pipe programming

Jerry Mcguire · 01-19-2010, 01:51 AM

Hi all,

We have seen and used a lot of piping, e.g. % A | B
in which the output of program A is fed to the input of program B.

If I am to write program B, do I simply read stdin? <I really doubt this>

Also, how does B know when the input is done and then it can finish up?

e.g. % ll | grep 'jerry'

grep does know when ll has finished. How?

Thanks.

Disillusionist · 01-19-2010, 02:00 AM

What happens, is that Program A runs (to completion) and writes it's output to stdout, the pipe passes that as input stdin to Program B.

The output is only passed to Program B once Program A has completed.

Jerry Mcguire · 01-19-2010, 02:08 AM

> The output is only passed to Program B once Program A has completed.

Then if Prog A outputs a huge amount of data or if Prog A doesn't stop writing, the pipe will overflow. Or isn't it the case?

/* Simple Prog A */
#include <stdio.h>
#include <unistd.h>
int main()
{
int i = 0;
do {
printf( "i = %d\n", i );
i ++;
usleep(10000);
} while(1);
return 0;
}

At the command line

% progA | grep '0'

gives output from time to time though.

Hko · 01-19-2010, 04:08 AM

Quote:

Originally Posted by Disillusionist

What happens, is that Program A runs (to completion) and writes it's output to stdout, the pipe passes that as input stdin to Program B.

The output is only passed to Program B once Program A has completed.

No. That is not what happens. That is the difference between using pipes or writing to a temp file and later read that back with some other program.

Hko · 01-19-2010, 04:20 AM

Quote:

Originally Posted by Jerry Mcguire

We have seen and used a lot of piping, e.g. % A | B
in which the output of program A is fed to the input of program B.

If I am to write program B, do I simply read stdin? <I really doubt this>

Yes, just read stdin. Until the read-function you use indicates end-of-file (EOF).

Quote:

Originally Posted by Jerry Mcguire

e.g. % ll | grep 'jerry'

grep does know when ll has finished. How?

ll closes his stdout, or if it doesn't close his stdout, the kernel will when ll exits. This makes the pipe send EOF to the reading end (grep), where to code should detect EOF and act accordingly.

Jerry Mcguire · 01-19-2010, 10:04 PM

Thanks. It works well.