On 3/16/21 11:51 AM, John Reiser wrote:
> On 3/16/21, David Howells wrote:
>> John Reiser <jrei...@bitwagon.com> wrote:
>>
>>> See the manual page "man 2 getdents".
>>
>> Um, which bit? I don't see anything obvious to that end.
>
> On that manual page:
> =====
> The system call getdents() reads several linux_dirent structures from the
> directory
> referred to by the open file descriptor fd into the buffer pointed to by dirp.
> [snip]]
> On success, the number of bytes read is returned.
But the original question was about the st_size returned stat, which is not
calling getdents.
Two different numbers, which mean 2 different things.
> =====
>
> So the return value is related to the size of the directory; the sum of the
> values
> returned before End-Of-File should be quite close to the .st_size of the
> directory.
Again, that's not at all correct. Counter-example on ext4:
# stat -c %s dir
2547712
# ls -a1 dir
.
..
file
# strace -v -egetdents ls dir
getdents(3, [{d_ino=524289, d_off=4294967296, d_reclen=24, d_name=".",
d_type=DT_DIR}, {d_ino=2, d_off=3358761300848251151, d_reclen=24, d_name="..",
d_type=DT_DIR}, {d_ino=534290, d_off=9223372036854775807, d_reclen=24,
d_name="file", d_type=DT_REG}], 32768) = 72
72 is not anywhere close to 2547712
> If a program is walking through the directory, reading all the entries via
> getdents64(),
> then .st_size of the directory is the only thing known in advance about the
> total size.
But it tells you nothing about how much is likely to be returned by getdents.
You should not use st_size to infer anything about the amount of data which
will be
returned by getdents. POSIX does not define the meaning of st_size for
directories,
and different filesystems can do wildly different things.
As Steve mentioned, st_blksize is your best hint for this purpose.
-Eric
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