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SHM_OPEN(2)                 BSD System Calls Manual                SHM_OPEN(2)

NAME
     shm_open -- open a shared memory object

SYNOPSIS
     #include <sys/mman.h>
     #include <fcntl.h>

     int
     shm_open(const char *name, int oflag, ...);

     The parameter "mode_t mode" is optional.

DESCRIPTION
     The shared memory object referenced by name is opened for reading and/or writing as specified by the
     argument oflag and the file descriptor returned to the calling process.  The returned file descriptor
     will be the lowest non-open file descriptor for the calling process, and is not shared with any other
     processes, as it is a new file descriptor. The new file descriptor will have the FD_CLOEXEC flag set.
     Repeated calls to shm_open with the same string value for name() will return a file descriptor refer-ring referring
     ring to the same shared memory object, provided that the object has not been unlinked by a call to
     shm_unlink().  The oflag argument may indicate the file is to be created if it does not exist (by spec-ifying specifying
     ifying the O_CREAT flag), in which case the file is created with mode mode as described in chmod(2) and
     modified by the process' umask value (see umask(2)).

     The value of oflag is formed by or'ing the following values:

           O_RDONLY        open for reading only
           O_RDWR          open for reading and writing
           O_CREAT         create object if it does not exist
           O_EXCL          error if create and object exists
           O_TRUNC         truncate size to 0

     Exactly one of O_RDONLY or O_RDWR must be specified.

     If O_TRUNC is specified and the file exists, the file is truncated to zero length.  If O_EXCL is set
     with O_CREAT and the file already exists, shm_open() returns an error.  This may be used to implement a
     simple exclusive access locking mechanism.

     If successful, shm_open() returns a non-negative integer, termed a file descriptor.  It returns -1 and
     sets errno on failure.  The file pointer used to mark the current position within the memory object is
     set to the beginning of the object.

     When a new shared memory object is created it is given the owner and group corresponding to the effec-tive effective
     tive user and group of the calling process. There is no visible entry in the file system for the cre-ated created
     ated object in this implementation.

     When a shared memory object is created, it persists until it it unlinked and all other references are
     gone. Objects do not persist across a system reboot.

     The system imposes a limit on the number of file descriptors open simultaneously by one process.
     getdtablesize(2) returns the current system limit.

ERRORS
     The named object is opened unless:

     [EACCES]           The required permissions (for reading and/or writing) are denied for the given
                        flags.

     [EACCES]           O_CREAT is specified, the object does not exist, and permission to create the object
                        is denied.

     [EEXIST]           O_CREAT and O_EXCL were specified and the object exists.

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

     [EINVAL]           The shm_open() operation is not supported.

     [EMFILE]           The process has already reached its limit for open file descriptors.

     [ENAMETOOLONG]     name exceeded the name size limit.  This is currently PSHMNAMLEN characters (defined
                        in <sys/posix_shm.h>), but this may change in the future.

     [ENFILE]           The system file table is full.

     [ENOENT]           O_CREAT is not set and the named object does not exist.

     [ENOSPC]           O_CREAT is specified, the file does not exist, and there is insufficient space
                        available to create the object.

SEE ALSO
     chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2)

HISTORY
     shm_open() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).

Darwin                          August 29, 2008                         Darwin

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