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From: "Ardhan Madras" <ajhwb@knac.com>
To: linux-c-programming@vger.kernel.org
Subject: Re: Help on pipe
Date: Fri, 10 Jul 2009 11:21:22 -0700	[thread overview]
Message-ID: <20090710112122.60FA2DF8@resin13.mta.everyone.net> (raw)

My demo can run. i use -Wall flag, the "const char *argv[2] = {
"sort", NULL }" as you said warned with:

pipe.c:35: warning: passing argument 2 of 'execve' from incompatible
pointer type.

but it's perfect for  "char * const argv[2] = { "sort", NULL }"

and i define "int ret" for both process, but i undestand forking
mechanism on GNU/Linux.

sorry i don't write the code cleanly.. My point is only to make the
"sort" working, im now understand that "sort" will always wait for
input until it's end (peer closed the input).

Thank you very much.



2009/7/10 Michał Nazarewicz <m.nazarewicz@samsung.com>:
> On Fri, 10 Jul 2009 08:22:54 +0200, Ardhan Madras <nightdecoder@gmail.com>
> wrote:
>
>> #include <sys/types.h>
>> #include <sys/wait.h>
>> #include <unistd.h>
>> #include <stdio.h>
>> #include <string.h>
>>
>> int main(void)
>> {
>>  pid_t pid;
>>  int fds[2], ret;
>>  const char *sort = "/usr/bin/sort";
>
> "static const char sort[]" would be better IMO.
>
>>  char *argv[2] = { "sort", NULL };
>
> "static const char *argv[]" is definitly better here and does not produce
> warnings.  Your code discards "const" from pointer (a string literal).
>
>>  ret = pipe(fds);
>>  if (ret == -1) {
>>    perror("pipe");
>>    return -1;
>>  }
>>  pid = fork();
>>  if (pid == -1) {
>>    perror("fork");
>>    return -1;
>>  }
>
> Personally I preffer something like:
>
>  switch (fork()) {
>  case -1: /* error */  break;
>  case  0: /* child */  break;
>  default: /* parent */
>  }
>
> but suit yourself.
>
>>  if (pid == 0) {
>>    int ret;
>
> No need to define this variable here.  You already have such variable.
>
>>
>>    close(fds[1]);
>>    dup2(fds[0], STDIN_FILENO);
>>
>>    ret = execve(sort, argv, NULL);
>>    if (ret == -1) {
>>      perror("execve");
>>    }
>>    close(fds[0]);
>>    _exit(0);
>
> No need to close(), and _exit(0) should be _exit(1) (or -1 if you will)
> since the chiled finished with error.
>
>>  }
>>  else {
>>    FILE *stream;
>>
>>    close(fds[0]);
>>    stream = fdopen(fds[1], "w");
>>    if (!stream) {
>>      perror("fdopen");
>>      return -1;
>>    }
>>    fprintf(stream, "this\n");
>>    fprintf(stream, "means\n");
>>    fprintf(stream, "war\n");
>>    fflush(stream);
>
> And here, replace fflush() with fclose().  It seems, data is buffered on
> kernel level, not only libc level and fflush() cannot handle that.
>
>>    waitpid(pid, &ret, 0);
>>    close(fds[1]);
>
> No need to close.
>
>>  }
>>  return 0;
>> }
>
> --
> Best regards,                                            _     _
>  .o. | Liege of Serenly Enlightened Majesty of         o' \,=./ `o
>  ..o | Computer Science,  Michał "mina86" Nazarewicz      (o o)
>  ooo +-<m.nazarewicz@samsung.com>-<mina86@jabber.org>-ooO--(_)--Ooo--
>
>
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next             reply	other threads:[~2009-07-10 18:21 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-07-10 18:21 Ardhan Madras [this message]
  -- strict thread matches above, loose matches on Subject: below --
2009-07-10  6:22 Help on pipe Ardhan Madras
2009-07-10  7:06 ` Michał Nazarewicz
2009-07-10  7:34   ` Ardhan Madras
2009-07-10 17:40   ` Glynn Clements
2009-07-11  3:41     ` David Lee
2009-07-13  6:45     ` Michał Nazarewicz
2009-07-10  7:27 ` David Lee

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