GNU bug report logs - #22769
In-place sorting of a file

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Yanyan Jiang <jiangyy <at> outlook.com>
To: bug-coreutils <at> gnu.org
Subject: In-place sorting of a file
Date: Mon, 22 Feb 2016 04:57:54 -0500

[Message part 1 (text/plain, inline)]

Hi,

I found someone talking about sorting a file in-place:

http://stackoverflow.com/questions/9117274/do-i-need-to-generate-a-second-file-to-sort-a-file <http://stackoverflow.com/questions/9117274/do-i-need-to-generate-a-second-file-to-sort-a-file>

However, sort file.txt -o file.txt is NOT safe. A strace sequence (sorting 1 2 3 4 5):

read(3, "1\n2\n3\n4\n5\n\n", 4096)      = 11
read(3, "", 4096)                       = 0
_llseek(3, 0, [11], SEEK_CUR)           = 0
close(3)                                = 0
munmap(0xb7743000, 4096)                = 0
ftruncate64(1, 0)                       = 0             <----------------------  (*)
fstat64(1, {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7743000
write(1, "\n1\n2\n3\n4\n5\n", 11)       = 11
close(1)                                = 0

If at this time, the system crashes (or later at write), the data is gone forever (or have a suffering recovery). Should we warn users of this, or add special treatment with this case (write a temp file and rename it)?

Thank your for your attention.

Regards,
Yanyan Jiang 蒋炎岩
Institute of Computer Software,
Dept. of Computer Science, Nanjing University

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Message #10 received at 22769-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Yanyan Jiang <jiangyy <at> outlook.com>, 22769-done <at> debbugs.gnu.org
Subject: Re: bug#22769: In-place sorting of a file
Date: Mon, 22 Feb 2016 23:53:25 -0800

[Message part 1 (text/plain, inline)]

Yanyan Jiang wrote:
> If at this time, the system crashes (or later at write), the data is gone forever (or have a suffering recovery). Should we warn users of this, or add special treatment with this case (write a temp file and rename it)?

Thanks for reporting the issue. Writing to a temp file and renaming would have 
problems, as it might change the file's ownership or permissions or etc., and 
also won't behave as the user expects if the file has multiple hard links. We 
should at least warn users about the problem in the documentation, though, so I 
installed the attached patch on Savannah at:

http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=4c1995f1b673b0e1eae73a9db4e28016b86dee9e

[0001-doc-document-sort-o-F-F-danger.txt (text/plain, attachment)]

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