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shm_open - Man Page

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:


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.


Open the object for read-write access.


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.


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.


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).

Return Value

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.



Permission to shm_unlink() the shared memory object was denied.


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.


Both O_CREAT and O_EXCL were specified to shm_open() and the shared memory object specified by name already exists.


The name argument to shm_open() was invalid.


The per-process limit on the number of open file descriptors has been reached.


The length of name exceeds PATH_MAX.


The system-wide limit on the total number of open files has been reached.


An attempt was made to shm_open() a name that did not exist, and O_CREAT was not specified.


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).

shm_open(), shm_unlink()Thread safetyMT-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.




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

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"

main(int argc, char *argv[])
    int            fd;
    char           *shmpath;
    struct shmbuf  *shmp;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);

    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)

    if (ftruncate(fd, sizeof(struct shmbuf)) == -1)

    /* 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)

    /* Initialize semaphores as process-shared, with value 0. */

    if (sem_init(&shmp->sem1, 1, 0) == -1)
    if (sem_init(&shmp->sem2, 1, 0) == -1)

    /* Wait for 'sem1' to be posted by peer before touching
       shared memory. */

    if (sem_wait(&shmp->sem1) == -1)

    /* 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)

    /* 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. */



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"

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]);

    shmpath = argv[1];
    string = argv[2];
    len = strlen(string);

    if (len > BUF_SIZE) {
        fprintf(stderr, "String is too long\n");

    /* 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)

    shmp = mmap(NULL, sizeof(*shmp), PROT_READ | PROT_WRITE,
                MAP_SHARED, fd, 0);
    if (shmp == MAP_FAILED)

    /* 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)

    /* Wait until peer says that it has finished accessing
       the shared memory. */

    if (sem_wait(&shmp->sem2) == -1)

    /* Write modified data in shared memory to standard output. */

    write(STDOUT_FILENO, &shmp->buf, len);
    write(STDOUT_FILENO, "\n", 1);


See Also

close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2), memfd_create(2), mmap(2), open(2), umask(2), shm_overview(7)

Referenced By

execve(2), file-hierarchy(7), memfd_create(2), mm(3), mmap(2), mmap2(2), sem_init(3), sem_overview(7), sem_post(3), shm_overview(7), tmpfs(5), truncate(2), umask(2).

The man page shm_unlink(3) is an alias of shm_open(3).

2024-06-15 Linux man-pages 6.9.1