An mbuf structure is a memory buffer for network data. Commonly, this data spans multiple mbuf structures, which are arranged into a linked list known as an mbuf chain.

struct mbuf is defined in the <sys/mbuf.h> header as follows:

struct mbuf {
      struct m_hdr m_hdr;
      union {
                struct {
                        struct pkthdr MH_pkthdr;
                        union {
                                struct m_ext MH_ext;
                                char MH_databuf[MHLEN];
                        } MH_dat;
                } MH;
                char M_databuf[MLEN];
        } M_dat;
};

Every mbuf structure contains a buffer for data and a header, which looks like this:

struct m_hdr { struct mbuf *mh_next; /* Next ...