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.