
                   | USHORT_CONST
                   | UINT_CONST
                   | UINT64_CONST
                   | FLOAT_CONST
                   | DOUBLE_CONST
                   | TERMSTRING

                   | intlist ',' constint

                   | UINT_CONST
                   | INT64_CONST
                   | UINT64_CONST

              conststring: TERMSTRING ;

                   | constint

              /* Push all idents thru here for tracking */
              ident: IDENT ;

Printed: 121-4-15        $Date: 2010/04/29 16:38:55 $                 ncgen(1)

netcdf 4.8.0 - Generated Thu Apr 15 18:18:17 CDT 2021
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