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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |VERSIONS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
CMSG(3) Library Functions ManualCMSG(3)CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary data
Standard C library (libc,-lc)
#include <sys/socket.h>struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh,struct cmsghdr *cmsg);size_t CMSG_ALIGN(size_tlength);size_t CMSG_SPACE(size_tlength);size_t CMSG_LEN(size_tlength);unsigned char *CMSG_DATA(struct cmsghdr *cmsg);
These macros are used to create and access control messages (also called ancillary data) that are not a part of the socket payload. This control information may include the interface the packet was received on, various rarely used header fields, an extended error description, a set of file descriptors, or UNIX credentials. For instance, control messages can be used to send additional header fields such as IP options. Ancillary data is sent by callingsendmsg(2) and received by callingrecvmsg(2). See their manual pages for more information. Ancillary data is a sequence ofcmsghdr structures with appended data. See the specific protocol man pages for the available control message types. The maximum ancillary buffer size allowed per socket can be set using/proc/sys/net/core/optmem_max; seesocket(7). Thecmsghdr structure is defined as follows: struct cmsghdr { size_t cmsg_len; /* Data byte count, including header (type is socklen_t in POSIX) */ int cmsg_level; /* Originating protocol */ int cmsg_type; /* Protocol-specific type */ /* followed by unsigned char cmsg_data[]; */ }; The sequence ofcmsghdr structures should never be accessed directly. Instead, use only the following macros:CMSG_FIRSTHDR() returns a pointer to the firstcmsghdr in the ancillary data buffer associated with the passedmsghdr. It returns NULL if there isn't enough space for acmsghdr in the buffer.CMSG_NXTHDR() returns the next validcmsghdr after the passedcmsghdr. It returns NULL when there isn't enough space left in the buffer. When initializing a buffer that will contain a series ofcmsghdr structures (e.g., to be sent withsendmsg(2)), that buffer should first be zero-initialized to ensure the correct operation ofCMSG_NXTHDR().CMSG_ALIGN(), given a length, returns it including the required alignment. This is a constant expression.CMSG_SPACE() returns the number of bytes an ancillary element with payload of the passed data length occupies. This is a constant expression.CMSG_DATA() returns a pointer to the data portion of acmsghdr. The pointer returned cannot be assumed to be suitably aligned for accessing arbitrary payload data types. Applications should not cast it to a pointer type matching the payload, but should instead usememcpy(3) to copy data to or from a suitably declared object.CMSG_LEN() returns the value to store in thecmsg_len member of thecmsghdr structure, taking into account any necessary alignment. It takes the data length as an argument. This is a constant expression. To create ancillary data, first initialize themsg_controllen member of themsghdr with the length of the control message buffer. UseCMSG_FIRSTHDR() on themsghdr to get the first control message andCMSG_NXTHDR() to get all subsequent ones. In each control message, initializecmsg_len (withCMSG_LEN()), the othercmsghdr header fields, and the data portion usingCMSG_DATA(). Finally, themsg_controllen field of themsghdr should be set to the sum of theCMSG_SPACE() of the length of all control messages in the buffer. For more information on themsghdr, seerecvmsg(2).For portability, ancillary data should be accessed using only the macros described here. In Linux,CMSG_LEN(),CMSG_DATA(), andCMSG_ALIGN() are constant expressions (assuming their argument is constant), meaning that these values can be used to declare the size of global variables. This may not be portable, however.
CMSG_FIRSTHDR()CMSG_NXTHDR()CMSG_DATA() POSIX.1-2008.CMSG_SPACE()CMSG_LEN()CMSG_ALIGN() Linux.
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD- Lite, the IPv6 advanced API described in RFC 2292 and SUSv2.CMSG_SPACE() andCMSG_LEN() will be included in the next POSIX release (Issue 8).
This code looks for theIP_TTLoption in a received ancillary buffer: struct msghdr msgh; struct cmsghdr *cmsg; int received_ttl; /* Receive auxiliary data in msgh */ for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL; cmsg = CMSG_NXTHDR(&msgh, cmsg)) { if (cmsg->cmsg_level == IPPROTO_IP && cmsg->cmsg_type == IP_TTL) { memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(received_ttl)); break; } } if (cmsg == NULL) { /* Error: IP_TTL not enabled or small buffer or I/O error */ } The code below passes an array of file descriptors over a UNIX domain socket usingSCM_RIGHTS: struct msghdr msg = { 0 }; struct cmsghdr *cmsg; int myfds[NUM_FD]; /* Contains the file descriptors to pass */ char iobuf[1]; struct iovec io = { .iov_base = iobuf, .iov_len = sizeof(iobuf) }; union { /* Ancillary data buffer, wrapped in a union in order to ensure it is suitably aligned */ char buf[CMSG_SPACE(sizeof(myfds))]; struct cmsghdr align; } u; msg.msg_iov = &io; msg.msg_iovlen = 1; msg.msg_control = u.buf; msg.msg_controllen = sizeof(u.buf); cmsg = CMSG_FIRSTHDR(&msg); cmsg->cmsg_level = SOL_SOCKET; cmsg->cmsg_type = SCM_RIGHTS; cmsg->cmsg_len = CMSG_LEN(sizeof(myfds)); memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds)); For a complete code example that shows passing of file descriptors over a UNIX domain socket, seeseccomp_unotify(2).recvmsg(2),sendmsg(2) RFC 2292
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17CMSG(3)Pages that refer to this page:memfd_create(2), recv(2), send(2), netlink(3), ipv6(7), netlink(7), packet(7), rtnetlink(7), sctp(7), socket(7), udp(7), unix(7), lslocks(8)
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