xenial (1) ncgen3.1.gz

Provided by: netcdf-bin_4.4.0-2_amd64 bug

NAME

       ncgen3  -  From  a  CDL  file  generate a netCDF classic or 64 bit classicfile, a C program, or a Fortran
       program

SYNOPSIS

       ncgen3 [-b] [-c] [-f] [-k kind_of_file] [-x] [-n] [-o netcdf_filename] input_file

DESCRIPTION

       ncgen3 generates either a netCDF file, or C or Fortran source code to create a netCDF file.  The input to
       ncgen3  is  a  description  of  a  netCDF file in a small language known as CDL (network Common Data form
       Language), described below.  If no options are specified in invoking ncgen3, it merely checks the  syntax
       of  the  input CDL file, producing error messages for any violations of CDL syntax.  Other options can be
       used to create the corresponding netCDF file, to generate a C program that uses the netCDF C interface to
       create the netCDF file, or to generate a Fortran program that uses the netCDF Fortran interface to create
       the same netCDF file.

       ncgen3 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  ncgen3  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 netCDF name (specified after the netcdf keyword in the  input)  by  appending
              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.

       -f     Generate Fortran source code that will create a netCDF file  matching  the  netCDF  specification.
              The Fortran source code is written to standard output.

       -o netcdf_file
              Name for the binary netCDF file created.  If this option is specified, it implies the "-b" option.
              (This option is necessary because netCDF files cannot be  written  directly  to  standard  output,
              since standard output is not seekable.)

       -k kind_of_file
              Using -k2 or -k "64-bit offset" specifies that generated file (or program) should use version 2 of
              format that employs 64-bit file offsets.  The default is to use version 1 ("classic") format  with
              32-bit  file  offsets, although this limits the size of the netCDF file, variables, and records to
              the sizes supported by the classic format.  (NetCDF-4 will  support  additional  kinds  of  netCDF
              files, "netCDF-4" and "netCDF-4 classic model".)  Note: -v is also accepted to mean the same thing
              as -k for backward compatibility, but -k is preferred, to match the corresponding ncdump option.

       -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.

EXAMPLES

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

              ncgen3 foo.cdl

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

              ncgen3 -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':

              ncgen3 -c -o x.nc foo.cdl

USAGE

   CDL Syntax Summary
       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

              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);

                   // 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";
                   Z:units = "geopotential meters";
                   Z:valid_range = 0., 5000.;
                   p:_FillValue = -9999.;
                   rh:_FillValue = -1;

              data:
                   lat   = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90;
                   lon   = -140, -118, -96, -84, -52;
              }

       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 three optional parts: dimensions, variables, and data, beginning with the
       keyword dimensions:, variables:,  and  data,  respectively.   The  variable  part  may  contain  variable
       declarations and attribute assignments.

       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.  At most one dimension in  a  netCDF  file
       can  have  the unlimited size, which means a variable using this dimension can grow to any length (like a
       record number in a file).

       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).

       In CDL, an attribute is designated by a variable and attribute name, separated by `:'.  It is possible to
       assign  global  attributes  not associated with any variable to the netCDF as a whole by using `:' before
       the attribute name.  The data type of an attribute in CDL is derived from the type of the value  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, and attributes must begin with an alphabetic character  or  `_',
       and subsequent characters may be alphanumeric or `_' or `-'.

       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.

   Primitive Data Types
              char characters
              byte 8-bit data
              short     16-bit signed integers
              long 32-bit signed integers
              int  (synonymous with long)
              float     IEEE single precision floating point (32 bits)
              real (synonymous with float)
              double    IEEE double precision floating point (64 bits)

       Except  for the added data-type byte and the lack of unsigned, CDL supports the same primitive data types
       as C.  The names for the primitive data types are reserved words in  CDL,  so  the  names  of  variables,
       dimensions,  and  attributes  must  not  be  type names.  In declarations, type names may be specified in
       either upper or lower case.

       Bytes differ from characters in that they are intended to hold a full eight bits of data,  and  the  zero
       byte  has  no  special significance, as it does for character data.  ncgen3 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.   ncgen3  converts  short  declarations  to  short
       declarations in the output C code and to the nonstandard INTEGER*2 declaration in output Fortran code.

       Longs  can  hold  values  between  -2147483648 and 2147483647.  ncgen3 converts long declarations to long
       declarations in the output C code and to INTEGER declarations in output Fortran code.   int  and  integer
       are  accepted  as  synonyms  for  long  in  CDL  declarations.   Now that there are platforms with 64-bit
       representations for C longs, it may be better to use the int synonym to avoid confusion.

       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.   ncgen3  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.  ncgen3 converts double declarations to
       double declarations in the output C code and to DOUBLE PRECISION declarations in output Fortran code.

   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 a single character or multiple character escape sequence enclosed in
       single quotes.  For example,
               'a'      // ASCII `a'
               '\0'          // a zero byte
               '\n'          // ASCII newline character
               '\33'         // ASCII escape character (33 octal)
               '\x2b'   // ASCII plus (2b hex)
               '\377'   // 377 octal = 255 decimal, non-ASCII

       Character constants are enclosed in double quotes.  A character array may  be  represented  as  a  string
       enclosed in double quotes.  The usual C string escape conventions are honored.  For example
              "a"       // ASCII `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.  NetCDF and CDL have no string type, but only fixed-length character arrays,
       which may be multi-dimensional.

       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

       Long  integer  constants  are  intended  for  representing  32-bit signed quantities.  The form of a long
       constant is an ordinary integer constant, although it is acceptable to append an optional `l' or `L'.  If
       a  long  constant  begins with `0', it is interpreted as octal, except that if it begins with `0x', it is
       interpreted as a hexadecimal constant.  Examples of valid long constants include:
              -2
              1234567890L
              0123      // octal
              0x7ff          // 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

BUGS

       The  programs  generated by ncgen3 when using the -c or -f use initialization 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 continuation 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  will simply be concatenated into a single array of characters, since netCDF
       cannot represent an array of variable-length strings in one netCDF variable.

       NetCDF and CDL do not yet support a type corresponding to a 64-bit integer.