When initializing a new buffer (e.g., that will be sent with
sendmsg(2)), that buffer must first be zero-initialized to
ensure the correct operation of CMSG_NXTHDR().
Verified by experiment, and also by inspection of the glibc
source code:
_EXTERN_INLINE struct cmsghdr *
__NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
{
if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
/* The kernel header does this so there may be a reason. */
return (struct cmsghdr *) 0;
[1] __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
+ CMSG_ALIGN (__cmsg->cmsg_len));
if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
+ __mhdr->msg_controllen)
[2] || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len) // <---
> ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
/* No more entries. */
return (struct cmsghdr *) 0;
return __cmsg;
}
At point [1], __cmsg has been updated to point to the next
cmsghdr. The subsequent check at [2] relies on 'cmsg_len'
in the next cmsghdr having some "sensible" value (e.g., 0).
See also https://stackoverflow.com/questions/27601849/cmsg-nxthdr-returns-null-even-though-there-are-more-cmsghdr-objects
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Make it cleared that all of the library functions
described on this page will use the getcwd() system call
if it is present. (The text previously implied that only
the getcwd() library function made use of the system call,
but looking in the glibc source code shows that all of the
functions make use of a generic implementation (__getcwd())
that uses the system call if it is present.)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
The existing text on some of the oddities of the Linux getcwd()
implementation was placed somewhat obtrusively in the DESCRIPTION.
Shift the text to NOTES, and at the same time move the related
discussion of glibc nonconformance to POSIX into BUGS.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
The example code currently passes `buflen - 1` to `strncpy`,
however the length parameter to `strncpy` is `size_t`, which is
unsigned. This means that when `buflen` is zero, the cast of `-1`
to unsigned will result in passing `UINT_MAX` as the length.
Obviously, that would be incorrect and could cause `strncpy` to
write well beyond the buffer passed.
The easy solution is to wrap the whole code in the `buflen > 0`
check, rather then just the part of the code that applies the null
terminator.
Signed-off-by: Matthew Kilgore <mattkilgore12@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Since the BSD header has been imported to other C libraries (including
glibc), note that here so people know it isn't BSD-specific.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
* man3/termios.3 (.B TABDLY): Reference to the BUGS section.
(.SH BUGS): New section. Describe an issue on alpha where the XTABS
macro was defined to a value outside TABDLY mask.
Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
The distribution may choose not to support _XOPEN_CRYPT in the
case that the distribution has transitioned from glibc crypt to
libxcrypt.
Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
With glibc fix 52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94 for
CVE-2018-1000001 (Sourceware BZ #22679) the implementation in the
just released glibc 2.27 has been changed such that instead of
returning "(unreachable)" the implementation now returns ENOENT
as it would have if the current directory had been unlinked.
I see that in 2015 the quirk was documented in commit
a2ac97c78b, and this is no longer
true with glibc 2.27, but may continue to be true in other C libraries,
so I reference NOTES from the paragraph in the central text.
Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
I am trying to fix a FIXME in the pthread_create.3 manpage.
It says info about default thread stack size should be put in
pthread_attr_setstacksize.3.
And pthread_attr_setstacksize.3 says "For details on the default
stack size of new threads, see pthread_create(3)".
So I list the default values for several architectures, starting
from glibc 2.12 (and still valid on current git glibc).
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
The manpage claimed that tsearch() returns a pointer to a data
item. This is incorrect; tsearch() returns a pointer to the
corresponding tree node, which can also be interpreted as a
pointer to a pointer to the data item.
Since this API is quite unintuitive, also add a clarifying
sentence.
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
The period after the function call is incorrectly marked with bold.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Michael Kerrisk