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Date: Wed, 18 Dec 2019 22:03:25 -0500
From: Rich Felker <>
To: "wangjianjian (C)" <>
Cc: "" <>,
	"Songyunlong (Euler)" <>
Subject: Re: 答复: [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

For details see 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."


> 发件人: Rich Felker [] 代表 Rich Felker
> 发送时间: 2019年12月18日 21:47
> 收件人: wangjianjian (C) <>
> 抄送:; Songyunlong (Euler) <>
> 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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