process communication via mmap

gistfile1.txt

/*************************************************************************\
 *                  Copyright (C) Michael Kerrisk, 2016.                   *
 *                                                                         *
 * This program is free software. You may use, modify, and redistribute it *
 * under the terms of the GNU General Public License as published by the   *
 * Free Software Foundation, either version 3 or (at your option) any      *
 * later version. This program is distributed without any warranty.  See   *
 * the file COPYING.gpl-v3 for details.                                    *
 \*************************************************************************/

/* t_mmap.c
   Demonstrate the use of mmap() to create a shared file mapping.
   */
#include <sys/mman.h>
#include <fcntl.h>
#include "tlpi_hdr.h"

#define MEM_SIZE 10

int
main(int argc, char *argv[])
{
	char *addr;
	int fd;

	if (argc < 2 || strcmp(argv[1], "--help") == 0)
		usageErr("%s file [new-value]\n", argv[0]);

	fd = open(argv[1], O_RDWR);
	if (fd == -1)
		errExit("open");

	addr = mmap(NULL, MEM_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED)
		errExit("mmap");

	if (close(fd) == -1)                /* No longer need 'fd' */
		errExit("close");

	printf("Current string=%.*s\n", MEM_SIZE, addr);
	/* Secure practice: output at most MEM_SIZE bytes */

	if (argc > 2) {                     /* Update contents of region */
		if (strlen(argv[2]) >= MEM_SIZE)
			cmdLineErr("'new-value' too large\n");

		memset(addr, 0, MEM_SIZE);      /* Zero out region */
		strncpy(addr, argv[2], MEM_SIZE - 1);
		if (msync(addr, MEM_SIZE, MS_SYNC) == -1)
			errExit("msync");

		printf("Copied \"%s\" to shared memory\n", argv[2]);
	}

	exit(EXIT_SUCCESS);
}
/*
usage: 
$dd if=/dev/zero of=s.txt bs=1 count=1024
1024+0 records in
1024+0 records out
1024 bytes (1.0 kB) copied, 0.002725 s, 376 kB/s
$./t_mmap s.txt hello
Current string=
Copied "hello" to shared memory
$./t_mmap s.txt goodbye
Current string=hello
Copied "goodbye" to shared memory
$od -c -w8 s.txt 
0000000   g   o   o   d   b   y   e  \0
0000010  \0  \0  \0  \0  \0  \0  \0  \0
*
0002000
*/