NCGEN(1) UNIDATA UTILITIES NCGEN(1)
NAME
ncgen - From a CDL file generate a netCDF file, a C program, or a Fortran program
SYNOPSIS
ncgen [-b] [-c] [-f] [-n] [-o output_file] input_file
DESCRIPTION
ncgen generates either a netCDF file, or C or Fortran source code to create a netCDF 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. 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 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.
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 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 outputfile
Name for the 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.)
-n Like -b option, except creates netCDF file with the obsolete `.cdf' extension instead of the `.nc' extension, in the absence of an
output filename specified by the -O option. This option is only supported for backward compatibility.
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 -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 ex-
ample 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 di-
mension 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 di-
mension 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 as-
sociated with any variable to the netCDF as a whole by using `:' before the attribute name. The data type of an attribute in CDL is de-
rived 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 matri-
ces. 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.
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. ncgen converts byte declarations to char declarations in the output C code and to the nonstandard BYTE decla-
ration 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.
Longs can hold values between -2147483648 and 2147483647. ncgen 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. 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.
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'
'