An example of using shared memory in a POSIX system. #c #linux #shmem #shm_open #shared-memory

use-shared-memory-1.c

// Note: Link with -lrt

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */
#include <unistd.h>
#include <string.h>

#include <assert.h>

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

	int oflags=O_RDWR;
	int opt;

	 while ((opt = getopt(argc, argv, "c")) != -1) {
		switch (opt) {
		case 'c': /* create it */
			oflags = O_RDWR | O_CREAT;
			break;
		default: /* '?' */
			fprintf(stderr, "Usage: %s -[c]\n", argv[0]);
			exit(EXIT_FAILURE);
		}
	}

	off_t   length = 2 * 1024;
	char   *name   = "/malex.hello03.01";
	int     fd = shm_open(name, oflags, 0644 );

	ftruncate(fd, length);

	fprintf(stderr,"Shared Mem Descriptor: fd=%d\n", fd);

	assert (fd>0);

	u_char *ptr = (u_char *) mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

	fprintf(stderr, "Shared Memory Address: %p [0..%lu]\n", ptr, length-1);
	fprintf(stderr, "Shared Memory Path: /dev/shm/%s\n", name );

	assert (ptr);

	char *msg = "hello world!!\n";

	strcpy((char*)ptr,msg);


	close(fd);
	exit(0);
}

use-shared-memory-2.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */
#include <unistd.h>
#include <string.h>

#include <assert.h>

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

	int oflags=O_RDWR;
	int i;

	char   *name   = "/malex.hello03.01";
	int     fd     = shm_open(name, oflags, 0644 );

	fprintf(stderr,"Shared Mem Descriptor: fd=%d\n", fd);

	assert (fd>0);

	struct stat sb;

	fstat(fd, &sb);
	off_t length = sb.st_size ;

	u_char *ptr = (u_char *) mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

	fprintf(stderr, "Shared Mem Address: %p [0..%lu]\n", ptr, length-1);
	assert (ptr);

	// hexdump first 100 bytes
	fprintf(stdout,"First 100 bytes:\n");
	for(i=0; i<100; i++)
		fprintf(stdout, "%02X%s", ptr[i], (i%25==24)?("\n"):(" ") );

	// change 1st byte
	ptr[ 0 ] = 'H' ;

	close(fd);
	exit(0);
}

@samuel100u
no, it uses a reference-count approach: the OS will delete the region only when all processes have been unlinked. See also the man page