NAME

       shm_open - open a shared memory object (REALTIME)

SYNOPSIS

       #include <sys/mman.h>

       int shm_open(const char *name, int oflag, mode_t mode);

DESCRIPTION

       The  shm_open() function shall establish a connection between a shared memory object and a
       file descriptor. It shall create an open file description that refers to the shared memory
       object  and  a  file  descriptor  that  refers  to  that  open  file description. The file
       descriptor is used by other functions to refer to that  shared  memory  object.  The  name
       argument  points  to a string naming a shared memory object. It is unspecified whether the
       name appears in the file system and is visible to other functions that take  pathnames  as
       arguments.  The  name  argument conforms to the construction rules for a pathname. If name
       begins with the slash character, then processes calling shm_open() with the same value  of
       name  refer  to  the same shared memory object, as long as that name has not been removed.
       If name does not begin with the slash character, the effect is implementation-defined. The
       interpretation  of  slash  characters  other  than  the leading slash character in name is
       implementation-defined.

       If successful, shm_open() shall return a file descriptor for the shared memory object that
       is  the lowest numbered file descriptor not currently open for that process. The open file
       description is new, and therefore the file descriptor does not share  it  with  any  other
       processes.  It  is  unspecified  whether  the  file  offset  is  set.  The FD_CLOEXEC file
       descriptor flag associated with the new file descriptor is set.

       The file status flags and file access modes of the open file description are according  to
       the  value of oflag. The oflag argument is the bitwise-inclusive OR of the following flags
       defined in the <fcntl.h> header. Applications specify exactly one of the first two  values
       (access modes) below in the value of oflag:

       O_RDONLY
              Open for read access only.

       O_RDWR Open for read or write access.

       Any combination of the remaining flags may be specified in the value of oflag:

       O_CREAT
              If  the shared memory object exists, this flag has no effect, except as noted under
              O_EXCL below. Otherwise, the shared memory object is created; the user  ID  of  the
              shared  memory  object  shall  be  set to the effective user ID of the process; the
              group ID of the shared memory object is set to a system default group ID or to  the
              effective  group ID of the process. The permission bits of the shared memory object
              shall be set to the value of the mode argument except those set in  the  file  mode
              creation mask of the process. When bits in mode other than the file permission bits
              are set, the effect is unspecified. The mode argument does not affect  whether  the
              shared  memory  object  is opened for reading, for writing, or for both. The shared
              memory object has a size of zero.

       O_EXCL If O_EXCL and O_CREAT are set, shm_open() fails if the shared memory object exists.
              The  check  for  the  existence of the shared memory object and the creation of the
              object if it does not exist is atomic with respect  to  other  processes  executing
              shm_open()  naming  the  same  shared memory object with O_EXCL and O_CREAT set. If
              O_EXCL is set and O_CREAT is not set, the result is undefined.

       O_TRUNC
              If the shared memory object exists, and  it  is  successfully  opened  O_RDWR,  the
              object  shall be truncated to zero length and the mode and owner shall be unchanged
              by this function call. The result of using O_TRUNC with O_RDONLY is undefined.

       When a shared memory object is created, the state of the shared memory  object,  including
       all data associated with the shared memory object, persists until the shared memory object
       is unlinked and all other references are gone. It is  unspecified  whether  the  name  and
       shared memory object state remain valid after a system reboot.

RETURN VALUE

       Upon  successful  completion,  the shm_open() function shall return a non-negative integer
       representing the lowest numbered unused file descriptor. Otherwise, it shall return -1 and
       set errno to indicate the error.

ERRORS

       The shm_open() function shall fail if:

       EACCES The  shared memory object exists and the permissions specified by oflag are denied,
              or the shared memory object does not exist and  permission  to  create  the  shared
              memory object is denied, or O_TRUNC is specified and write permission is denied.

       EEXIST O_CREAT and O_EXCL are set and the named shared memory object already exists.

       EINTR  The shm_open() operation was interrupted by a signal.

       EINVAL The shm_open() operation is not supported for the given name.

       EMFILE Too many file descriptors are currently in use by this process.

       ENAMETOOLONG
              The  length  of  the  name  argument  exceeds {PATH_MAX} or a pathname component is
              longer than {NAME_MAX}.

       ENFILE Too many shared memory objects are currently open in the system.

       ENOENT O_CREAT is not set and the named shared memory object does not exist.

