The mailbox used for the pipe is a temporary mailbox. The mailbox
    is not deleted until all processes that have open channels to
    that mailbox close those channels. The last process that closes a
    pipe writes a message to the mailbox, indicating the end-of-file.

    The mailbox is created by using the $CREMBX system service,
    specifying the following characteristics:

    o  A maximum message length of 512 characters

    o  A buffer quota of 512 characters

    o  A protection mask granting all privileges to USER and GROUP
       and no privileges to SYSTEM or WORLD

    The buffer quota of 512 characters implies that you cannot write
    more than 512 characters to the mailbox before all or part of the
    mailbox is read. Since a mailbox record is slightly larger than
    the data part of the message that it contains, not all of the
    512 characters can be used for message data. You can increase the
    size of the buffer by specifying an alternative size using the
    optional, third argument to the pipe function. A pipe under the
    OpenVMS system is a stream-oriented file with no carriage-control
    attributes. It is fully buffered by default in the Compaq C RTL.
    A mailbox used as a pipe is different than a mailbox created by
    the application. A mailbox created by the application defaults
    to a record-oriented file with carriage return, carriage control.
    Additionally, writing a zero-length record to a mailbox writes an
    EOF, as does each close of the mailbox. For a pipe, only the last
    close of a pipe writes an EOF.

    The pipe is created by the parent process before vfork and
    an exec function are called. By calling pipe first, the child
    inherits the open file descriptors for the pipe. You can then use
    the getname function to return the name of the mailbox associated
    with the pipe, if this information is desired. The mailbox name
    returned by getname has the format _MBAnnnn: (Alpha only) or _
    MBAnnnnn: (Integrity servers(ONLY)) , where nnnn or nnnnn is a
    unique number.

    Both the parent and the child need to know in advance which file
    descriptors will be allocated for the pipe. This information
    cannot be retrieved at run time. Therefore, it is important to
    understand how file descriptors are used in any Compaq C for
    OpenVMS program.

    File descriptors 0, 1, and 2 are open in a Compaq C for OpenVMS
    program for stdin (SYS$INPUT), stdout (SYS$OUTPUT), and stderr
    (SYS$ERROR), respectively. Therefore, if no other files are open
    when pipe is called, pipe assigns file descriptor 3 for writing
    and file descriptor 4 for reading. In the array returned by pipe,
    4 is placed in element 0 and 3 is placed in element 1.

    If other files have been opened, pipe assigns the first
    available file descriptor for writing and the next available
    file descriptor for reading. In this case, the pipe does not
    necessarily use adjacent file descriptors. For example, assume
    that two files have been opened and assigned to file descriptors
    3 and 4 and the first file is then closed. If pipe is called at
    this point, file descriptor 3 is assigned for writing and file
    descriptor 5 is assigned for reading. Element 0 of the array will
    contain 5 and element 1 will contain 3.

    In large applications that do large amounts of I/O, it gets
    more difficult to predict which file descriptors are going to
    be assigned to a pipe; and, unless the child knows which file
    descriptors are being used, it will not be able to read and write
    successfully from and to the pipe.

    One way to be sure that the correct file descriptors are being
    used is to use the following procedure:

    1. Choose two descriptor numbers that will be known to both the
       parent and the child. The numbers should be high enough to
       account for any I/O that might be done before the pipe is
       created.

    2. Call pipe in the parent at some point before calling an exec
       function.

    3. In the parent, use dup2 to assign the file descriptors
       returned by pipe to the file descriptors you chose. This now
       reserves those file descriptors for the pipe; any subsequent
       I/O will not interfere with the pipe.

    You can read and write through the pipe using the UNIX I/O
    functions read and write, specifying the appropriate file
    descriptors. As an alternative, you can issue fdopen calls to
    associate file pointers with these file descriptors so that you
    can use the Standard I/O functions (fread and fwrite).

    Two separate file descriptors are used for reading from and
    writing to the pipe, but only one mailbox is used so some I/O
    synchronization is required. For example, assume that the parent
    writes a message to the pipe. If the parent is the first process
    to read from the pipe, then it will read its own message back as
    shown in Reading and Writing to a Pipe.

                                   NOTE

       For added UNIX portability, you can use the following
       feature logicals to control the behavior of the C RTL pipe
       implementation:

       o  Define the DECC$STREAM_PIPE feature logical name to
          ENABLE to direct the pipe function to use stream I/O
          instead of record I/O.

       o  Define the DECC$POPEN_NO_CRLF_REC_ATTR feature logical
          to ENABLE to prevent CR/LF carriage control from being
          added to pipe records for pipes opened with the popen
          function. Be aware that enabling this feature might
          result in undesired behavior from other functions such
          as gets that rely on the carriage-return character.

    Figure REF-1  Reading and Writing to a Pipe