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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.
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
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;
}
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
`_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.
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 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
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.
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
'a' // ASCII `a'
' ' // a zero byte
'
' // ASCII newline character
'' // ASCII escape character (33 octal)
'+' // ASCII plus (2b hex)
'ÿ' // 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
lines
" // a 10-character string with two embedded newlines
"a bell:" // 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
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 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.