shm_open(3) — Linux manual page


shm_open(3)             Library Functions Manual             shm_open(3)

NAME         top

       shm_open, shm_unlink - create/open or unlink POSIX shared memory

LIBRARY         top

       Real-time library (librt, -lrt)

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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

       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.

       O_RDWR 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.

       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.

              If the shared memory object already exists, truncate it to
              zero bytes.

       Definitions of these flag values can be obtained by including

       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

RETURN VALUE         top

       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.

ERRORS         top

       EACCES Permission to shm_unlink() the shared memory object was

       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.

              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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                    Attribute     Value          │
       │ shm_open(), shm_unlink()     │ Thread safety │ MT-Safe locale │

VERSIONS         top

       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

       The POSIX shared memory object implementation on Linux makes use
       of a dedicated tmpfs(5) filesystem that is normally mounted under

STANDARDS         top


HISTORY         top

       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.

EXAMPLES         top

       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         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                    shm_open(3)

Pages that refer to this page: 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)