int shm_open(const char *name, int oflag, mode_t mode);
int shm_unlink(const char *name);
The operation of shm_open() is analogous to that of open(2). name specifies the shared memory object to be created or opened. For portable use, a shared memory object should be identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX (i.e., 255) characters consisting of an initial slash, followed by one or more characters, none of which are slashes.
oflag is a bit mask created by ORing together exactly one of O_RDONLY or O_RDWR and any of the other flags listed here:
Definitions of these flag values can be obtained by including <fcntl.h>.
On successful completion shm_open() returns a new file descriptor referring to the shared memory object. This file descriptor is guaranteed to be the lowest-numbered file descriptor not previously opened within the process. The FD_CLOEXEC flag (see fcntl(2)) is set for the file descriptor.
The file descriptor is normally used in subsequent calls to ftruncate(2) (for a newly created object) and mmap(2). After a call to mmap(2) the file descriptor may be closed without affecting the memory mapping.
The operation of shm_unlink() is analogous to unlink(2): it removes a shared memory object name, and, once all processes have unmapped the object, de-allocates and destroys the contents of the associated memory region. After a successful shm_unlink(), attempts to shm_open() an object with the same name fail (unless O_CREAT was specified, in which case a new, distinct object is created).
| Interface | Attribute | Value |
| shm_open(), shm_unlink() | Thread safety | MT-Safe locale |
POSIX.1-2001 says that the group ownership of a newly created shared memory object is set to either the calling process's effective group ID or "a system default group ID". POSIX.1-2008 says that the group ownership may be set to either the calling process's effective group ID or, if the object is visible in the filesystem, the group ID of the parent directory.
The POSIX shared memory object implementation on Linux makes use of a dedicated tmpfs(5) filesystem that is normally mounted under /dev/shm.
$ ./pshm_ucase_bounce /myshm & [1] 270171 $ ./pshm_ucase_send /myshm hello HELLO
Further detail about these programs is provided below.
#include <sys/mman.h> #include <fcntl.h> #include <semaphore.h> #include <sys/stat.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
#define BUF_SIZE 1024 /* Maximum size for exchanged string */
/* Define a structure that will be imposed on the shared
memory object */
struct shmbuf {
sem_t sem1; /* POSIX unnamed semaphore */
sem_t sem2; /* POSIX unnamed semaphore */
size_t cnt; /* Number of bytes used in 'buf' */
char buf[BUF_SIZE]; /* Data being transferred */
};
After the "send" program has posted the first of the semaphores, the "bounce" program upper cases the data that has been placed in the memory by the "send" program and then posts the second semaphore to tell the "send" program that it may now access the shared memory.
/* pshm_ucase_bounce.c
Licensed under GNU General Public License v2 or later.
*/
#include <ctype.h>
#include "pshm_ucase.h"
int
main(int argc, char *argv[])
{
if (argc != 2) {
fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);
exit(EXIT_FAILURE);
}
char *shmpath = argv[1];
/* Create shared memory object and set its size to the size
of our structure */
int fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR,
S_IRUSR | S_IWUSR);
if (fd == -1)
errExit("shm_open");
if (ftruncate(fd, sizeof(struct shmbuf)) == -1)
errExit("ftruncate");
/* Map the object into the caller's address space */
struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
if (shmp == MAP_FAILED)
errExit("mmap");
/* Initialize semaphores as process-shared, with value 0 */
if (sem_init(&shmp->sem1, 1, 0) == -1)
errExit("sem_init-sem1");
if (sem_init(&shmp->sem2, 1, 0) == -1)
errExit("sem_init-sem2");
/* Wait for 'sem1' to be posted by peer before touching
shared memory */
if (sem_wait(&shmp->sem1) == -1)
errExit("sem_wait");
/* Convert data in shared memory into upper case */
for (int j = 0; j < shmp->cnt; j++)
shmp->buf[j] = toupper((unsigned char) shmp->buf[j]);
/* Post 'sem2' to tell the to tell peer that it can now
access the modified data in shared memory */
if (sem_post(&shmp->sem2) == -1)
errExit("sem_post");
/* Unlink the shared memory object. Even if the peer process
is still using the object, this is okay. The object will
be removed only after all open references are closed. */
shm_unlink(shmpath);
The program opens the shared memory object and maps the object into its address space. It then copies the data specified in its second argument into the shared memory, and posts the first semaphore, which tells the "bounce" program that it can now access that data. After the "bounce" program posts the second semaphore, the "send" program prints the contents of the shared memory on standard output.
/* pshm_ucase_send.c
Licensed under GNU General Public License v2 or later.
*/
#include <string.h>
#include "pshm_ucase.h"
int
main(int argc, char *argv[])
{
if (argc != 3) {
fprintf(stderr, "Usage: %s /shm-path string\n", argv[0]);
exit(EXIT_FAILURE);
}
char *shmpath = argv[1];
char *string = argv[2];
size_t len = strlen(string);
if (len > BUF_SIZE) {
fprintf(stderr, "String is too long\n");
exit(EXIT_FAILURE);
}
/* Open the existing shared memory object and map it
into the caller's address space */
int fd = shm_open(shmpath, O_RDWR, 0);
if (fd == -1)
errExit("shm_open");
struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
if (shmp == MAP_FAILED)
errExit("mmap");
/* Copy data into the shared memory object */
shmp->cnt = len;
memcpy(&shmp->buf, string, len);
/* Tell peer that it can now access shared memory */
if (sem_post(&shmp->sem1) == -1)
errExit("sem_post");
/* Wait until peer says that it has finished accessing
the shared memory */
if (sem_wait(&shmp->sem2) == -1)
errExit("sem_wait");
/* Write modified data in shared memory to standard output */
write(STDOUT_FILENO, &shmp->buf, len);
write(STDOUT_FILENO, "\n", 1);