From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Sun,  2 Apr 2017 15:33:59 +0300
From: Antons Suspans <antox@ml.lv>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Message-ID: <20170402123359.GA18347@ax.s16>
References: <57E90700-85A0-47F4-9156-DE732859A74C@gmail.com>
	<CAJSxfmKMhyh=R-9FVCkzQ1V9yoGQK2H6B1TT_jTBb5T72zdtGg@mail.gmail.com>
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Subject: Re: [9fans] pipe: bug or feature?
Topicbox-Message-UUID: b914fdc0-ead9-11e9-9d60-3106f5b1d025

This was writing a \0 just past end of buf[], into memory alloted for pfd=
[0].
See the explanation by Alex in the 9front list:
/n/9front.org/lists/9front/1490668028.00

On Sun, Apr 02, 2017 at 07:34:52AM +0000, Skip Tavakkolian wrote:
> this is either a bug in devpipe or the documentation is misleading. the=
 man
> page seems to say that the pair of fd's returned by pipe(2) are symmetr=
ical,
> but it matters which fd of the pair is used for writing. =C2=A0if you s=
witch around
> pfd[0] and pfd[1], it works as you'd expect.
>=20
> On Fri, Mar 31, 2017 at 5:29 PM arisawa <karisawa@gmail.com> wrote:
>=20
>   Hello,
>=20
>   I was playing with an experimental code on pipe and met with a proble=
m which
>   I don=E2=80=99t understand.
>=20
>   the program reads a file and writes it to one end of pipe and then re=
ads it
>   from another end of pipe.
>   the buffer for writing pipe is named buf0, and for reading pipe is na=
med buf.
>   and I found the program does not finish unless sizeof(buf) > sizeof(b=
uf0).
>   is this a bug or feature of pipe?
>=20
>   Kenji Arisawa
>=20
>   =3D=3D=3D BEGIN a.c =3D=3D=3D
>   #include <u.h>
>   #include <libc.h>
>=20
>   char *argv0;
>=20
>   void
>   usage(void)
>   {
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 fprint(2,"usage: %s file\n",argv0);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 exits("usage");
>   }
>=20
>   void
>   main(int argc, char *argv[])
>   {
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 int fd,pfd[2];
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 char buf[256];
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 char buf0[256];
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 /* need to be sizeof(buf) > sizeof(buf0)
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0* but this condition is very curiou=
s to me */
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 int n;
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 char *file;
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 argv0 =3D argv[0];
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 argc--;argv++;
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 USED(argc);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 if(argv[0] =3D=3D nil)
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 usage();
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 file =3D argv[0];
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 fd =3D open(file,OREAD);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 if(fd < 0)
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 sysfatal("no =
such file");
>=20
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 if(pipe(pfd) < 0)
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 sysfatal("pip=
e error");
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 print("pfd: %d %d\n",pfd[0],pfd[1]);
>=20
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 while((n =3D read(fd,buf0,sizeof(buf0))) =
> 0){
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 print("read: =
%d %s\n",n,file);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 n =3D write(p=
fd[1],buf0,n);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 print("write:=
 %d\n",n);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 }
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 close(pfd[1]);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 while((n =3D read(pfd[0],buf,sizeof(buf))=
) > 0){
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 buf[n] =3D 0;
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 print("%d %s\=
n",n,buf);
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 }
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 print("%d\n",n);
>=20
>   =C2=A0 =C2=A0 =C2=A0 =C2=A0 exits(nil);
>   }
>   =3D=3D=3D END a.c =3D=3D=3D
>=20