GNU bug report logs - #36455
Question on reading from socket(pair) instead from pipe

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: "Dr. Werner Fink" <werner <at> suse.de>
Subject: bug#36455: closed (Re: bug#36455: Question on reading from
 socket(pair) instead from pipe)
Date: Thu, 04 Jul 2019 06:49:02 +0000

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Your bug report

#36455: Question on reading from socket(pair) instead from pipe

which was filed against the grep package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 36455 <at> debbugs.gnu.org.

-- 
36455: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=36455
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: "Dr. Werner Fink" <werner <at> suse.de>
Cc: 36455-done <at> debbugs.gnu.org
Subject: Re: bug#36455: Question on reading from socket(pair) instead from pipe
Date: Wed, 3 Jul 2019 23:47:50 -0700

Right, closing the bug report.

[Message part 3 (message/rfc822, inline)]

From: "Dr. Werner Fink" <werner <at> suse.de>
To: bug-grep <at> gnu.org
Subject: Question on reading from socket(pair) instead from pipe
Date: Mon, 1 Jul 2019 08:22:36 +0200

[Message part 4 (text/plain, inline)]

Hi,

with grep 3.3 I see for a FIFO this

  echo XXX | strace grep -E XXX
  [...]
  fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0xc), ...}) = 0
  stat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x3), ...}) = 0
  fstat(0, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
  lseek(0, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
  read(0, "XXX\n", 98304)                 = 4
  fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0xc), ...}) = 0
  write(1, "XXX\n", 4XXX)                    = 4
  read(0, "", 98304)                      = 0
  close(1)                                = 0
  close(2)                                = 0
  exit_group(0)                           = ?
  +++ exited with 0 +++

prefect but with libpipeline configured to use much faster socketpairs I see
this:

  strace -f ./mgrep -e XXX
  clone(strace: Process 2020 attached
  child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fb632fb4a10) = 2020
  [...]
  [pid  2020] execve("/usr/bin/grep", ["grep", "-e XXX"], 0x7ffd31f42ea8 /* 128 vars */ <unfinished ...>
  [...]
  [pid  2020] fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0xc), ...}) = 0
  [pid  2020] stat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x3), ...}) = 0
  [pid  2020] fstat(0, {st_mode=S_IFSOCK|0400, st_size=0, ...}) = 0
  [pid  2020] lseek(0, 0, SEEK_CUR)       = -1 ESPIPE (Illegal seek)
  [pid  2020] read(0, "XXX\n", 98304)     = 4
  [pid  2020] read(0, "", 98304)          = 0
  [pid  2020] close(1)                    = 0
  [pid  2020] close(2)                    = 0
  [pid  2020] exit_group(1)               = ?
  [pid  2020] +++ exited with 1 +++

IMHO reading data from a FIFO/pipe or from a FSOCK/socket(pair) should result
in the same result.  The mgrep is a simple test C code which uses libpipeline
to write "XXX\n" to a FILE stream used on the input fd of the sub process for
the grep command:

  FILE *xy;
  int fd = dup(1);
  pipeline *p = pipeline_new();
  pipeline_want_in(p, -1);
  pipeline_want_out(p, fd);
  pipecmd *grep = pipecmd_new("/usr/bin/grep");
  [... handle options and arguments ...]
  xy = pipeline_get_infile (p);
  fprintf(xy, "XXX\n");
  fflush(xy);
  fclose(xy);

Nevertheless in both cases stdout for grep is 136/12 aka /dev/pts/12

Werner

-- 
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr

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