threads/79887: [patch] freopen() isn't thread-safe

[Vasil Dimov]

The following reply was made to PR threads/79887; it has been noted by GNATS.

From: Vasil Dimov <vd at FreeBSD.org>
To: bug-followup at FreeBSD.org, tejblum at yandex-team.ru
Cc:  
Subject: Re: threads/79887: [patch] freopen() isn't thread-safe
Date: Tue, 7 Dec 2010 11:52:00 +0200

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 Attached is a prog which demonstrates what happens when another thread
 calls open() when freopen() is executing. The output is:
 
 open(): /tmp/f1: 3
 trx1: freopen() begins, will associate fd=3 with /tmp/f2
 trx1: freopen(): open(): /tmp/f2: 4
 trx1: freopen(): close(): 3: 0
 trx2: open(): /tmp/f_other: 3
 trx1: freopen(): dup2(): 4,3: 3
 trx1: freopen(): end, now fd=3 is associated with /tmp/f2
 trx1: write to fd=3 (supposed to be /tmp/f2)
 trx2: write to fd=3 (supposed to be /tmp/f_other)
 
 At the end /tmp/f2 contains "ABCXYZ", /tmp/f_other is empty.
 
 -- 
 Vasil Dimov
 gro.DSBeerF at dv
 %
 The difference between life and the movies is that a script has to
 make sense, and life doesn't.
                 -- Joseph L. Mankiewicz
 
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 #include <sys/types.h>
 #include <sys/uio.h>
 
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
 
 int
 main(int argc, char **argv)
 {
 	int	fd1;
 	int	fd1_orig;
 	int	fd_other;
 	int	ret;
 
 	fd1 = open("/tmp/f1", O_RDWR | O_CREAT, 0644);
 	printf("open(): /tmp/f1: %d\n", fd1);
 
 	printf("trx1: freopen() begins, will associate fd=%d with /tmp/f2\n",
 	       fd1);
 
 	fd1_orig = fd1;
 
 	fd1 = open("/tmp/f2", O_RDWR | O_CREAT, 0644);
 	printf("trx1: freopen(): open(): /tmp/f2: %d\n", fd1);
 
 	ret = close(fd1_orig);
 	printf("trx1: freopen(): close(): %d: %d\n", fd1_orig, ret);
 
 	/* another thread executes in the meantime, opening some file */
 	fd_other = open("/tmp/f_other", O_RDWR | O_CREAT, 0644);
 	printf("trx2: open(): /tmp/f_other: %d\n", fd_other);
 	/* end of another thread, freopen() continues */
 
 	ret = dup2(fd1, fd1_orig);
 	printf("trx1: freopen(): dup2(): %d,%d: %d\n", fd1, fd1_orig, ret);
 	fd1 = fd1_orig;
 
 	printf("trx1: freopen(): end, now fd=%d is associated with /tmp/f2\n",
 	       fd1);
 
 	printf("trx1: write to fd=%d (supposed to be /tmp/f2)\n", fd1);
 	write(fd1, "ABC", 3);
 
 	printf("trx2: write to fd=%d (supposed to be /tmp/f_other)\n", fd_other);
 	write(fd_other, "XYZ", 3);
 
 	close(fd1);
 	close(fd_other);
 
 	return(0);
 }
 
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