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
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.
Printed: 110-$Date: 1996-03-26 16:43:35 -0600 (Tue, 26 Mar 1996) $ NCGEN(1)