From: Rich Felker <dalias@libc.org>
To: "wangjianjian (C)" <wangjianjian3@huawei.com>
Cc: "musl@lists.openwall.com" <musl@lists.openwall.com>,
"Songyunlong (Euler)" <yunlong.song@huawei.com>
Subject: Re: 答复: [musl] [PATCH] ftello: Need adjust file offset before switching to write
Date: Wed, 18 Dec 2019 22:03:25 -0500 [thread overview]
Message-ID: <20191219030325.GR1666@brightrain.aerifal.cx> (raw)
In-Reply-To: <c468c360c46e4175a300d90277df1c37@huawei.com>
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@aerifal.cx] 代表 Rich Felker
> 发送时间: 2019年12月18日 21:47
> 收件人: wangjianjian (C) <wangjianjian3@huawei.com>
> 抄送: musl@lists.openwall.com; Songyunlong (Euler) <yunlong.song@huawei.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
next prev parent reply other threads:[~2019-12-19 3:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-18 10:18 wangjianjian (C)
2019-12-18 13:46 ` Rich Felker
2019-12-19 2:47 ` 答复: [musl] " wangjianjian (C)
2019-12-19 3:03 ` Rich Felker [this message]
2019-12-19 3:19 ` 答复: " wangjianjian (C)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191219030325.GR1666@brightrain.aerifal.cx \
--to=dalias@libc.org \
--cc=musl@lists.openwall.com \
--cc=wangjianjian3@huawei.com \
--cc=yunlong.song@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).