1NCGEN3(1) UNIDATA UTILITIES NCGEN3(1)
2
6 ncgen3 - From a CDL file generate a netCDF classic or 64 bit classic‐
7 file, a C program, or a Fortran program
8
10 ncgen3 [-b] [-c] [-f] [-k kind_of_file] [-x] [-n] [-o netcdf_filename]
11 input_file
12
14 ncgen3 generates either a netCDF file, or C or Fortran source code to
15 create a netCDF file. The input to ncgen3 is a description of a netCDF
16 file in a small language known as CDL (network Common Data form Lan‐
17 guage), described below. If no options are specified in invoking nc‐
18 gen3, it merely checks the syntax of the input CDL file, producing er‐
19 ror messages for any violations of CDL syntax. Other options can be
20 used to create the corresponding netCDF file, to generate a C program
21 that uses the netCDF C interface to create the netCDF file, or to gen‐
22 erate a Fortran program that uses the netCDF Fortran interface to cre‐
23 ate the same netCDF file.
24 ncgen3 may be used with the companion program ncdump to perform some
26 simple operations on netCDF files. For example, to rename a dimension
27 in a netCDF file, use ncdump to get a CDL version of the netCDF file,
28 edit the CDL file to change the name of the dimensions, and use ncgen3
29 to generate the corresponding netCDF file from the edited CDL file.
30
32 -b Create a (binary) netCDF file. If the -o option is absent, a
33 default file name will be constructed from the netCDF name
34 (specified after the netcdf keyword in the input) by appending
35 the `.nc' extension. If a file already exists with the speci‐
36 fied name, it will be overwritten.
37 -c Generate C source code that will create a netCDF file matching
39 the netCDF specification. The C source code is written to stan‐
40 dard output.
41 -f Generate Fortran source code that will create a netCDF file
43 matching the netCDF specification. The Fortran source code is
44 written to standard output.
45 -o netcdf_file
47 Name for the binary netCDF file created. If this option is
48 specified, it implies the "-b" option. (This option is neces‐
49 sary because netCDF files cannot be written directly to standard
50 output, since standard output is not seekable.)
51 -k kind_of_file
53 Using -k2 or -k "64-bit offset" specifies that generated file
54 (or program) should use version 2 of format that employs 64-bit
55 file offsets. The default is to use version 1 ("classic") for‐
56 mat with 32-bit file offsets, although this limits the size of
57 the netCDF file, variables, and records to the sizes supported
58 by the classic format. (NetCDF-4 will support additional kinds
59 of netCDF files, "netCDF-4" and "netCDF-4 classic model".)
60 Note: -v is also accepted to mean the same thing as -k for back‐
61 ward compatibility, but -k is preferred, to match the corre‐
62 sponding ncdump option.
63 -x Don't initialize data with fill values. This can speed up cre‐
65 ation of large netCDF files greatly, but later attempts to read
66 unwritten data from the generated file will not be easily de‐
67 tectable.
68
70 Check the syntax of the CDL file `foo.cdl':
71 ncgen3 foo.cdl
73 From the CDL file `foo.cdl', generate an equivalent binary netCDF file
75 named `x.nc':
76 ncgen3 -o x.nc foo.cdl
78 From the CDL file `foo.cdl', generate a C program containing the netCDF
80 function invocations necessary to create an equivalent binary netCDF
81 file named `x.nc':
82 ncgen3 -c -o x.nc foo.cdl
84
86 CDL Syntax Summary
87 Below is an example of CDL syntax, describing a netCDF file with sever‐
88 al named dimensions (lat, lon, and time), variables (Z, t, p, rh, lat,
89 lon, time), variable attributes (units, long_name, valid_range, _Fill‐
90 Value), and some data. CDL keywords are in boldface. (This example is
91 intended to illustrate the syntax; a real CDL file would have a more
92 complete set of attributes so that the data would be more completely
93 self-describing.)
94 netcdf foo { // an example netCDF specification in CDL
96 dimensions:
98 lat = 10, lon = 5, time = unlimited ;
99 variables:
101 long lat(lat), lon(lon), time(time);
102 float Z(time,lat,lon), t(time,lat,lon);
103 double p(time,lat,lon);
104 long rh(time,lat,lon);
105 // variable attributes
107 lat:long_name = "latitude";
108 lat:units = "degrees_north";
109 lon:long_name = "longitude";
110 lon:units = "degrees_east";
111 time:units = "seconds since 1992-1-1 00:00:00";
112 Z:units = "geopotential meters";
113 Z:valid_range = 0., 5000.;
114 p:_FillValue = -9999.;
115 rh:_FillValue = -1;
116 data:
118 lat = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90;
119 lon = -140, -118, -96, -84, -52;
120 }
121 All CDL statements are terminated by a semicolon. Spaces, tabs, and
123 newlines can be used freely for readability. Comments may follow the
124 characters `//' on any line.
