[Caml-list] process conundrum

[Caml-list] process conundrum

[David Fox]

Suppose I open a process to reverse the lines of a file like so:

# let readable, writable = 
  Unix.open_process "cat -n | sort -n -r | sed 's:^[ 0-9]*[^ 0-9]::'";;

Then I write a few lines to it:

# output_string writable "a\n";;
# output_string writable "b\n";;
# output_string writable "c\n";;

Now I close the write end to cause an EOF, so the output arrives on readable:

# close_out writable;;
# input_line readable;;
- : string = "c"
# input_line readable;;
- : string = "b"
# input_line readable;;
- : string = "a"
# input_line readable;;
Exception: End_of_file.

How do I cleanly terminate the process?

# Unix.close_process (readable, writable);;
Exception: Sys_error "Bad file descriptor".
# 
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Re: [Caml-list] process conundrum

[Nicolas FRANCOIS]

On 13 Feb 2002 13:43:14 -0800
David Fox <david@lindows.com> wrote:

> Suppose I open a process to reverse the lines of a file like so:
> 
> # let readable, writable = 
>   Unix.open_process "cat -n | sort -n -r | sed 's:^[ 0-9]*[^ 0-9]::'";;
> 
> Then I write a few lines to it:
> 
> # output_string writable "a\n";;
> # output_string writable "b\n";;
> # output_string writable "c\n";;
> 
> Now I close the write end to cause an EOF, so the output arrives on
> readable:
> 
> # close_out writable;;
> # input_line readable;;
> - : string = "c"
> # input_line readable;;
> - : string = "b"
> # input_line readable;;
> - : string = "a"
> # input_line readable;;
> Exception: End_of_file.

This can be avoided with a

try ... with End_of_file -> ...
 
> How do I cleanly terminate the process?
> 
> # Unix.close_process (readable, writable);;
> Exception: Sys_error "Bad file descriptor".

I tried your example and only got a 

# Unix.close_process (readable, writable);;
- : Unix.process_status = WEXITED 0

So what's the problem ?

\bye

-- 

                   Nicolas FRANCOIS
            http://nicolas.francois.free.fr
 A TRUE Klingon programmer does NOT comment his code
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Re: [Caml-list] process conundrum

[Xavier Leroy]

> Suppose I open a process to reverse the lines of a file like so:
> 
> # let readable, writable = 
>   Unix.open_process "cat -n | sort -n -r | sed 's:^[ 0-9]*[^ 0-9]::'";;
> 
> Then I write a few lines to it:
> Now I close the write end to cause an EOF, so the output arrives on readable:
> How do I cleanly terminate the process?
> # Unix.close_process (readable, writable);;
> Exception: Sys_error "Bad file descriptor".

This is indeed a bug in Unix.close_process: when it closes both
channels, it should not fail if the output channel is already closed.

Notice however a potential problem in your code: Unix pipes are
fixed-size buffers, so if you write a lot of data to one pipe, the
output pipe may fill up and the underlying process will block.  Thus,
Unix.open_process is usable only if you can alternate writes (of small
quantities) and reads, e.g. via two threads.  In other cases, it is
necessary to use an intermediate file, to hold either the input or the
output of the command.

- Xavier Leroy
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