On Friday 27 April 2018 10:58 AM, Thomas Stüfe wrote:
On Fri, Apr 27, 2018 at 5:57 AM, vyom tewari <vyom.tew...@oracle.com> wrote:
Hi Christoph,
On Tuesday 24 April 2018 04:45 PM, Langer, Christoph wrote:
Hi Vyom,
I think, it is intentional to handle case where return "hostname" is to
large to
fit in array. if you see the man page(http://man7.org/linux/man-
pages/man2/gethostname.2.html) it says that it is unspecified whether
returned buffer includes a terminating null byte.
current code will put null in case of large "hostname", What do you think ?
yes, I had read the man page and saw this point of the spec. But exactly for
this purpose there's this code:
// make sure string is null-terminated
hostname[NI_MAXHOST] = '\0';
If we only hand 'NI_MAXHOST' as size value into gethostname, then the
function might only write NI_MAXHOST - 1 characters of the hostname into the
buffer.
doc says it will copy len bytes into buffer and will not copy null character
into the buffer.
################################
C library/kernel differences
The GNU C library does not employ the gethostname() system call;
instead, it implements gethostname() as a library function that calls
uname(2) and copies up to len bytes from the returned nodename field
into name. Having performed the copy, the function then checks if
the length of the nodename was greater than or equal to len, and if
it is, then the function returns -1 with errno set to ENAMETOOLONG;
in this case, a terminating null byte is not included in the returned
name.
############################################################
This is shared code, so we should refer to Posix, not linux specific man pages.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/gethostname.html
<quote>
DESCRIPTION
The gethostname() function shall return the standard host name for the
current machine. The namelen argument shall specify the size of the
array pointed to by the name argument. The returned name shall be
null-terminated, except that if namelen is an insufficient length to
hold the host name, then the returned name shall be truncated and it
is unspecified whether the returned name is null-terminated.
Host names are limited to {HOST_NAME_MAX} bytes.
RETURN VALUE
Upon successful completion, 0 shall be returned; otherwise, -1 shall
be returned.
</quote>
Note that there is no indication what happens if the buffer is too
small. It may zero-terminate, it may not. It may return an error, it
may not. Decision is left to the platform implementors.
So from that, I would pass in a large-enough buffer and always
zero-terminate on success. According to Posix, a large-enough buffer
means HOST_NAME_MAX bytes.
I do not understand why we use NI_MAXHOST instead (and we we define it
to an arbitrary 1025 byte if undefined). Were there platforms where
HOST_NAME_MAX was too short? If yes, one should at least check that
NI_MAXHOST >= HOST_NAME_MAX.
Even i noticed this, why we use our own NI_MAXHOST instead HOST_NAME_MAX ?
Just for curiosity, are we facing any issues with the current code ?. Your
code change looks logical but if nothing is broken then why to change code.
If it can be proven by looking at the API description that it is
broken for some corner case, why keep it broken?
:) Agreed, as i said Christoph change is logically correct but i
don't know the history behind current code, so just wanted to be sure
that we are not missing any corner case.
Thanks,
Vyom
Thanks, Thomas
Thanks,
Vyom
Best regards
Christoph
