Michael, http://man7.org/linux/man-pages/man3/cmsg.3.html currently says this: To create ancillary data, first initialize the msg_controllen member of the msghdr with the length of the control message buffer. Use CMSG_FIRSTHDR() on the msghdr to get the first control message and CMSG_NXTHDR() to get all subsequent ones. In each control message, initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields, and the data portion using CMSG_DATA(). Finally, the msg_controllen field of the msghdr should be set to the sum of the CMSG_SPACE() of the length of all control messages in the buffer. For more information on the msghdr, see recvmsg(2). This does not mention that the memory that CMSG_NXTHDR() is used on to construct a cmsg data must be zero-initialized, but that's apparently the case as the macro verifies the cmsg_len field after increasing the pointers. The example at the end of the man page (which constructs a cmsg, but does not actually use CMSG_NXTHDR) does not initialize the memory to NUL, hence one would normally assume that NUL-initialization is not necessary when constructing cmsgs when reading the man page. We ran into this in systemd: https://github.com/systemd/systemd/pull/1540 https://github.com/systemd/systemd/issues/1505 I figure the man page should mention the requirement to NUL initialize. Also, the example should explicitly NUL-initialize the memory it uses. Even better of course would be an example that actually uses CMSG_NXTHDR(). Also, while we are at it, I figure the last sentence in the paragraph should probably mention sendmsg() rather than recvmsg(), as you construct cmsg yourself only for the fromer, not the latter... Thanks, and keep up the great work on the man pages! Lennart -- Lennart Poettering, Red Hat -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
