square16_par.c File Reference

Demonstrates parallel NetCDF-4 I/O with MPI using a 2x2 rank decomposition. More...

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

Macros
#define	NDIMS   2
#define	QUAD_SIZE   8
#define	DIM_SIZE   16
#define	ERR(e)

Functions
int	main (int argc, char **argv)

Detailed Description

Demonstrates parallel NetCDF-4 I/O with MPI using a 2x2 rank decomposition.

This example shows how to use NetCDF-4's parallel I/O capabilities with MPI to write and read a shared dataset concurrently from multiple processes. Four MPI ranks cooperate to write a single 16x16 integer array, each contributing an 8x8 quadrant filled with its own rank number.

The program demonstrates the fundamental parallel I/O pattern: open a file collectively, define the full dataset shape once, then have each rank write only its local portion using hyperslab selections (start/count). After writing, it performs a parallel read-back to verify correctness.

Learning Objectives:

• Open/create NetCDF files in parallel with nc_create_par() and nc_open_par()
• Use nc_var_par_access() to enable collective I/O mode
• Compute per-rank hyperslab offsets for a 2D domain decomposition
• Write and read variable subsets with nc_put_vara_int() / nc_get_vara_int()
• Verify parallel write correctness with a coordinated read-back

Key Concepts:

• Collective I/O: All ranks participate in every I/O call (NC_COLLECTIVE), which allows the MPI-IO layer to optimize access patterns
• Independent I/O: Alternative mode (NC_INDEPENDENT) where each rank calls I/O functions independently; less common for performance-critical code
• Domain Decomposition: Dividing a global array among MPI ranks; here a simple 2x2 block decomposition maps rank → quadrant
• Hyperslab: A rectangular sub-region of a variable selected by start[] and count[] arrays passed to nc_put_vara / nc_get_vara functions
• MPI_INFO_NULL: Passed when no MPI-IO hints are needed; production code may supply hints (e.g., striping) via MPI_Info objects for better performance

Rank Layout (2×2 grid):

 col 0-7    col 8-15
+----------+----------+
| rank 0   | rank 2   |  rows 0-7
+----------+----------+
| rank 1   | rank 3   |  rows 8-15
+----------+----------+

Prerequisites:

• simple_nc4.c - NetCDF-4 format basics
• An MPI installation and a NetCDF-C library built with parallel support

Related Examples:

• f_square16_par.f90 - Fortran equivalent

Compilation:

mpicc -o square16_par square16_par.c -lnetcdf

Usage:

mpirun -n 4 ./square16_par

Expected Output:

Parallel I/O write and read-back successful. File: square16_par.nc

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, Intelligent Data Design, Inc.

Date

June 3, 2026

Macro Definition Documentation

DIM_SIZE

#define DIM_SIZE   16

ERR

#define ERR

(

e

)

Value:

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

NDIMS

#define NDIMS   2

QUAD_SIZE

#define QUAD_SIZE   8

Function Documentation

main()

int main	(	int	argc,
		char **	argv
	)