opendap_constraint.c File Reference

OPeNDAP constraint expression example - server-side subsetting. More...

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

Macros
#define	ERR(e)

Functions
int	main (void)

Detailed Description

OPeNDAP constraint expression example - server-side subsetting.

This example demonstrates server-side subsetting using OPeNDAP constraint expressions appended to URLs. The server extracts only the requested data, significantly reducing network bandwidth compared to downloading full files.

What this example does:

1. Constructs a constrained URL by appending ?sst[0:2][0:88][0:179] to the base URL, requesting only the first 3 time steps of the SST variable
2. Opens the constrained dataset - the server sees only the subset
3. Demonstrates that dimension sizes reflect the CONSTRAINED subset, not the original full dataset dimensions
4. Reads the entire (already subsetted) variable data
5. Reopens with a stride constraint [0:2:12] to sample every 2nd time step

Constraint expression syntax: ?variable[start:stop] - Range selection (inclusive) ?variable[start:stride:stop] - Range with stride ?var1[...],var2[...] - Multiple variables

Key concept: With constraints, nc_inq_dimlen() returns the SUBSET size, not the original dataset size. This is different from client-side subsetting where the full dataset is opened and start/count arrays select subsets.

When to use constraints:

• When you know exactly what subset you need before opening
• For large reductions in data size (e.g., one region from global data)
• When making a single request for a specific time/space subset

Learning Objectives:

• Construct OPeNDAP constraint expressions appended to URLs
• Understand that constrained dimensions report subset sizes, not full sizes
• Use range selection [start:stop] and stride [start:stride:stop] syntax
• Compare server-side constraints with client-side subsetting (start/count)
• Recognize when server-side subsetting is more bandwidth-efficient

Key Concepts:

• Constraint Expression: URL suffix (?var[start:stop]) that tells the server to extract and return only the specified subset
• Server-Side Subsetting: The OPeNDAP server processes the constraint and sends only the requested data — reduces network transfer significantly
• Dimension Size After Constraint: nc_inq_dimlen() returns the constrained subset size, not the original full dimension size
• Stride Selection: [start:stride:stop] samples every Nth element along a dimension, useful for quick-look previews of large datasets
• Multiple Variables: Comma-separated constraints (?var1[...],var2[...]) can select subsets of multiple variables in one open

Prerequisites:

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

Related Examples:

• opendap_simple.c - Simplest OPeNDAP access (no constraints)
• opendap_subset.c - Client-side subsetting (multiple requests, one open)

Compilation:

gcc -o opendap_constraint opendap_constraint.c -lnetcdf

Usage:

./opendap_constraint

Expected Output:

• Opens a constrained URL requesting first 3 time steps of SST
• Prints constrained dimension sizes (subset, not full dataset)
• Reads the entire constrained variable
• Reopens with a stride constraint to sample every 2nd 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

)