SHM_OPEN(3)                 Linux Programmer's Manual                SHM_OPEN(3)

       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:

              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

       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

       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.

              The length of name exceeds PATH_MAX.

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

       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

       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.

           #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 */

   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 "pshm_ucase.h"

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

               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)

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

               /* 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)

               /* 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 (int 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 <string.h>
           #include "pshm_ucase.h"

           main(int argc, char *argv[])
               if (argc != 3) {
                   fprintf(stderr, "Usage: %s /shm-path string\n", argv[0]);

               char *shmpath = argv[1];
               char *string = argv[2];
               size_t 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. */

               int fd = shm_open(shmpath, O_RDWR, 0);
               if (fd == -1)

               struct shmbuf *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);


       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 release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                        SHM_OPEN(3)