125 A CDL description consists of three optional parts: dimensions, vari‐
127 ables, and data, beginning with the keyword dimensions:, variables:,
128 and data, respectively. The variable part may contain variable decla‐
129 rations and attribute assignments.
130 A netCDF dimension is used to define the shape of one or more of the
132 multidimensional variables contained in the netCDF file. A netCDF di‐
133 mension has a name and a size. At most one dimension in a netCDF file
134 can have the unlimited size, which means a variable using this dimen‐
135 sion can grow to any length (like a record number in a file).
136 A variable represents a multidimensional array of values of the same
138 type. A variable has a name, a data type, and a shape described by its
139 list of dimensions. Each variable may also have associated attributes
140 (see below) as well as data values. The name, data type, and shape of
141 a variable are specified by its declaration in the variable section of
142 a CDL description. A variable may have the same name as a dimension;
143 by convention such a variable is one-dimensional and contains coordi‐
144 nates of the dimension it names. Dimensions need not have correspond‐
145 ing variables.
146 A netCDF attribute contains information about a netCDF variable or
148 about the whole netCDF dataset. Attributes are used to specify such
149 properties as units, special values, maximum and minimum valid values,
150 scaling factors, offsets, and parameters. Attribute information is
151 represented by single values or arrays of values. For example, "units"
152 is an attribute represented by a character array such as "celsius". An
153 attribute has an associated variable, a name, a data type, a length,
154 and a value. In contrast to variables that are intended for data, at‐
155 tributes are intended for metadata (data about data).
156 In CDL, an attribute is designated by a variable and attribute name,
158 separated by `:'. It is possible to assign global attributes not asso‐
159 ciated with any variable to the netCDF as a whole by using `:' before
160 the attribute name. The data type of an attribute in CDL is derived
161 from the type of the value assigned to it. The length of an attribute
162 is the number of data values assigned to it, or the number of charac‐
163 ters in the character string assigned to it. Multiple values are as‐
164 signed to non-character attributes by separating the values with com‐
165 mas. All values assigned to an attribute must be of the same type.
166 The names for CDL dimensions, variables, and attributes must begin with
168 an alphabetic character or `_', and subsequent characters may be al‐
169 phanumeric or `_' or `-'.
170 The optional data section of a CDL specification is where netCDF vari‐
172 ables may be initialized. The syntax of an initialization is simple: a
173 variable name, an equals sign, and a comma-delimited list of constants
174 (possibly separated by spaces, tabs and newlines) terminated with a
175 semicolon. For multi-dimensional arrays, the last dimension varies
176 fastest. Thus row-order rather than column order is used for matrices.
177 If fewer values are supplied than are needed to fill a variable, it is
178 extended with a type-dependent `fill value', which can be overridden by
179 supplying a value for a distinguished variable attribute named `_Fill‐
180 Value'. The types of constants need not match the type declared for a
181 variable; coercions are done to convert integers to floating point, for
182 example. The constant `_' can be used to designate the fill value for
183 a variable.
184 Primitive Data Types
186 char characters
187 byte 8-bit data
188 short 16-bit signed integers
189 long 32-bit signed integers
190 int (synonymous with long)
191 float IEEE single precision floating point (32 bits)
192 real (synonymous with float)
193 double IEEE double precision floating point (64 bits)
194 Except for the added data-type byte and the lack of unsigned, CDL sup‐
196 ports the same primitive data types as C. The names for the primitive
197 data types are reserved words in CDL, so the names of variables, dimen‐
198 sions, and attributes must not be type names. In declarations, type
199 names may be specified in either upper or lower case.
200 Bytes differ from characters in that they are intended to hold a full
202 eight bits of data, and the zero byte has no special significance, as
203 it does for character data. ncgen3 converts byte declarations to char
204 declarations in the output C code and to the nonstandard BYTE declara‐
205 tion in output Fortran code.
206 Shorts can hold values between -32768 and 32767. ncgen3 converts short
208 declarations to short declarations in the output C code and to the non‐
209 standard INTEGER*2 declaration in output Fortran code.
210 Longs can hold values between -2147483648 and 2147483647. ncgen3 con‐
212 verts long declarations to long declarations in the output C code and
213 to INTEGER declarations in output Fortran code. int and integer are
214 accepted as synonyms for long in CDL declarations. Now that there are
215 platforms with 64-bit representations for C longs, it may be better to
216 use the int synonym to avoid confusion.
