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Full Discussion: How does pipe work?
Top Forums UNIX for Dummies Questions & Answers How does pipe work? Post 302268886 by siba.s.nayak on Tuesday 16th of December 2008 01:25:43 PM
Old 12-16-2008
Quote:
Originally Posted by Perderabo
That is not correct. It is undefined whether A or B starts first. They might start at the exactly the same time if there are multiple cpu's. A pipe can hold an undefined but finite amount of data.

If B tries to read from the pipe, but no data is available, B will wait until the data arrives. If B was reading from a disk, B might have the same problem and need to wait until a disk read finishes. A closer analogy would be reading from a keyboard. There, B would need to wait for a user to type. But in all of these cases, B has started a "read" operation and must wait until it finishes.

If A tries to write to the pipe, and the pipe is full, A must wait for some room in the pipe to become free. A could have the same problem if A was writing to a terminal. A terminal has flow control and can moderate the pace of data. In any event, to A, it has started a "write" operation and will wait until the write operation finishes.

A and B are behaving as co-processes, although not all co-processes while be communicating with a pipe. Neither is in full control of the other.

In a case, like:
A | sort
The sort command cannot output anything until it reads all of the data. So the sort command will do that, just as it would if it was reading from a file. Many other programs strive to read and write data if they can. This allows them to be used in long pipelines with data continuously flowing though the entire pipeline.
Thanks a lot. Though I knew these things, I could not map the same things with the question. This is purely lack of understanding on IPC. Thanks a lot again for refreshing up me.
 

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PIPE(2) 							System Calls Manual							   PIPE(2)

NAME
pipe - create an interprocess channel SYNOPSIS
#include <u.h> #include <libc.h> int pipe(int fd[2]) DESCRIPTION
Pipe creates a buffered channel for interprocess I/O communication. Two file descriptors are returned in fd. Data written to fd[1] is available for reading from fd[0] and data written to fd[0] is available for reading from fd[1]. After the pipe has been established, cooperating processes created by subsequent fork(2) calls may pass data through the pipe with read and write calls. The bytes placed on a pipe by one write are contiguous even if many processes are writing. Write boundaries are preserved: each read terminates when the read buffer is full or after reading the last byte of a write, whichever comes first. The number of bytes available to a read(2) is reported in the Length field returned by fstat or dirfstat on a pipe (see stat(2)). When all the data has been read from a pipe and the writer has closed the pipe or exited, read(2) will return 0 bytes. Writes to a pipe with no reader will generate a note sys: write on closed pipe. SOURCE
/sys/src/libc/9syscall SEE ALSO
intro(2), read(2), pipe(3) DIAGNOSTICS
Sets errstr. BUGS
If a read or a write of a pipe is interrupted, some unknown number of bytes may have been transferred. When a read from a pipe returns 0 bytes, it usually means end of file but is indistinguishable from reading the result of an explicit write of zero bytes. PIPE(2)
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