user_types.c File Reference

Demonstrates NetCDF-4 user-defined types (compound, enum, vlen, opaque) More...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <netcdf.h>

Data Structures
struct	WeatherObs

Macros
#define	FILE_NAME   "user_types.nc"
#define	NOBS   5
#define	NDAYS   3
#define	NSTATIONS   4
#define	CALIB_SIZE   16
#define	ERRCODE   2
#define	ERR(e)   {printf("Error: %s\n", nc_strerror(e)); exit(ERRCODE);}

Enumerations
enum	CloudCover { CLEAR = 0 , PARTLY_CLOUDY = 1 , CLOUDY = 2 , OVERCAST = 3 }

Functions
int	main ()

Detailed Description

Demonstrates NetCDF-4 user-defined types (compound, enum, vlen, opaque)

This example showcases NetCDF-4's powerful type system, which extends beyond the basic atomic types (int, float, etc.) to include user-defined types similar to C structures and enumerations. User-defined types enable more expressive and efficient data models for complex scientific data.

The program demonstrates four types of user-defined types:

• Compound types: Structured records like C structs (weather observations)
• Enum types: Named integer constants (cloud cover categories)
• Variable-length types: Arrays of varying length (daily measurements)
• Opaque types: Binary blobs for arbitrary data (calibration data)
• String type: Variable-length strings (NC_STRING, station names)

Learning Objectives:

• Understand NetCDF-4 user-defined type system
• Learn to create compound types with nc_def_compound()
• Master enum type definition with nc_def_enum()
• Work with variable-length arrays using nc_def_vlen()
• Use opaque types for binary data with nc_def_opaque()
• Use NC_STRING for variable-length string data
• Write and read complex structured data

Key Concepts:

• Compound Type: Structured type with named fields (like C struct)
• Enum Type: Integer type with named values (like C enum)
• Variable-Length Type: Array whose length varies per element
• Opaque Type: Uninterpreted binary data of fixed size
• String Type: Variable-length character strings (NC_STRING)
• Type ID: Unique identifier for each user-defined type

User-Defined Type Details:

Compound Types:

• Define with nc_def_compound(), add fields with nc_insert_compound()
• Fields can be atomic types or other user-defined types
• Useful for observations, records, structured metadata
• Example: Weather observation with time, temp, pressure, humidity

Enum Types:

• Define with nc_def_enum(), add members with nc_insert_enum()
• Based on integer type (NC_BYTE, NC_SHORT, NC_INT, etc.)
• Provides named constants for categorical data
• Example: Cloud cover (CLEAR, PARTLY_CLOUDY, CLOUDY, OVERCAST)

Variable-Length Types:

• Define with nc_def_vlen() specifying base type
• Each array element can have different length
• Useful for ragged arrays, particle lists, event sequences
• Example: Daily measurements (different counts per day)

Opaque Types:

• Define with nc_def_opaque() specifying byte size
• Store arbitrary binary data (images, encrypted data, proprietary formats)
• NetCDF doesn't interpret the content
• Example: Instrument calibration data

Prerequisites:

• simple_nc4.c - NetCDF-4 format basics
• coord_vars.c - Variables and attributes
• var4d.c - Multi-dimensional data

Related Examples:

• f_user_types.f90 - Fortran equivalent
• simple_nc4.c - NetCDF-4 format requirement
• compression.c - Compression with user-defined types

Compilation:

gcc -o user_types user_types.c -lnetcdf -lm

Usage:

./user_types
ncdump user_types.nc

Expected Output: Creates user_types.nc containing:

• Compound type: WeatherObs (time, temperature, pressure, humidity)
• Enum type: CloudCover (CLEAR=0, PARTLY_CLOUDY=1, CLOUDY=2, OVERCAST=3)
• Vlen type: DailyMeasurements (variable-length float arrays)
• Opaque type: CalibrationData (16-byte binary blobs)
• String type: Station names (variable-length strings)
• Variables demonstrating each type with realistic data

Author

Edward Hartnett, Intelligent Data Design, Inc.

Date

2026

Macro Definition Documentation

CALIB_SIZE

#define CALIB_SIZE   16

ERR

#define ERR

(

e

)

{printf("Error: %s\n", nc_strerror(e)); exit(ERRCODE);}

ERRCODE

#define ERRCODE   2

FILE_NAME

#define FILE_NAME   "user_types.nc"

NDAYS

#define NDAYS   3

NOBS

#define NOBS   5

NSTATIONS

#define NSTATIONS   4

Enumeration Type Documentation

CloudCover

enum CloudCover

Enumerator
CLEAR
PARTLY_CLOUDY
CLOUDY
OVERCAST

Function Documentation

main()

int main

(

)