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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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shm_open(3) Library Functions Manual shm_open(3)
shm_open, shm_unlink - create/open or unlink POSIX shared memory
objects
Real-time library (librt, -lrt)
#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);
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, deallocates 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 file descriptor (a nonnegative
integer). On success, shm_unlink() returns 0. On failure, both
functions return -1 and set errno to indicate the error.
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.
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 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.
POSIX.1-2008.
glibc 2.2. POSIX.1-2001.
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 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.
Program source: pshm_ucase.h
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.
#ifndef PSHM_UCASE_H
#define PSHM_UCASE_H
#include <semaphore.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.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 */
};
#endif // include guard
Program source: pshm_ucase_bounce.c
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.
/* pshm_ucase_bounce.c
Licensed under GNU General Public License v2 or later.
*/
#include <ctype.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
#include "pshm_ucase.h"
int
main(int argc, char *argv[])
{
int fd;
char *shmpath;
struct shmbuf *shmp;
if (argc != 2) {
fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);
exit(EXIT_FAILURE);
}
shmpath = argv[1];
/* Create shared memory object and set its size to the size
of our structure. */
fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR, 0600);
if (fd == -1)
errExit("shm_open");
if (ftruncate(fd, sizeof(struct shmbuf)) == -1)
errExit("ftruncate");
/* Map the object into the caller's address space. */
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 (size_t j = 0; j < shmp->cnt; j++)
shmp->buf[j] = toupper((unsigned char) shmp->buf[j]);
/* Post 'sem2' to tell the 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);
}
Program source: pshm_ucase_send.c
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.
/* pshm_ucase_send.c
Licensed under GNU General Public License v2 or later.
*/
#include <fcntl.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include "pshm_ucase.h"
int
main(int argc, char *argv[])
{
int fd;
char *shmpath, *string;
size_t len;
struct shmbuf *shmp;
if (argc != 3) {
fprintf(stderr, "Usage: %s /shm-path string\n", argv[0]);
exit(EXIT_FAILURE);
}
shmpath = argv[1];
string = argv[2];
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. */
fd = shm_open(shmpath, O_RDWR, 0);
if (fd == -1)
errExit("shm_open");
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. */
if (write(STDOUT_FILENO, &shmp->buf, len) == -1)
errExit("write");
if (write(STDOUT_FILENO, "\n", 1) == -1)
errExit("write");
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 shm_open(3)
Pages that refer to this page: ipcrm(1), execve(2), memfd_create(2), mmap2(2), mmap(2), truncate(2), umask(2), sem_init(3), sem_post(3), tmpfs(5), file-hierarchy(7), sem_overview(7), shm_overview(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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