unbuffered pipe

[Oliver]

I'm using a pipe to talk to a forked child process -

I do not want any data to get lost if the child process exits
unexpectedly -

- so I was hoping to turn off buffering so that there was at most 1
piece of data (i.e. one line) in the pipe.

I thought if I did:

select(PIPEHANDLE);
$| = 1;

then my pipe would not do buffering, and I was expecting that that
would lead to blocking writes - i.e. if the data hadn't been read from
the pipe, then a write to the pipe would block until it was.

however - after doing the above - I find I can still write data to the
pipe - regardless of how much is read out of the other end.

could someone enlighten me as to what's going on here please?
thanks muchly,
Oliver.

 

[Ben Morrow]

Quoth (e-mail address removed) (Oliver):

I'm using a pipe to talk to a forked child process -

I do not want any data to get lost if the child process exits
unexpectedly -

- so I was hoping to turn off buffering so that there was at most 1
piece of data (i.e. one line) in the pipe.

I thought if I did:

select(PIPEHANDLE);
$| = 1;

then my pipe would not do buffering, and I was expecting that that
would lead to blocking writes - i.e. if the data hadn't been read from
the pipe, then a write to the pipe would block until it was.

however - after doing the above - I find I can still write data to the
pipe - regardless of how much is read out of the other end.

could someone enlighten me as to what's going on here please?

$| turns off perl's buffering of the data before it gets sent down the
pipe. The data is still buffered inside the pipe: AFAIK, there is no way
to prevent this.

Ben

 

[Ben Morrow]

Quoth (e-mail address removed) (Oliver):

$| turns off perl's buffering of the data before it gets sent down the
pipe. The data is still buffered inside the pipe: AFAIK, there is no way
to prevent this.

....however, having thought some more, you could use a socketpair instead
and set the socketopts SO_SNDBUF and SO_RCVBUF to 1 byte.

Ben

 

[ctcgag]

I'm using a pipe to talk to a forked child process -

I do not want any data to get lost if the child process exits
unexpectedly -

- so I was hoping to turn off buffering so that there was at most 1
piece of data (i.e. one line) in the pipe.

I thought if I did:

select(PIPEHANDLE);
$| = 1;

then my pipe would not do buffering, and I was expecting that that
would lead to blocking writes - i.e. if the data hadn't been read from
the pipe, then a write to the pipe would block until it was.

however - after doing the above - I find I can still write data to the
pipe - regardless of how much is read out of the other end.

You have prevented Perl from buffing up it's output before handing it to
the OS, but the OS still has it's own pipe buffer.

If you want the two processes to proceed in lock-stop, you could require
an acknowledgement from the child before the parent attempts the next
print.

Xho