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Date: Thu, 19 Dec 2019 02:47:46 +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: [PATCH] ftello: Need adjust file offset before switching to write
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.
-----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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