SHM_OPEN(3)               (2020-11-01)                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) charac-
          ters 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

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               of macro constants which can be used to define mode is
               listed in open(2).  (Symbolic definitions of these con-
               stants 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 atomi-

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

          Definitions of these flag values can be obtained by includ-
          ing <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 descrip-

          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 con-
          tents 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 nonnega-
          tive integer).  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:

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               Permission to shm_unlink() the shared memory object was

               Permission was denied to shm_open() name in the speci-
               fied 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

               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.

          These functions are provided in glibc 2.2 and later.

          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lbw24 lb lb l l l.
          Interface Attribute Value T{ shm_open(), shm_unlink()
          T}   Thread safety  MT-Safe locale

          POSIX.1-2001, POSIX.1-2008.

          POSIX.1-2001 says that the group ownership of a newly cre-
          ated shared memory object is set to either the calling
          process's effective group ID or "a system default group ID".

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          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 trun-
          cate 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 mem-
          ory.  An example execution of the two programs is the fol-

              $ ./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

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                 memory object */

              struct shmbuf {
                  sem_t  sem1;            /* POSIX unnamed semaphore */
                  sem_t  sem2;            /* POSIX unnamed semaphore */
                  size_t cnt;             /* Number of bytes used in aqbufaq */
                  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 sema-
          phores, 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 calleraqs address space */

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                  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 aqsem1aq 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 aqsem2aq to tell the to tell 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

          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" pro-
          gram posts the second semaphore, the "send" program prints
          the contents of the shared memory on standard output.

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              /* 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 calleraqs 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)

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                  /* 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),

          This page is part of release 5.10 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

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