shm_open(3p) — Linux manual page


SHM_OPEN(3P)              POSIX Programmer's Manual             SHM_OPEN(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       shm_open — open a shared memory object (REALTIME)

SYNOPSIS         top

       #include <sys/mman.h>

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

DESCRIPTION         top

       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,
       except that the interpretation of <slash> characters other than the
       leading <slash> character in name is implementation-defined, and that
       the length limits for the name argument are implementation-defined
       and need not be the same as the pathname limits {PATH_MAX} and
       {NAME_MAX}.  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

       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 shall be set to the effective group ID of the
                   process; however, if the name argument is visible in the
                   file system, the group ID may be set to the group ID of
                   the containing directory. 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

       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         top

       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         top

       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

       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 All file descriptors available to the process are currently

       ENFILE Too many shared memory objects are currently open in the

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

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

       The shm_open() function may fail if:

              The length of the name argument exceeds {_POSIX_PATH_MAX} on
              systems that do not support the XSI option or exceeds
              {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
              that is longer than {_POSIX_NAME_MAX} on systems that do not
              support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI

       The following sections are informative.

EXAMPLES         top

   Creating and Mapping a Shared Memory Object
       The following code segment demonstrates the use of shm_open() to
       create a shared memory object which is then sized using ftruncate()
       before being mapped into the process address space using mmap():

           #include <unistd.h>
           #include <sys/mman.h>

           #define MAX_LEN 10000
           struct region {        /* Defines "structure" of shared memory */
               int len;
               char buf[MAX_LEN];
           struct region *rptr;
           int fd;

           /* Create shared memory object and set its size */

           fd = shm_open("/myregion", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
           if (fd == −1)
               /* Handle error */;

           if (ftruncate(fd, sizeof(struct region)) == −1)
               /* Handle error */;

           /* Map shared memory object */

           rptr = mmap(NULL, sizeof(struct region),
                  PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
           if (rptr == MAP_FAILED)
               /* Handle error */;

           /* Now we can refer to mapped region using fields of rptr;
              for example, rptr->len */



RATIONALE         top

       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

       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

       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
       POSIX.1‐2008 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 POSIX.1‐2008 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.


       A future version might require the shm_open() and shm_unlink()
       functions to have semantics similar to normal file system operations.

SEE ALSO         top

       close(3p), dup(3p), exec(1p), fcntl(3p), mmap(3p), shmat(3p),
       shmctl(3p), shmdt(3p), shm_unlink(3p), umask(3p)

       The Base Definitions volume of POSIX.1‐2008, fcntl.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see .

IEEE/The Open Group                 2013                        SHM_OPEN(3P)

Pages that refer to this page: sys_mman.h(0p)shmat(3p)shmctl(3p)shmdt(3p)shmget(3p)shm_unlink(3p)