As tested on Linux:
- An SCM_RIGHTS ancillary message is "attached" to the range of data bytes sent in the same sendmsg() call.
- However, as always, recvmsg() calls on the receiving end don't necessarily map 1:1 to sendmsg() calls. Messages can be coalesced or split.
- The recvmsg() call that receives the first byte of the ancillary message's byte range also receives the ancillary message itself.
- To prevent multiple ancillary messages being delivered at once, the recvmsg() call that receives the ancillary data will be artifically limited to read no further than the last byte in the range, even if more data is available in the buffer after that byte, and even if that later data is not actually associated with any ancillary message.
- However, if the recvmsg() that received the first byte does not provide enough buffer space to read the whole message, the next recvmsg() will be allowed to read past the end of the mesage range and even into a new ancillary message's range, returning the ancillary data for the later message.
- Regular read()s will show the same pattern of potentially ending early even though they cannot receive ancillary messages at all. This can mess things up when using edge triggered I/O if you assumed that a short read() indicates no more data is available.
- A single SCM_RIGHTS message may contain up to SCM_MAX_FD (253) file descriptors.
- If the recvmsg() does not provide enough ancillary buffer space to fit the whole descriptor array, it will be truncated to fit, with the remaining descriptors being discarded and closed. You cannot split the list over multiple calls.
@kentonv Thanks for your response!
It's not the division I was worried about, rather having to subtract
CMSG_ALIGN (sizeof (struct cmsghdr))
which looked presumably non-portable for me, I mean, is it guaranteed that that's whatCMSG_LEN()
adds to the payload length?Your approach of writing
CMSG_LEN(0)
instead is definitely nicer, the nicest so far. You still rely on the (pretty reasonable) assumption that the function is linear with a slope ofsizeof(int)
, but don't make an assumption on the overhead's size. Nice!I really think there should be a macro doing this. I'm not sure if glibc / freebsd / etc. developers would be open to this idea; or if it should be brought up with POSIX / Austin Group or who else exactly.