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