On Tue, Dec 31, 2024 at 08:32:47PM +0000, Diego Nieto Cid wrote:
>
> Adding the following line to the program:
>
> printf("cmsghdr: %lu\n", sizeof (struct cmsghdr));
>
> shows 12 on Hurd and 16 on Linux. I think I'm closer :)
The only difference in the structure definition is the cmsg_len field type
which happens to be the 4 missing bytes.
--- >8 --- >8 ---
Linux
=====
/* Structure used for storage of ancillary data object information. */
struct cmsghdr
{
size_t cmsg_len; /* Length of data in cmsg_data plus length
of cmsghdr structure.
!! The type should be socklen_t but the
definition of the kernel is incompatible
with this. */
int cmsg_level; /* Originating protocol. */
int cmsg_type; /* Protocol specific type. */
#if __glibc_c99_flexarr_available
__extension__ unsigned char __cmsg_data __flexarr; /* Ancillary data.
*/
#endif
};
Hurd
====
/* Structure used for storage of ancillary data object information. */
struct cmsghdr
{
socklen_t cmsg_len; /* Length of data in cmsg_data plus length
of cmsghdr structure. */
int cmsg_level; /* Originating protocol. */
int cmsg_type; /* Protocol specific type. */
#if __glibc_c99_flexarr_available
__extension__ unsigned char __cmsg_data __flexarr; /* Ancillary data.
*/
#endif
};
--- >8 --- >8 ---
So, while CMSG_LEN aligns the cmsghdr structure, CMSG_DATA don't. I think
that's the problem.
