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Date: Fri, 08 Jan 2021 11:09:26 +0100
From: Florian Weimer <>
To: enh <>
Subject: Re: Future directions for *_r functions

> iirc basename(3) was the place where we caused problems in
> practice. basically with the GNU basename(3) [but not the POSIX one!]
> it's a valid assumption that you can call basename(3) on two different
> strings and compare the results, say, whereas on Android the second
> call will have overwritten the first result, and both calls will have
> returned the same pointer to TLS. (see
> for example.)


| bionic's "POSIX" basename had the wrong CV-qualifiers since at least
| cupcake (there was no NDK before that, so it didn't matter that it was
| wrong until then), so even today it never modifies its arguments and
| uses a thread-local buffer for backwards compatibility (with Android;
| obviously this is an incompatibility with glibc).

I think neither the POSIX nor GNU definition *require* a static buffer,
although POSIX permits it, and even deals with the TLS case (“The
returned pointer might also be invalidated if the calling thread is
terminated.”, first time I see language like that in POSIX.)

> readdir_r(3) has long been my favorite example of the "why _r is
> terrible" side of the argument. almost every use of readdir_r(3) i've
> ever seen has been wrong, not least in Android, internal Google code,
> and all the third-party open source code they pull in. (i assume
> everyone on this list is familiar with the horror that is
> readdir_r(3), but if not, see
> for
> more.)

The readdir_r deprecation has been rather painful, with lots of
developers working around -Werror failures rather than switching to

In glibc, we patched readdir_r to check agains NAME_MAX, and return
ENAMETOOLONG at the end of the directory stream if it encounters an
overly long name.

I think POSIX has agreed to remove the interface in a future version.

I believe this is all the result of an old implementation of fdopendir

DIR *fdopendir(fd)
  return fd;

For that to work, readdir has to use a static buffer.

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