shm_open, shm_unlink - create/open or unlink POSIX shared memory objects
#include <sys/mman.h>
#include <sys/stat.h> /* For mode constants */
#include <fcntl.h> /* For O_* constants */
int shm_open(const char *name, int oflag, mode_t
mode);
int shm_unlink(const char *name);
Link with
-lrt.
shm_open() creates and opens a new, or opens an existing, POSIX shared
memory object. A POSIX shared memory object is in effect a handle which can be
used by unrelated processes to
mmap(2) the same region of shared
memory. The
shm_unlink() function performs the converse operation,
removing an object previously created by
shm_open().
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:
- O_RDONLY
- Open the object for read access. A shared memory object opened in this way
can be mmap(2)ed only for read (PROT_READ) access.
- O_RDWR
- Open the object for read-write access.
- O_CREAT
- Create the shared memory object if it does not exist. The user and group
ownership of the object are taken from the corresponding effective IDs of
the calling process, and the object's permission bits are set according to
the low-order 9 bits of mode, except that those bits set in the
process file mode creation mask (see umask(2)) are cleared for the
new object. A set of macro constants which can be used to define
mode is listed in open(2). (Symbolic definitions of these
constants can be obtained by including <sys/stat.h>.)
- A new shared memory object initially has zero length—the size of
the object can be set using ftruncate(2). The newly allocated bytes
of a shared memory object are automatically initialized to 0.
- O_EXCL
- If O_CREAT was also specified, and a shared memory object with the
given name already exists, return an error. The check for the
existence of the object, and its creation if it does not exist, are
performed atomically.
- O_TRUNC
- If the shared memory object already exists, truncate it to zero
bytes.
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).
On success,
shm_open() returns a nonnegative file descriptor. On failure,
shm_open() returns -1.
shm_unlink() returns 0 on success, or -1
on error.
On failure,
errno is set to indicate the cause of the error. Values which
may appear in
errno include the following:
- EACCES
- Permission to shm_unlink() the shared memory object was
denied.
- EACCES
- Permission was denied to shm_open() name in the specified
mode, or O_TRUNC was specified and the caller does not have
write permission on the object.
- EEXIST
- Both O_CREAT and O_EXCL were specified to shm_open()
and the shared memory object specified by name already exists.
- EINVAL
- The name argument to shm_open() was invalid.
- EMFILE
- The per-process limit on the number of open file descriptors has been
reached.
- ENAMETOOLONG
- The length of name exceeds PATH_MAX.
- ENFILE
- The system-wide limit on the total number of open files has been
reached.
- ENOENT
- An attempt was made to shm_open() a name that did not exist,
and O_CREAT was not specified.
- ENOENT
- An attempt was to made to shm_unlink() a name that does not
exist.
These functions are provided in glibc 2.2 and later.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| shm_open (), shm_unlink () |
Thread safety |
MT-Safe locale |
POSIX.1-2001, POSIX.1-2008.
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.
POSIX leaves the behavior of the combination of
O_RDONLY and
O_TRUNC unspecified. On Linux, this will successfully truncate an
existing shared memory object—this may not be so on other UNIX systems.
The POSIX shared memory object implementation on Linux makes use of a dedicated
tmpfs(5) filesystem that is normally mounted under
/dev/shm.
The programs below employ POSIX shared memory and POSIX unnamed semaphores to
exchange a piece of data. The "bounce" program (which must be run
first) raises the case of a string that is placed into the shared memory by
the "send" program. Once the data has been modified, the
"send" program then prints the contents of the modified shared
memory. An example execution of the two programs is the following:
$ ./pshm_ucase_bounce /myshm &
[1] 270171
$ ./pshm_ucase_send /myshm hello
HELLO
Further detail about these programs is provided below.
The following header file is included by both programs below. Its primary
purpose is to define a structure that will be imposed on the memory object
that is shared between the two programs.
#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 */
};
The "bounce" program creates a new shared memory object with the name
given in its command-line argument and sizes the object to match the size of
the
shmbuf structure defined in the header file. It then maps the
object into the process's address space, and initializes two POSIX semaphores
inside the object to 0.
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.
#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(struct shmbuf),
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);
exit(EXIT_SUCCESS);
}
The "send" program takes two command-line arguments: the pathname of a
shared memory object previously created by the "bounce" program and
a string that is to be copied into that object.
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.
#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(struct shmbuf),
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);
exit(EXIT_SUCCESS);
}
close(2),
fchmod(2),
fchown(2),
fcntl(2),
fstat(2),
ftruncate(2),
memfd_create(2),
mmap(2),
open(2),
umask(2),
shm_overview(7)