pipe() function in syscalls.cc

[Steve Lipa]

"John W. Eaton" <address@hidden> wrote:

>Why do you need the number?  It's probably best to just pretend that
>the octave_file object is special and just pass it around to functions
>that expect to handle file objects (fprintf, fscanf, fread, etc.).
>
>But, if you really need it, if a file object is used in numeric
>contexts, it is automatically converted to a number, so something like
>
>  nth (x, 1) + 0
>
>should do what you want (but I would still like to know why you need
>to have the number -- maybe it would be useful to have a fileno()
>function).


Dr. Eaton:

Thank you very much for responding to my question.   I want to pipe data
from Octave to a program that allows two forms of interprocess communication:
either a named pipe or an anonymous pipe.   I have been using the named
pipe approach, but I consider it kludgey because I have to have a cleanup
function that deletes the named pipe from the filesystem.

To use the anonymous pipe approach, I need to pass the actual system file
descriptor as a command line argument to the program.  This number happened
to be the same as the file number for simple cases, and everything worked
at first, so I mistakenly thought they were the same or at least related.

It turns out that fid[0] in syscalls.cc consistently contains the correct
descriptor.   My problem was that I was telling the other process to read
from fid[0] and telling Octave to write to fid[1] instead of telling it to 
write to the file associated with fid[1].    I simply didn't think hard 
enough about the error message which was telling me exactly what I was 
doing wrong.

So the end result is that I *do* need to use gpipe.oct because the pipe()
function does not return fid[0].  I don't know how common it is to need the 
file descriptor associated with a stream.  I suspect that this application 
is rare enough that you don't need to bother with fileno() or filedes() 
functions.  

Thank you again for your help!

Steve

-- 

Steve Lipa
address@hidden
gpg fingerprint = 8B68 77D7 9E09 9991 C97E  25FF 6A12 D2B9 EC7D 66C1