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Date: Thu, 19 Dec 2019 03:19:15 +0000
From: "wangjianjian (C)" <wangjianjian3@...wei.com>
To: Rich Felker <dalias@...c.org>
CC: "musl@...ts.openwall.com" <musl@...ts.openwall.com>, "Songyunlong (Euler)"
<yunlong.song@...wei.com>
Subject: 答复: 答复: [PATCH] ftello: Need adjust file offset before switching to write
Gotcha. Thanks for detailed explanation.
-----邮件原件-----
发件人: Rich Felker [mailto:dalias@...ifal.cx] 代表 Rich Felker
发送时间: 2019年12月19日 11:03
收件人: wangjianjian (C) <wangjianjian3@...wei.com>
抄送: musl@...ts.openwall.com; Songyunlong (Euler) <yunlong.song@...wei.com>
主题: Re: 答复: [musl] [PATCH] ftello: Need adjust file offset before switching to write
On Thu, Dec 19, 2019 at 02:47:46AM +0000, wangjianjian (C) wrote:
> I don't think this has something to do with ungetc or UB.
> If don't call ungetc and replace fputs with others, like fwrite or
> fputc, the result of ftell or final file content is not correct.
You cannot switch between reading and writing on a stream that's open for both without a successful seek in between, unless the read ended with hitting EOF. You're right that if you remove the ungetc, the UB is still there because fgetc is still there. Removing both, or putting a seek after them but before the fputs, will make things work as expected.
For details see 7.21.5.3 The fopen function, ¶7:
"When a file is opened with update mode ('+' as the second or
third character in the above list of mode argument values), both
input and output may be performed on the associated stream.
However, output shall not be directly followed by input without an
intervening call to the fflush function or to a file positioning
function (fseek, fsetpos, or rewind), and input shall not be
directly followed by output without an intervening call to a file
positioning function, unless the input operation encounters end-
of-file. Opening (or creating) a text file with update mode may
instead open (or create) a binary stream in some implementations."
http://port70.net/~nsz/c/c11/n1570.html#7.21.5.3
Rich
-----邮件原件-----
> 发件人: Rich Felker [mailto:dalias@...ifal.cx] 代表 Rich Felker
> 发送时间: 2019年12月18日 21:47
> 收件人: wangjianjian (C) <wangjianjian3@...wei.com>
> 抄送: musl@...ts.openwall.com; Songyunlong (Euler)
> <yunlong.song@...wei.com>
> 主题: Re: [musl] [PATCH] ftello: Need adjust file offset before
> switching to write
>
> On Wed, Dec 18, 2019 at 10:18:16AM +0000, wangjianjian (C) wrote:
> > Consider below code flow:
> >
> > FILE *fp = fopen("foobar", "w+");
> > fputs("hello", fp);
> > rewind(fp);
> > fgetc(fp);
> > ungetc('x', fp);
> > fputs("world", fp);
> ^^^^^^^^^^^^^^^^^^^
>
> This line produces UB. You can't perform a write after ungetc without an intervening successful seek (or hitting EOF).
>
> Rich
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