Re: C-c in guile

[Alex Vong <alexvong1995@gmail.com>]

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Hello,


I manage to find a solution: define a new procedure 'system**' which is
implemented in C using fork-exec-waitpid. The code is in the attchment.


To compile the C extension, run:

  $ gcc -o libguile-system_star_star.so -shared -fPIC \
  `pkg-config --cflags guile-2.2` -O2 -Wall -Wextra system_star_star.c


To test the C extension, try running something like:

  (use-modules (ice-9 threads))
  (load-extension "./libguile-system_star_star" "init_system_star_star")

  (define n 10000)
  (define s (make-string n #\s))
  (define l (make-list n s))

  (define main
    (lambda _
      (parallel (system** "echo" "foo" "bar")
                (apply system** l)
                (system** "echo" "foo" "bar")
                (apply system** l))))


C-c should terminate the program.


My first solution was to use 'primitive-fork'. But it didn't work,
because guile warned me that 'primitive-fork' shouldn't be called in a
multithreaded program. I want to find out why. So I read the man-page
and it says after forking in a multithread program, it is only safe to
call async-safe functions. So I re-implement the whole thing in C and
exec right after forking. This is how I come up with the current
solution.



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/* Copyright (C) 2017 Alex Vong <alexvong1995@gmail.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 3 of
 * the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301 USA
 */

#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <libguile.h>

static SCM
system_star_star(SCM prog, SCM args)
{
  SCM prog_args = scm_cons(prog, args);
  size_t len = scm_to_size_t(scm_length(prog_args));
  char **argv = scm_gc_malloc((len + 1) * sizeof(char *), "argv");

  for (size_t k = 0; k < len; ++k)
    {
      SCM arg = SCM_CAR(prog_args);
      size_t len = scm_c_string_length(arg);

      argv[k] = scm_gc_malloc_pointerless(len + 1, "argv[k]");
      scm_to_locale_stringbuf(arg, argv[k], len);
      argv[k][len] = '\0';

      prog_args = SCM_CDR(prog_args);
    }
  argv[len] = NULL;

  int status = 0;
  pid_t pid = fork();
  if (pid == -1)
    scm_syserror("system**");

  if (pid == 0) /* child  */
    {
      execvp(argv[0], argv);
      _exit(127);
    }
  else /* parent  */
    {
      if (waitpid(pid, &status, 0) == -1)
        scm_syserror("system**");
    }

  return scm_from_int(status);
}

void
init_system_star_star(void)
{
  scm_c_define_gsubr("system**", 1, 0, 1, system_star_star);
}

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Cheers,
Alex


Alex Vong <alexvong1995@gmail.com> writes:

> Hello,
>
> When running the external program "yes" in shell,
>
>   $ yes
>
> We can terminate the process by pressing C-c.
>
> However, when running the external program "yes" in guile,
>
>   $ guile -c '(system* "yes")'
>
> We cannot terminate the process by pressing C-c,
> but we can suspend it by pressing C-z.
>
> Why is that? Is there any way I can terminate the process by pressing
> C-c?
>
> Thanks,
> Alex

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