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);
}

What does 0644 mean in shm_open(name, oflags, 0644 );?

@pharapeti
it is the ownership mode, just like in the open system call

Thank you for this example. It was really helpful.
However, I have a question. Any particular reason for not using shm_unlink or munmap? Is it not necessary?

@DharminB
shm_unlink would delete the shared memory region from the OS (it appears under /dev/shm); in many cases we want it to remain for other processes to read/write to it.
munmap would destroy (invalidate) the memory mapping for the pointer pointing to the shm region. This affects only the current process and it automatically happens when the process terminates.

@drmalex07

shm_unlink would delete the file under /dev/shm/xxxxx even there are other processes using it ?

@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