Re: [PATCH] stdio-common: Add the fgetln function

[Florian Weimer <fweimer@redhat.com>]

* Andreas Schwab:

> On Mai 03 2022, Florian Weimer via Libc-alpha wrote:
>
>> +/* Reads the next line from STREAM and returns a pointer to the start of
>       Read                                return
>
>> +   an array containing it.  The array is not null-terminated.  The
>
> That makes it a rather clunky interface.  Why do we need that, given
> getline et al?

Unfortunately, some software uses this function.  It cannot be reliably
implemented outside of libc, and there are some really problematic
portability wrappers out there.

For example, the libbsd implementation is not thread-safe, and has a
use-after-free condition if more than 32 streams are used:

| static struct filebuf fb_pool[FILEBUF_POOL_ITEMS];
| static int fb_pool_cur;
[…]
|
|	/* Try to diminish the possibility of several fgetln() calls being
|	 * used on different streams, by using a pool of buffers per file. */
|	fb = &fb_pool[fb_pool_cur];
|	if (fb->fp != stream && fb->fp != NULL) {
|		fb_pool_cur++;
|		fb_pool_cur %= FILEBUF_POOL_ITEMS;
|		fb = &fb_pool[fb_pool_cur];
|	}
|	fb->fp = stream;

<https://github.com/freedesktop/libbsd/blob/master/src/fgetln.c>

Thanks,
Florian