Date: Tue, 27 Mar 2001 13:47:51 +0100 From: David Malone <dwmalone@maths.tcd.ie> To: freebsd-hackers@freebsd.org, freebsd-alpha@freebsd.org Cc: wpaul@freebsd.org, brian@freebsd.org Subject: Control messages. Message-ID: <200103271347.aa40230@salmon.maths.tcd.ie>
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I was looking at our implimention of passing descriptors and
credentials over unix domain sockets (I want to add the ability to
pass more than one message at a time). According to Steven's book
you should use the CMSG_DATA macro to find the data in associated
with a struct cmsghdr. We define this macro as:
#define CMSG_DATA(cmsg) ((u_char *)(cmsg) + \
_ALIGN(sizeof(struct cmsghdr)))
However, the kernel seems to get the address of the data by looking
at (void *)(&cmsg[1]). Some applications either find the data either
using the kernel's method or by using:
struct blah {
struct cmsghdr header;
sturct wibble data;
};
and taking the address of "data". Fortunately all these methods are
the same on the i386 'cos the sizeof(sizeof(struct cmsghdr)) is a
multiple of the alighment size. However on the alpha the alignment
requirement is 8 bytes and so:
CMSG_DATA(cmsg) != (&cmsg[1])
If you use the struct method then offset of "data" will line up
with one or the other of these depending on the alignment requirements
of data.
I'd like to change the code to always use the CMSG_DATA method,
which will change the ABI on the alpha. This shouldn't cause too
much disruption as descriptor and credential passing is rarely
used. (I think ppp and maybe some rpc stuff use it - not much
else does).
(As far as I can tell descriptor passing for a large number of
descriptors will panic an alpha running FreeBSD too. If anyone can
give access to an alpha that it's OK for me to crash I can test
this).
David.
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