       ENOSPC There is insufficient space for the creation of the new shared memory object.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       When the Memory Mapped Files option is supported, the normal open() call is used to obtain
       a  descriptor to a file to be mapped according to existing practice with mmap().  When the
       Shared Memory Objects  option  is  supported,  the  shm_open()  function  shall  obtain  a
       descriptor to the shared memory object to be mapped.

       There  is ample precedent for having a file descriptor represent several types of objects.
       In the POSIX.1-1990 standard, a file descriptor can represent a file, a pipe,  a  FIFO,  a
       tty,  or  a  directory.   Many  implementations simply have an operations vector, which is
       indexed by the file descriptor type and does very different operations. Note that in  some
       cases  the file descriptor passed to generic operations on file descriptors is returned by
       open() or creat() and in some cases returned by alternate functions, such as  pipe().  The
       latter technique is used by shm_open().

       Note  that such shared memory objects can actually be implemented as mapped files. In both
       cases, the size can be set after the  open  using  ftruncate().  The  shm_open()  function
       itself does not create a shared object of a specified size because this would duplicate an
       extant function that set the size of an object referenced by a file descriptor.

       On implementations where memory objects are implemented using the  existing  file  system,
       the  shm_open()  function  may  be  implemented using a macro that invokes open(), and the
       shm_unlink() function may be implemented using a macro that invokes unlink().

       For implementations without a permanent file system, the definition of  the  name  of  the
       memory  objects  is  allowed not to survive a system reboot. Note that this allows systems
       with a permanent file system to implement memory objects as data  structures  internal  to
       the implementation as well.

       On  implementations  that  choose  to  implement  memory  objects using memory directly, a
       shm_open() followed by an ftruncate() and close() can be  used  to  preallocate  a  shared
       memory  area and to set the size of that preallocation.  This may be necessary for systems
       without virtual memory hardware support in order to ensure that the memory is contiguous.

       The set of valid open flags to shm_open() was restricted to O_RDONLY, O_RDWR, O_CREAT, and
       O_TRUNC  because  these  could  be easily implemented on most memory mapping systems. This
       volume of IEEE Std 1003.1-2001 is silent on  the  results  if  the  implementation  cannot
       supply  the  requested  file  access  because of implementation-defined reasons, including
       hardware ones.

       The error conditions [EACCES] and [ENOTSUP] are provided to inform  the  application  that
       the implementation cannot complete a request.

       [EACCES] indicates for implementation-defined reasons, probably hardware-related, that the
       implementation cannot comply with a requested  mode  because  it  conflicts  with  another
       requested  mode.  An  example might be that an application desires to open a memory object
       two times, mapping different areas with different access  modes.   If  the  implementation
       cannot  map  a  single area into a process space in two places, which would be required if
       different access modes were required for the two areas, then the implementation may inform
       the application at the time of the second open.

       [ENOTSUP]  indicates  for  implementation-defined reasons, probably hardware-related, that
       the implementation cannot comply with a requested mode at all. An example  would  be  that
       the hardware of the implementation cannot support write-only shared memory areas.

       On all implementations, it may be desirable to restrict the location of the memory objects
       to specific file systems for performance (such as a RAM  disk)  or  implementation-defined
       reasons  (shared  memory supported directly only on certain file systems).  The shm_open()
       function may be used to enforce  these  restrictions.   There  are  a  number  of  methods
       available  to the application to determine an appropriate name of the file or the location
       of an appropriate directory. One way is from the environment via getenv().  Another  would
       be from a configuration file.

       This volume of IEEE Std 1003.1-2001 specifies that memory objects have initial contents of
       zero when created. This is consistent with current  behavior  for  both  files  and  newly
       allocated memory. For those implementations that use physical memory, it would be possible
       that such implementations could simply use available memory and give  it  to  the  process
       uninitialized.   This,   however,  is  not  consistent  with  standard  behavior  for  the
       uninitialized data area, the stack, and of course, files. Finally, it is highly  desirable
       to  set  the  allocated  memory  to  zero  for security reasons. Thus, initializing memory
       objects to zero is required.

FUTURE DIRECTIONS

       None.

SEE ALSO

       close() , dup() , exec() , fcntl() , mmap() , shmat() , shmctl() , shmdt() ,  shm_unlink()
       , umask() , the Base Definitions volume of IEEE Std 1003.1-2001, <fcntl.h>, <sys/mman.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .