NCGEN(1)

💖 NCGEN(1) 💖 — UNIDATA UTILITIES 🦋

ok so like, welcome to the UNIDATA UTILITIES section bestie, we're talking about ncgen-hdf today and honestly? she's iconic. 💅

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🌸 NAME 🌸

ncgen-hdf — takes a CDL file and generates a netCDF file, or a C program, or a Fortran program. three vibes, one tool, no cap. ✨

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✨ SYNOPSIS ✨

ncgen-hdf [-b] [-c] [-f] [-n] [-o output_file] input_file

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🦋 DESCRIPTION 🦋

okay so here's the tea 🍓 — ncgen-hdf generates either a netCDF file, or C or Fortran source code that creates a netCDF file. like, it's giving "I contain multitudes" fr fr.

the input to ncgen-hdf is a description of a netCDF file, written in a little language called CDL (which stands for network Common Data form Language, and it's described below, iykyk).

now here's the part where she's lowkey just a vibe checker: if you invoke ncgen-hdf with no options at all, she merely checks the syntax of your input CDL file and produces error messages for any violations of CDL syntax. that's it. that's the whole job. a little syntax bouncer at the door. 😭

but the other options? they're the main character energy ✨ — you can create the corresponding netCDF file, generate a C program that uses the netCDF C interface to create the netCDF file, or generate a Fortran program that uses the netCDF Fortran interface to create the same netCDF file.

also, ncgen-hdf may be used with the companion program ncdump to perform some simple operations on netCDF files 👑. for example — say you wanna rename a dimension in a netCDF file:

1. use ncdump to get a CDL version of the netCDF file 🎀
2. edit the CDL file to change the name of the dimensions ✏️
3. use ncgen-hdf to generate the corresponding netCDF file from the edited CDL file 🌈

that's the whole workflow, obsessed honestly.

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👑 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. and heads up bestie 🚨 — if a file already exists with the specified name, it will be overwritten. no warning, no asking, just gone. 💀
• -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 — that's the whole reason, periodt.) 🎀
• -n — Like the -b option, except creates a 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, ngl she's a legacy queen. 🫶

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🍓 EXAMPLES 🍓

check the syntax of the CDL file foo.cdl:

ncgen-hdf foo.cdl

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

ncgen-hdf -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-hdf -c -o x.nc foo.cdl

💖 USAGE 💖

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🎀 CDL Syntax Summary 🎀

Okay bestie, here's the whole vibe laid out. 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, do whatever looks cute. 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. Slay.

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). Main character dimension energy, honestly. 👑

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. fr fr.

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). It's giving ✨data about the 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. No mixing, bestie, it's not that kind of party.

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. Cozy. 😌

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🌈 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-hdf 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-hdf 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-hdf 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. Pro tip energy, but from the original docs, not from me.

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-hdf 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-hdf converts double declarations to double declarations in the output C code and to DOUBLE PRECISION declarations in output Fortran code.

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💅 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. Suffixes are the moment, iykyk.

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. 😭 no strings bestie, only vibes and fixed lengths.

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
.1f

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 💀 (read this bestie, it's giving "we tried our best") 💀

Okay so ngl there are some ✨known quirks✨ and you deserve to know about them before you get got.

• When ncgen-hdf is doing its thing with -c or -f, the programs it generates store data in variables using initialization statements. That's fine and cute for small stuff 🎀 but if you point it at a large dataset, it's gonna fail to produce a compilable program, because those resulting statements can blow past the line length or the number of continuation statements your compiler is willing to allow. It will not compile. No cap. 😭
• The CDL syntax makes it so easy to assign something that looks like an array of variable-length strings to a netCDF variable 🦋 — but surprise, the strings just get concatenated into a single array of characters, because netCDF straight up cannot represent an array of variable-length strings inside one netCDF variable. It's giving "syntax said yes, data model said no" 💀. Not a bug you can fix with vibes, it's a limitation.
• And the big one: NetCDF and CDL do not yet support a type corresponding to a 64-bit integer. That's it, that's the whole note. No 64-bit int type yet, bestie. Plan accordingly 🙈

Like, none of these are "your fault" bugs — they're known limits of the tool and the formats. Just don't be shocked when they show up, okay? 🫶

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🌸 a little sign-off 🌸

This whole thing came from the NCGEN(1) manual page, printed on 126-10-12, with the date placeholder $Date$ doing its thing where dates usually go. So yeah — source is NCGEN(1), that's the version stamp, that's the date, and that's a wrap 💅✨

Go forth and generate some netCDF files, you absolute icon 👑🌈