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 $CREMBX system service,
specifying the following characteristics:
o A maximum message length of 512 characters
o A buffer quota of 512 characters
o A protection mask granting all privileges to USER and GROUP
and no privileges to SYSTEM or WORLD
The buffer quota of 512 characters implies that you cannot write
more than 512 characters to the mailbox before all or part of the
mailbox is read. Since a mailbox record is slightly larger than
the data part of the message that it contains, not all of the
512 characters can be used for message data. You can increase the
size of the buffer by specifying an alternative size using the
optional, third argument to the pipe function. A pipe under the
OpenVMS system is a stream-oriented file with no carriage-control
attributes. It is fully buffered by default in the C RTL.
A mailbox used as a pipe is different than a mailbox created by
the application. A mailbox created by the application defaults
to a record-oriented file with carriage return, carriage control.
Additionally, writing a zero-length record to a mailbox writes an
EOF, as does each close of the mailbox. For a pipe, only the last
close of a pipe writes an EOF.
The pipe is created by the parent process before vfork and
an exec function are called. By calling pipe first, the child
inherits the open file descriptors for the pipe. You can then use
the getname function to return the name of the mailbox associated
with the pipe, if this information is desired. The mailbox name
returned by getname has the format _MBAnnnn: (Alpha only) or _
MBAnnnnn: (Integrity servers(ONLY)) , where nnnn or nnnnn is a
unique number.
Both the parent and the child need to know in advance which file
descriptors will be allocated for the pipe. This information
cannot be retrieved at run time. Therefore, it is important to
understand how file descriptors are used in any VSI C for
OpenVMS program.
File descriptors 0, 1, and 2 are open in a VSI C for OpenVMS
program for stdin (SYS$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