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 gen-
erate 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 per-
form 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 ab-
sent, 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 al-
ready 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 ob-
solete `.cdf' extension instead of the `.nc' extension,
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NCGEN(1) UNIDATA UTILITIES NCGEN(1)
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), vari-
ables (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;
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NCGEN(1) UNIDATA UTILITIES NCGEN(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. Com-
ments may follow the characters `//' on any line.
A CDL description consists of three optional parts: dimen-
sions, variables, and data, beginning with the keyword di-
mensions:, variables:, and data, respectively. The variable
part may contain variable declarations and attribute assign-
ments.
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. Dimen-
sions need not have corresponding variables.
A netCDF attribute contains information about a netCDF vari-
able 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 at-
tribute 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 attri-
bute name, separated by `:'. It is possible to assign glo-
bal 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 attri-
bute is the number of data values assigned to it, or the
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NCGEN(1) UNIDATA UTILITIES NCGEN(1)
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 ini-
tialization 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 ma-
trices. 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 un-
signed, 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 out-
put C code and to the nonstandard BYTE declaration in output
Fortran code.
Shorts can hold values between -32768 and 32767. ncgen con-
verts short declarations to short declarations in the output
C code and to the nonstandard INTEGER*2 declaration in out-
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NCGEN(1) UNIDATA UTILITIES NCGEN(1)
put 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 ap-
pended to shorts and floats to distinguish them from longs
and doubles.
A byte constant is represented by a single character or mul-
tiple 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 char-
acter array may be represented as a string enclosed in dou-
ble 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 in-
terpreted as the end of the significant characters by the
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NCGEN(1) UNIDATA UTILITIES NCGEN(1)
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 ar-
rays 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 in-
teger constant with an `s' or `S' appended. If a short con-
stant begins with `0', it is interpreted as octal, except
that if it begins with `0x', it is interpreted as a hexade-
cimal 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 ordi-
nary 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 signifi-
cant 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 signifi-
cant 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 ac-
ceptable double constants:
-2.0
3.141592653589793
1.0e-20
1.d
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NCGEN(1) UNIDATA UTILITIES NCGEN(1)
BUGS
The programs generated by ncgen 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: 97-12-16 Last change: 1996-03-26 7
⌐ 1994 Man-cgi 1.15, Panagiotis Christias <christia@theseas.ntua.gr>