Date: Wed, 12 Jul 2000 18:46:01 +0200 From: petervd@vuurwerk.nl To: FreeBSD-gnats-submit@freebsd.org Subject: gnu/19871: select bug on named pipes Message-ID: <963420361.560147.96633.nullmailer@daemon.vuurwerk.nl>
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>Number: 19871
>Category: gnu
>Synopsis: select on named pipes always returns 'available for reading'
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Wed Jul 12 09:50:00 PDT 2000
>Closed-Date:
>Last-Modified:
>Originator: Peter van Dijk
>Release: FreeBSD 4.0-STABLE i386
>Organization:
Vuurwerk Internet, Haarlem, The Netherlands
>Environment:
Standard FreeBSD 4.0-STABLE kernel/world (last cvsup+new world/kernel around
3 months ago).
>Description:
When select()'ing on a named pipe for reading, it will *always* return
immediately even if no data is waiting. *also* openening the pipe for writing
will fix this behaviour.
>How-To-Repeat:
The following snippet of code is stolen from the qmail-1.03 sources and
changed slightly to compile standalone:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <unistd.h>
#define FN "temp-trynpbg1.fifo"
void main()
{
int flagbug;
struct timeval instant;
fd_set rfds;
flagbug = 0;
if (mkfifo(FN,0600) != -1) {
close(0);
if (open(FN,O_RDONLY|O_NDELAY) == 0) {
FD_ZERO(&rfds);
FD_SET(0,&rfds);
instant.tv_sec = instant.tv_usec = 0;
if (select(1,&rfds,(fd_set *) 0,(fd_set *) 0,&instant) > 0)
flagbug = 1;
}
unlink(FN);
}
_exit(!flagbug);
}
Compile with cc -o namedpipebug namedpipebug.c
Run with './namedpipebug && echo buggy'
Will echo 'buggy' on FreeBSD, and nothing on, for example, Linux.
>Fix:
The workaround is to also open the socket for writing, as can be seen in this
piece of qmail-1.03 code (from trigger.c):
static int fd = -1;
#ifdef HASNAMEDPIPEBUG1
static int fdw = -1;
#endif
void trigger_set()
{
if (fd != -1)
close(fd);
#ifdef HASNAMEDPIPEBUG1
if (fdw != -1)
close(fdw);
#endif
fd = open_read("lock/trigger");
#ifdef HASNAMEDPIPEBUG1
fdw = open_write("lock/trigger");
#endif
}
>Release-Note:
>Audit-Trail:
>Unformatted:
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