cmsg_nxthdr(3) — Linux manual page


CMSG(3)                   Linux Programmer's Manual                  CMSG(3)

NAME         top


SYNOPSIS         top

       #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_t length);
       size_t CMSG_SPACE(size_t length);
       size_t CMSG_LEN(size_t length);
       unsigned char *CMSG_DATA(struct cmsghdr *cmsg);

DESCRIPTION         top

       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 calling
       sendmsg(2) and received by calling recvmsg(2).  See their manual
       pages for more information.

       Ancillary data is a sequence of cmsghdr 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; see socket(7).

       The cmsghdr 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 of cmsghdr structures should never be accessed directly.
       Instead, use only the following macros:

       *  CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the
          ancillary data buffer associated with the passed msghdr.  It
          returns NULL if there isn't enough space for a cmsghdr in the buf‐

       *  CMSG_NXTHDR() returns the next valid cmsghdr after the passed cms‐
          ghdr.  It returns NULL when there isn't enough space left in the

          When initializing a buffer that will contain a series of cmsghdr
          structures (e.g., to be sent with sendmsg(2)), that buffer should
          first be zero-initialized to ensure the correct operation of

       *  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

       *  CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
          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
          use memcpy(3) to copy data to or from a suitably declared object.

       *  CMSG_LEN() returns the value to store in the cmsg_len member of
          the cmsghdr structure, taking into account any necessary align‐
          ment.  It takes the data length as an argument.  This is a con‐
          stant expression.

       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).

CONFORMING TO         top

       This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-
       Lite, the IPv6 advanced API described in RFC 2292 and SUSv2.
       CMSG_FIRSTHDR(), CMSG_NXTHDR(), and CMSG_DATA() are specified in
       POSIX.1-2008.  CMSG_SPACE() and CMSG_LEN() will be included in the
       next POSIX release (Issue 8).

       CMSG_ALIGN() is a Linux extension.

NOTES         top

       For portability, ancillary data should be accessed using only the
       macros described here.  CMSG_ALIGN() is a Linux extension and should
       not be used in portable programs.

       In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_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.

EXAMPLES         top

       This code looks for the IP_TTL option 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(int));

           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 using SCM_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(int) * NUM_FD);
           memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));

SEE ALSO         top

       recvmsg(2), sendmsg(2)

       RFC 2292

COLOPHON         top

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Linux                            2020-06-09                          CMSG(3)