Provided by: hdf4-tools_4.2.16-4build1_amd64 
NAME
ncgen-hdf - From a CDL file generate a netCDF file, a C program, or a Fortran program
SYNOPSIS
ncgen-hdf [-b] [-c] [-f] [-n] [-o output_file] input_file
DESCRIPTION
ncgen-hdf generates either a netCDF file, or C or Fortran source code to create a netCDF file. The input
to ncgen-hdf 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-hdf, it merely checks the syn‐
tax 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-hdf 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-hdf 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 construct‐
ed 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-hdf foo.cdl
From the CDL file `foo.cdl', generate an equivalent binary netCDF file named `x.nc':
ncgen-hdf -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-hdf -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, _Fil‐
lValue), and some data. CDL keywords are in boldface. (This example is intended to illustrate the syn‐
tax; a real CDL file would have a more complete set of attributes so that the data would be more com‐
pletely 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 declara‐
tions 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 dimen‐
sion; 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. At‐
tributes 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 ar‐
rays 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 charac‐
ters 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 syn‐
tax of an initialization is simple: a variable name, an equals sign, and a comma-delimited list of con‐
stants (possibly separated by spaces, tabs and newlines) terminated with a semicolon. For multi-dimen‐
sional 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 at‐
tribute named `_FillValue'. The types of constants need not match the type declared for a variable; co‐
ercions 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, di‐
mensions, 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-hdf 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-hdf converts short declarations to short declara‐
tions 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-hdf 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 repre‐
sentations 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-hdf converts float declarations to float dec‐
larations 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-hdf 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 dis‐
tinguish 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 en‐
closed 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 signif‐
icant 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 con‐
stant 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 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 con‐
stant 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 in‐
terpreted 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
.1f
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 ncgen-hdf 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.
Printed: 125-7-18 $Date$ NCGEN(1)