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

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

       #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 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
                  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 <fc-

       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

       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 ob-
       ject  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 nonnegative file descriptor.  On fail-
       ure,  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:

       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.

              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 ex-

       These functions are provided in glibc 2.2 and later.

       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |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 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.

       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.

       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 5.05 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                             2017-09-15                       SHM_OPEN(3)
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