Description

   The shm_open function establishes a connection between a shared
   memory object and a file descriptor. It creates 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. The name can be a pathname, in which case
   other processes referring to the same pathname refer to the same
   shared memory object.

   When a shared memory object is created, its state and all data
   associated with it persist until the shared memory is unlinked.

   The shm_open function returns a file descriptor for the shared
   memory object that is the lowest numbered file descriptor not
   currently open for that process.

   The file status flags and file access modes of the open file
   description are set according to the value of oflag, and can have
   zero or more of the following values:

      O_RDONLY - Open for read access only.

      O_RDWR - Open for read or write access.


      O_CREAT - Create the shared memory if the memory object does
      not exist already. The user ID and group ID of the shared
      memory object are identical to those of the calling process.
      The shared memory object's permission bits are set to the
      value of mode, except those set in the file mode creation mask
      of the process.


      O_EXCL - Prevent the opening of a shared memory object if O_
      CREAT is set and the shared memory object already exists. Use
      this option only in combination with O_CREAT.


      O_TRUNC - Truncate the shared memory object to zero length if
      it is successfully opened for read or write access (O_RDWR).

   The initial contents of the shared memory object are binary
   zeros.