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shm_open(3)                Library Functions Manual                shm_open(3)

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

LIBRARY
       Real-time library (librt, -lrt)

SYNOPSIS
       #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
       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 opera-
       tion, 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 porta-
       ble 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 correspond-
              ing  effective IDs of the calling process, and the object's per-
              mission 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  ob-
              tained 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 ex-
              ist, 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  <fc-
       ntl.h>.

       On  successful  completion shm_open() returns a new file descriptor re-
       ferring to the shared memory object.  This file descriptor  is  guaran-
       teed  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 ftrun-
       cate(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 ob-
       ject 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 inte-
       ger).  On success, shm_unlink() returns 0.  On failure, both  functions
       return -1 and set errno to indicate the error.

ERRORS
       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 per-
              mission 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 ex-
              ist.

VERSIONS
       These functions are provided in glibc 2.2 and later.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       ┌─────────────────────────────────────┬───────────────┬────────────────┐
       │InterfaceAttributeValue          │
       ├─────────────────────────────────────┼───────────────┼────────────────┤
       │shm_open(), shm_unlink()             │ Thread safety │ MT-Safe locale │
       └─────────────────────────────────────┴───────────────┴────────────────┘

STANDARDS
       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 own-
       ership 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.

NOTES
       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.

EXAMPLES
       The  programs  below employ POSIX shared memory and POSIX unnamed sema-
       phores 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 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 pri-
       mary  purpose is to define a structure that will be imposed on the mem-
       ory object that is shared between the two programs.

           #include <fcntl.h>
           #include <semaphore.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/mman.h>
           #include <sys/stat.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 mem-
       ory 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"

           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  argu-
       ment 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"

           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. */

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

               exit(EXIT_SUCCESS);
           }

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)

Linux man-pages 6.03              2023-02-05                       shm_open(3)

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