opendap_simple.c File Reference

Simple OPeNDAP example - open, read, and close a remote dataset. More...

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

Macros
#define	ERR(e)

Functions
int	main (void)

Detailed Description

Simple OPeNDAP example - open, read, and close a remote dataset.

This is the most basic OPeNDAP example, demonstrating the core concept: remote datasets are accessed using the same NetCDF API calls as local files.

What this example does:

1. Opens a remote sea surface temperature dataset from the public OPeNDAP test server at test.opendap.org
2. Queries dataset metadata (dimensions, variables, attributes)
3. Reads information about the 'sst' variable and its dimensions
4. Reads a small 5x5 spatial subset from the first time step
5. Closes the remote connection

Key OPeNDAP concepts demonstrated:

• Using an HTTP URL instead of a local file path in nc_open()
• The NetCDF library handles all HTTP communication transparently
• Data is transferred over the network only when read functions are called
• Subsetting with start/count arrays reduces data transfer

The dataset accessed is sst.mnmean.nc.gz, a classic netCDF-3 file containing NOAA Extended Reconstructed Sea Surface Temperature data. The server transparently decompresses and serves the data via OPeNDAP.

Learning Objectives:

• Open a remote dataset by passing an HTTP URL to nc_open()
• Query remote dataset metadata (dimensions, variables, attributes)
• Read a spatial subset using start/count arrays to minimize data transfer
• Understand that the netCDF API is transport-agnostic (local file vs remote URL)
• Recognize that data transfer occurs only on nc_get_var_*() calls, not on open

Key Concepts:

• OPeNDAP: Open-source protocol for accessing remote scientific data via HTTP
• Transparent Access: nc_open() with a URL triggers the OPeNDAP dispatch layer; all subsequent nc_*() calls work identically to local file access
• Lazy Data Transfer: Opening a URL fetches only metadata (DAS/DDS); actual array data is transferred only when nc_get_var_*() is called
• Client-Side Subsetting: Use start[] and count[] arrays in nc_get_vara_*() to request only needed data, reducing network bandwidth
• NC_NOWRITE: Remote datasets are always read-only; this flag is required

Prerequisites:

• simple_2D.c - Basic nc_open/nc_inq/nc_get_var workflow
• coord_vars.c - Understanding dimension and variable relationships
• A NetCDF-C library built with OPeNDAP support (NC_HAS_DAP2 or NC_HAS_DAP4)

Related Examples:

• opendap_subset.c - Client-side subsetting with multiple access patterns
• opendap_constraint.c - Server-side subsetting via constraint expressions

Compilation:

gcc -o opendap_simple opendap_simple.c -lnetcdf

Usage:

./opendap_simple

Expected Output:

• Prints dataset metadata (dimensions, variables, attributes)
• Prints dimension names and sizes
• Reads and displays a 5x5 spatial subset of SST from the first time step

Note

Companion code for "The NetCDF Developer's Handbook: The Authoritative Guide to Writing High-Performance Programs for Scientific Data Management, Second Edition" (https://www.amazon.com/dp/B0H7Q1Z75L)

Author

Edward Hartnett

Date

6/15/26

Macro Definition Documentation

ERR

#define ERR

(

e

)

Value:

    do { \
    if (e) { \
        fprintf(stderr, "Error: %s at line %d\n", nc_strerror(e), __LINE__); \
        return 1; \
    } \
} while (0)

Function Documentation

main()

int main

(

void

)