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


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


     #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
     On  successful  completion  shm_open()  returns  a  new file descriptor
     referring 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
     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 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


     These functions are provided in glibc 2.2 and later.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     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  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 4.16 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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