Re: lib/55836: popen(3)'s type "r+" doesn't work correctly

[Michael Scholz <mike%fth-devel.net@localhost>]

The following reply was made to PR lib/55836; it has been noted by GNATS.

From: Michael Scholz <mike%fth-devel.net@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: lib-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost
Subject: Re: lib/55836: popen(3)'s type "r+" doesn't work correctly
Date: Wed, 2 Dec 2020 23:20:11 +0100 (CET)

 Hi, Wolfgang!
 
 On Wed, 2 Dec 2020, Wolfgang Solfrank wrote:
 
 > The following reply was made to PR lib/55836; it has been noted by GNATS.
 >
 > From: Wolfgang Solfrank <Wolfgang%Solfrank.net@localhost>
 > To: gnats-bugs%netbsd.org@localhost, lib-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
 > netbsd-bugs%netbsd.org@localhost
 > Cc:
 > Subject: Re: lib/55836: popen(3)'s type "r+" doesn't work correctly
 > Date: Wed, 2 Dec 2020 16:55:44 +0100
 >
 > Hi,
 >
 > > While I was playing with read/write pipes with popen(command, "r+") I
 > > realized that this doesn't work as expected.  The plus triggers creating
 > > the pipe object with socketpair(2) instead of pipe2(2) in
 > > src/lib/libc/gen/popen.c, but socketpair doesn't seem to work properly.
 >
 > This seems to be a problem with your expectations.  The "r+" tells
 > popen to open the pipe "for reading and writing" (see popen(3)).
 > I.e. with your test program, the forked "cat" gets its stdin *and*
 > stdout connected to the one end of the socketpair.  So the cat
 > doesn't read the output from the ls command, but sits there waiting
 > for your program to write something into fp.
 >
 > So this works exactly as designed.
 >
 > Ciao,
 > Wolfgang
 > --
 > Wolfgang%Solfrank.net@localhost				Wolfgang Solfrank
 
 Thank you for your input.  According to the man pages "the command's 
 standard input is the same as that of the process that called popen()".
 
 Okay, if you don't trust this, try that after the pipe is opened:
 
  	fputs("abcdefg\n", fp);
 
 fgets(buf, sizeof(buf), fp) returns this string but after that it hangs, 
 no NULL will be returned.
 
 Mike