Date: Wed, 21 Apr 2021 21:19:46 +1000 From: andrew clarke <mail@ozzmosis.com> To: Frank Leonhardt <freebsd-doc@fjl.co.uk> Cc: freebsd-questions@freebsd.org Subject: Re: Bug in current versions of /usr/include/dirent.h ? Message-ID: <20210421111946.w2qsqvom5tse4w6m@ozzmosis.com> In-Reply-To: <fa2c4f47-f8e6-7553-6fff-2454f3cac2d8@fjl.co.uk> References: <fa2c4f47-f8e6-7553-6fff-2454f3cac2d8@fjl.co.uk>
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On 2021-04-20 19:21:45, Frank Leonhardt (freebsd-doc@fjl.co.uk) wrote:
> Weird one this! I'm currently using 12.2, but this appears to be a problem
> in recent versions.
>
> In FreeBSD 8, dirent.h contains the following:
>
> /* structure describing an open directory. */
> typedef struct _dirdesc {
> int dd_fd; /* file descriptor associated with directory
> */
> long dd_loc; /* offset in current buffer */
> long dd_size; /* amount of data returned by getdirentries
> */
> char *dd_buf; /* data buffer */
> int dd_len; /* size of data buffer */
> long dd_seek; /* magic cookie returned by getdirentries */
> long dd_rewind; /* magic cookie for rewinding */
> int dd_flags; /* flags for readdir */
> struct pthread_mutex *dd_lock; /* lock */
> struct _telldir *dd_td; /* telldir position recording */
> } DIR;
>
> Nothing wrong there. It's the structure used by opendir() etc in the
> standard C library.
FWIW opendir() is a POSIX extension, not standard C.
> In 12.2 we've not got a structure definition, but instead a forward
> reference at line 87. DIR is typedefed on the following line. However,
> nowhere can I find where this forward reference is later resolved - meaning
> it isn't.
>
> Has anyone got the faintest idea what's going on? I've had a look through
> the source code to see where a DIR structure is used, and it may as well be
> a (void *) - it's used as a handle returned by opendir() in subsequent
> operations but never dereferenced.
POSIX doesn't guarantee DIR is a struct, just a "directory stream" type.
I suspect the FreeBSD folks have deprecated dereferencing DIR, perhaps in an effort to discourage people writing non-portable code.
See this commit from 2012:
"Hide DIR definition by making it an opaque struct typedef."
https://github.com/freebsd/freebsd-src/commit/0bb2aabf26842b91fbf14efa8e4e2030f0f4d2e4#diff-e2affeccb18763e4ad00c347282781dda3f60646e1e3f6c8fd534932fdc0ac8d
> And before anyone questions why de-referencing it is necessary - first off,
> it's always been a published structure so it's fair game. Secondly, full
> access to the dd_fd field was handy. If access to this structure is
> "deprecated" after 40 years, at the very least the handle should have been
> typedefed into something safer than a forward reference.
There is the dirfd() function if you really need access to the file descriptor.
DIR *dir;
dir = opendir(".");
if (dir)
{
int fd = dirfd(dir);
closedir(dir);
}
Andrew
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