The Pipe File Manager

(PIPEMAN)

The Pipe file manager handles control of processes that use

paths to pipes. Pipes allow concurrently executing processes to

send each other data by using the output of one process (the

writer) as input to a second process (the reader). The reader gets

input from the standard input. The exclamation point (!) opera

tor specifies that the input or output is from or to a pipe. The

Pipe file manager allocates a 256-byte block and a path descrip

tor for data transfer. The Pipe file manager also determines

which process has control of the pipe. The Pipe file manager has

the standard file manager branch table at its entry point:

PipeEnt lbra Create

lbra Open

lbra MakDir

lbra ChgDir

lbra Delete

lbra Seek

' lbra PRead

lbra PWrite

lbra PRdLn

lbra PWrLn

lbra Getstat

lbra Putstat

lbra Close

You cannot use MakDir, ChgDir, Delete, and Seek with pipes. If

you try to do so, the system returns E$UNKSVC (unknown ser

vice request). Getstat and Putstat are also no-action service rou

tines. They return without error.

Create and Open perform the same functions. They set up the

256-byte data exchange buffer, and save several addresses in the

path descriptor.

The Close request checks to see if any process is reading or writ

ing through the pipe. If not, OS-9 returns the buffer.

PRead, PWrite, PRdLn, and PWrLn read data from the buffer

and write data to it.

7-1

OS-9 Technical Reference

The ! operator tells the Shell that processes wish to communicate
through a pipe. For example:

proc1 ! Proc2 ENTER

In this example, shell forks Procl with the standard output path
to a pipe, and forks Proc2 with the Standard input path from a
pipe.

Shell can also handle a series of processes using pipes. Example:

proc1 ! Proc2 ! proc3 ! proc4 ENTER

The following outline shows how to set up pipes between
processes:

Open /Pipe save path in variable x

Dup path #1 save 5tdout in variable y

Dup x make path available

Fork proc1 put pipe in stdout.

CDup uses lowest available)

Close #1 make path available

Dup y restore stdout

Close y make path available

Dup path #0 save stdin in Y

Close #0 make path available

Dup x put pipe in stdin

Fork 2 fork process 2

Close #0 make path available

Dup y restore 5tdin

Close x no longer needed

Close y no longer needed

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The Pipe File Manager (PIPEMAN) / 7

Example: The following example shows how an application can
initiate another process with the stdin and stdout routed
through a pipe.

Open /pipe1 save path in variable a
Open /pipe2 save path in variable b
Dup 0 save stdin in variable x
Dup 1 save stdout in variable y
Close 0 make path available
Close 1 make path available
Dup a make pipe1 stdin
Dup b make pipe2 5tdout
Fork new process
Close 0 make path available
Close 1 make path available
Dup x restore stdin
Dup y restore stdout
Return a&b return pipe path numbers to caller

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7-3