Date: Wed, 14 Oct 1998 07:25:14 -0700 From: Wayne Scott <wscott@ichips.intel.com> To: current@FreeBSD.ORG Subject: simple popen() bug and solution (PATCH) Message-ID: <199810141425.HAA26802@pdxcs565.pdx.intel.com>
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This is a restatement of PR misc/7810. Since that PR is making NO progress
I thought I would restate the problem in terms everyone can understand.
The Problem
-----------
The following program hangs on all current versions of FreeBSD, but
works on Linux, AIX, and HPUX.
#include <stdio.h>
#include <fcntl.h>
main()
{
FILE *file1;
FILE *file2;
file1 = popen("gzip > file1.gz", "w");
file2 = popen("gzip > file2.gz", "w");
fprintf(file1, "This is a test\n");
fprintf(file2, "This is also a test\n");
pclose(file1);
pclose(file2);
}
The Cause
---------
When popen exec's the second gzip process, that process inherits
the open filehandle 'file1'. When 'pclose(file1)' is executed the
first gzip does not exit because the second gzip is still holding
an open handle for its input.
The Workaround
--------------
If you reverse the order of the 'pclose()' calls then the program works
correctly, but this is not possible in the big program I am really trying
to get work.
The other Workaround
--------------------
If you set the 'close_on_exec' flag on the 'file1' then the second gzip
will not inheret open file handle.
ex. fcntl(fileno(file1), F_SETFD, 1)
This works, but is changes the semantics of popen().
What POSIX says
---------------
/* Posix.2: "popen() shall ensure that any streams from previous
popen() calls that remain open in the parent process are closed
in the new child process." */
The Solution
------------
popen() already maintains a list of filehandles that have be previously
returned. We just need to walk that list and close all the files.
No need to deallocate the list because we are just about the exec()
anyway.
Index: popen.c
===================================================================
RCS file: /home/ncvs/src/lib/libc/gen/popen.c,v
retrieving revision 1.9
diff -u -r1.9 popen.c
--- popen.c 1997/04/22 09:44:06 1.9
+++ popen.c 1998/09/02 18:47:49
@@ -62,6 +62,7 @@
struct pid *cur;
FILE *iop;
int pdes[2], pid, twoway;
+ struct pid *p;
/*
* Lite2 introduced two-way popen() pipes using socketpair().
@@ -115,6 +116,9 @@
(void)close(pdes[0]);
}
(void)close(pdes[1]);
+ }
+ for (p = pidlist; p; p = p->next) {
+ close(fileno(p->fp));
}
execl(_PATH_BSHELL, "sh", "-c", command, NULL);
I would love to have this fixed in the 3.0 release of FreeBSD. Note I sent the
PR with the patch in plenty of time...
Wayne Scott MD6 Architecture - Intel Corp.
wscott@ichips.intel.com Work #: (503) 613-5063
Disclaimer: All views expressed are my own opinions, and not necessarily
those of Intel Corporation.
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