217 Floats can hold values between about -3.4+38 and 3.4+38. Their exter‐
219 nal representation is as 32-bit IEEE normalized single-precision float‐
220 ing point numbers. ncgen3 converts float declarations to float decla‐
221 rations in the output C code and to REAL declarations in output Fortran
222 code. real is accepted as a synonym for float in CDL declarations.
223 Doubles can hold values between about -1.7+308 and 1.7+308. Their ex‐
225 ternal representation is as 64-bit IEEE standard normalized double-pre‐
226 cision floating point numbers. ncgen3 converts double declarations to
227 double declarations in the output C code and to DOUBLE PRECISION decla‐
228 rations in output Fortran code.
229 CDL Constants
231 Constants assigned to attributes or variables may be of any of the ba‐
232 sic netCDF types. The syntax for constants is similar to C syntax, ex‐
233 cept that type suffixes must be appended to shorts and floats to dis‐
234 tinguish them from longs and doubles.
235 A byte constant is represented by a single character or multiple char‐
237 acter escape sequence enclosed in single quotes. For example,
238 'a' // ASCII `a'
239 '\0' // a zero byte
240 '\n' // ASCII newline character
241 '\33' // ASCII escape character (33 octal)
242 '\x2b' // ASCII plus (2b hex)
243 '\377' // 377 octal = 255 decimal, non-ASCII
244 Character constants are enclosed in double quotes. A character array
246 may be represented as a string enclosed in double quotes. The usual C
247 string escape conventions are honored. For example
248 "a" // ASCII `a'
249 "Two\nlines\n" // a 10-character string with two embedded newlines
250 "a bell:\007" // a string containing an ASCII bell
251 Note that the netCDF character array "a" would fit in a one-element
252 variable, since no terminating NULL character is assumed. However, a
253 zero byte in a character array is interpreted as the end of the signif‐
254 icant characters by the ncdump program, following the C convention.
255 Therefore, a NULL byte should not be embedded in a character string un‐
256 less at the end: use the byte data type instead for byte arrays that
257 contain the zero byte. NetCDF and CDL have no string type, but only
258 fixed-length character arrays, which may be multi-dimensional.
259 short integer constants are intended for representing 16-bit signed
261 quantities. The form of a short constant is an integer constant with
262 an `s' or `S' appended. If a short constant begins with `0', it is in‐
263 terpreted as octal, except that if it begins with `0x', it is inter‐
264 preted as a hexadecimal constant. For example:
265 -2s // a short -2
266 0123s // octal
267 0x7ffs //hexadecimal
268 Long integer constants are intended for representing 32-bit signed
270 quantities. The form of a long constant is an ordinary integer con‐
271 stant, although it is acceptable to append an optional `l' or `L'. If
272 a long constant begins with `0', it is interpreted as octal, except
273 that if it begins with `0x', it is interpreted as a hexadecimal con‐
274 stant. Examples of valid long constants include:
275 -2
276 1234567890L
277 0123 // octal
278 0x7ff // hexadecimal
279 Floating point constants of type float are appropriate for representing
281 floating point data with about seven significant digits of precision.
282 The form of a float constant is the same as a C floating point constant
283 with an `f' or `F' appended. For example the following are all accept‐
284 able float constants:
285 -2.0f
286 3.14159265358979f // will be truncated to less precision
287 1.f
288 Floating point constants of type double are appropriate for represent‐
291 ing floating point data with about sixteen significant digits of preci‐
292 sion. The form of a double constant is the same as a C floating point
293 constant. An optional `d' or `D' may be appended. For example the
294 following are all acceptable double constants:
295 -2.0
296 3.141592653589793
297 1.0e-20
298 1.d
299
302 The programs generated by ncgen3 when using the -c or -f use initial‐
303 ization statements to store data in variables, and will fail to produce
304 compilable programs if you try to use them for large datasets, since
305 the resulting statements may exceed the line length or number of con‐
306 tinuation statements permitted by the compiler.
307 The CDL syntax makes it easy to assign what looks like an array of
309 variable-length strings to a netCDF variable, but the strings will sim‐
310 ply be concatenated into a single array of characters, since netCDF
311 cannot represent an array of variable-length strings in one netCDF
312 variable.
313 NetCDF and CDL do not yet support a type corresponding to a 64-bit in‐
315 teger.
316Printed: 119-6-20 $Date: 2009/09/24 18:19:10 $ NCGEN3(1)