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From: Rich Felker <dalias@libc.org>
Newsgroups: gmane.linux.lib.musl.general
Subject: Re: [PATCH] ftello: Need adjust file offset before switching
 to write
Date: Wed, 18 Dec 2019 08:46:36 -0500
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To: "wangjianjian (C)" <wangjianjian3@huawei.com>
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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