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NAME

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

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);

       Link with -lrt.

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 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:

       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  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.

       O_TRUNC
              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,  de-allocates  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 failure,
       shm_open() returns -1.  shm_unlink() returns 0 on success, or -1 on error.

ERRORS

       On failure, errno is set to indicate the cause of the error.  Values which may  appear  in
       errno include the following:

       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.

       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 exist.

VERSIONS

       These functions are provided in glibc 2.2 and later.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

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

CONFORMING TO

       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.

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 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
           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"

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

               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)
                   errExit("shm_open");

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

               /* 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)
                   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 (int j = 0; j < shmp->cnt; j++)
                   shmp->buf[j] = toupper((unsigned char) shmp->buf[j]);

               /* Post 'sem2' to tell the to tell 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 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"

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

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

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

               struct shmbuf *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)

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.