opendap_subset.c File Reference

OPeNDAP client-side subsetting example. More...

#include <netcdf.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

Macros
#define	ERR(e)

Functions
int	main (void)

Detailed Description

OPeNDAP client-side subsetting example.

This example demonstrates client-side subsetting: opening the full remote dataset once, then making multiple targeted data requests using start/count arrays. This approach offers more flexibility than server-side constraints.

What this example does:

1. Opens the full remote dataset (no constraint expression)
2. Queries the variable dimensions to understand the full dataset shape
3. Demonstrates four different data access patterns:
   • Single time slice: Read all lat/lon for one time step
   • Time series: Read all time points at a single lat/lon location
   • Regional subset: Read a small 3D cube (3 time steps, 10x10 spatial)
   • Multiple scattered requests: Show overhead of many small requests
4. Closes the dataset

Client-side vs Server-side subsetting trade-offs:

Client-side (this example):

• Can make multiple different subset requests from one open
• Flexible exploration - change start/count without reopening
• Transfers more data over network for each request
• Server still reads full data then extracts subset

Server-side (constraint expressions):

• Server only sends the requested data
• Most bandwidth-efficient for single known subset
• Must reopen to change the subset
• Dimension queries return constrained sizes only

The 3D array layout is [time][lat][lon] with dimensions approximately [time~200][lat=89][lon=180] depending on the dataset version.

Learning Objectives:

• Open a full remote dataset without constraint expressions
• Use start[] and count[] arrays for multiple targeted data requests
• Implement four different access patterns (time slice, time series, regional, scattered)
• Understand the trade-offs between client-side and server-side subsetting
• Recognize the overhead of many small requests vs fewer large requests

Key Concepts:

• Client-Side Subsetting: Open the full dataset once, then use start/count in nc_get_vara_*() calls to select different subregions on each read
• Time Slice Access: Read all spatial points for one time step (start=[t,0,0], count=[1,nlat,nlon])
• Time Series Access: Read all time points at one spatial location (start=[0,lat,lon], count=[ntime,1,1])
• Regional Subset: Read a small 3D cube from the full dataset (start=[t0,lat0,lon0], count=[nt,nlat_sub,nlon_sub])
• Multiple Requests: One nc_open() supports unlimited nc_get_vara_*() calls with different start/count values — no need to reopen for each subset

Prerequisites:

• opendap_simple.c - Basic OPeNDAP access and metadata query
• simple_2D.c - Understanding start/count subsetting for local files
• A NetCDF-C library built with OPeNDAP support (NC_HAS_DAP2 or NC_HAS_DAP4)

Related Examples:

• opendap_simple.c - Simplest OPeNDAP access (single subset)
• opendap_constraint.c - Server-side subsetting via URL constraints

Compilation:

gcc -o opendap_subset opendap_subset.c -lnetcdf

Usage:

./opendap_subset

Expected Output:

• Opens remote SST dataset and prints full dimension sizes
• Demonstrates four access patterns with timing and data summaries: single time slice, time series at a point, regional 3D cube, and multiple scattered requests

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

)