Re: [patch]spawn-pipe.c / spawni.c compile on OpenVMS.

[John E. Malmberg]

On 7/3/2017 7:58 PM, Bruno Haible wrote:
> John E. Malmberg wrote:
> > The OpenVMS Mailboxes work for data up to about 65K and then have buffer
> > full problems.
> > ...
> > For Bash and Coreutils, I have a close to a pipe replacement that uses a
> > slightly different API in that you have to know if the pipe is parent to
> > child or child to parent when you create the pipes.  This handles pipes
> > up to the amount of virtual memory available to the process.
>
> The point of pipes on Unix is that the buffer size is limited (to 4 KB
> usually, nowadays), and that the writer blocks when the reader is not
> reading fast enough. You have to
>    1. make the writer block when it recognizes that the buffer is nearly
>       full,
>    2. make the reader notify (wake up) the writer when it has read a significant
>       amount of data from the buffer. (How? You know the interprocess
>       communication facilities of OpenVMS better than me.)

The developers that mapped the pipe API to the Mailbox API used 
bi-directional channels instead of unidirectional channels.

Unidirectional mailbox channels were also a new feature of that version 
of OpenVMS.

A channel is the native OpenVMS equivalent to a file descriptor.

With unidirectional channels to the mailbox it is trivial for the parent 
to monitor the child's reading and writing or if there even is a child 
connected.

With bi-directional channels it is not possible at all.

Since only the side that uses my code to create the "pipe" uses the 
unidirectional channel, I have to do everything on that side.

Until I changed the OpenVMS GNV Bash port to use my replacement pipe 
code, I could not run most configure scripts as something would write 
>65 KB of data into a pipe that did not yet have anything reading from 
it, and then everything would stop.

So while the existing pipe stuff works in a lot of cases, some programs 
stress it more than others.

I am not expecting any significant changes in the VMS C library before 
OpenVMS 9.0 on x86_64 is released.

Regards,
-John