[plt-scheme] the make-pipe

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Thu Dec 6 08:39:07 EST 2007

At Thu, 6 Dec 2007 18:20:07 +0800, yug at ios.ac.cn wrote:
> (module test-mem mzscheme
>   (require (lib "process.ss"))
>   (define null-port (open-output-file "/dev/null" 'append))
>   (define null-iport (open-input-file "/dev/null"))
>   (define std-oport (current-output-port))
>   (define (x) (let-values ([(r w) (make-pipe)])
>                 (process/ports w null-iport null-port "/sbin/sysctl -a")
>                 (process/ports std-oport r  null-port "grep
> vm.min_free_kbytes")
>               ))
>   (define (y) (process/ports std-oport null-iport null-port "/sbin/sysctl
> -a| grep vm.min_free.kbytes"))
>   (define (z)
>     (let-values ([(r w) (make-pipe)])
>       (process/ports w null-iport null-port "/sbin/sysctl -a")
>       (let loop ()
>         (let ([v (read-line r)])
>           (if (eof-object? r)
>               (void)
>             (begin
>               (fprintf std-oport "~a~n" v)
>               (loop))
>             )))))
>   )
> ------------------------------------
> Welcome to DrScheme, version 371.3-svn7nov2007 [3m].
> Language: (module ...).
> > (x)
> (#f #f 11744 #f #<procedure:control>)
> > (y)
> (#f #f 11745 #f #<procedure:control>)
> vm.min_free_kbytes = 5858
> 1.why (x) and (y) is different?

I'm not sure. On my machine, they work the same when I run in plain
MzScheme, but differently in DrScheme. I'll have to investigate more.

> 2. why (z) could not terminate?

Nothing closes the write side of the pipe.

In both `x' and `z', I think you want to use a returned port from
`process/ports' instead of supplying the write end of a pipe (which
creates extra layer of indirection):

  (define (x) 
    ;; this is what `/sbin/sysctl -a | grep ...' does in the shell:
    (let ([p (process/ports #f null-iport null-port "/sbin/sysctl -a")])
      (process/ports std-oport (car p) null-port "grep vm.min_free_kbytes")
      (close-input-port (car p))))

  (define (z)
    (let* ([p (process/ports #f null-iport null-port "/sbin/sysctl -a")]
           [r (car p)])
      (let loop ()
        (let ([v (read-line r)])
          (if (eof-object? v)
                (fprintf std-oport "~a~n" v)
      (close-input-port r)))

When the process ends, the OS closes the write end of the process's
stdout is closed, so the read end see an EOF.


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