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Date: Sun, 3 Jan 2010 17:58:46 +0100
From: Enrico Forestieri <forenr AT lyx DOT org>
To: cygwin AT cygwin DOT com
Subject: Re: select() and named pipes
Message-ID: <20100103165846.GA3284@GIOVE>
References: <20091222224309 DOT GA3504 AT GIOVE> <20091223003714 DOT GA4491 AT ednor DOT casa DOT cgf DOT cx> <20091223021219 DOT GA5836 AT GIOVE>
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On Wed, Dec 23, 2009 at 03:12:19AM +0100, Enrico Forestieri wrote:
> On Tue, Dec 22, 2009 at 07:37:14PM -0500, Christopher Faylor wrote:
> > On Tue, Dec 22, 2009 at 11:43:09PM +0100, Enrico Forestieri wrote:
> > >I am experiencing a problem with select() and named pipes in cygwin
> > >1.7.  The attached test case segfaults on the select() call, but works
> > >fine with both cygwin 1.5 and linux.
> > 
> > Thanks for the test case.  This should be fixed in the next Cygwin
> > snapshot but it will not be fixed (of course?) in 1.7.1 which is now
> > available from the Cygwin web site.
> 
> I confirm that with the 20091222 snapshot the crash does not occur
> anymore. However, there's still something wrong. Indeed, running the
> test case in a terminal and issuing "echo foo > /tmp/pipe" from another
> one, the program exits without printing the expected "foo".

Apparently, the problem is due to the O_NONBLOCK flag used in the open()
call. Taking away O_NONBLOCK, everything works.

This bug is causing an infinite loop in an application. The loop can
be reproduced by the slightly modified test case attached here.

When O_NONBLOCK is used, after issuing "echo foo > /tmp/pipe", read()
always returns -1 and sets errno to EAGAIN. However, according to
http://www.opengroup.org/onlinepubs/009695399/functions/read.html
this should only occur when reading from an empty pipe or fifo
and some process has the pipe open for writing and O_NONBLOCK is set.
Here, the fifo is not empty (as reported by select), and thus that
should not happen.

-- 
Enrico

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#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define FIFONAME "/tmp/pipe"

int main(void)
{
    int fd;
    int nsel;
    fd_set readfds;
    FD_ZERO(&readfds);

    if (mkfifo(FIFONAME, 0600) < 0) {
	perror("mkfifo");
	exit(1);
    }

    fd = open(FIFONAME, O_RDONLY | O_NONBLOCK);

    if (fd < 0) {
	perror("open");
	remove(FIFONAME);
	exit(2);
    }

    FD_SET(fd, &readfds);
    do {
	nsel = select(fd + 1, &readfds, 0, 0, 0);
    } while (nsel == -1 && (errno == EINTR || errno == EAGAIN));

    if (nsel == -1) {
	perror("select");
	exit(3);
    }

    if (FD_ISSET(fd, &readfds)) {
	char buf[100];
	int status;
	int count = 0;
	printf("%d: ", ++count);
	while ((status = read(fd, buf, sizeof(buf) - 1))) {
	    if (status > 0) {
		buf[status] = '\0';
		printf("%s", buf);
	    } else if (errno == EAGAIN) {
		    printf("\n%d: ", ++count);
	    } else {
		perror("read");
		break;
	    }
	}
    }

    close(fd);
    remove(FIFONAME);
    return 0;
}


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