Giovani, you will find the answer to your question in "tcp/ip illustrated, volume 2: the implementation" in chapter 2.
But to briefly answer your question, yes, there are 4 different types of mbufs, depending on the m_flags value. 1) m_flags = 0 and mbuf contains only data up to 108 bytes. 2) m_flags = M_PKTHDR to designate a packet header. 3)m_flags = M_EXT. In a situation where a user process write() in a buffer > 256 bytes, the system allocates a cluster to hold that data. 4) m_flags = M_EXT|M_PKTHDR and yes when using clusters, the memory in the mbuf is unsed. hop that helped. Jerry On Tuesday 23 September 2003 07:12 pm, Giovanni P. Tirloni wrote: > Hi, > > I have been reading the D&I chapter about IPC, specially the mbuf > section and *many* doubts were raised by my mind. I sending them here > in the hope that someone can clarify some bits for me so I can proceed. > > Reading sys/param.h and sys/mbuf.h I came to the conclusion that there > are four types of mbufs regarding it's allocation of memory for data. > Is the listing below correct? > > struct mbuf *m; > > 1. Normal mbuf using m->M_databuf > 2. Normal mbuf with external storage (cluster?) in m->m_hdr->mh_data > 3. Header mbuf using m->m_pktdat; > 4. Header mbuf with ext. storage (cluster?) in m->m_ext->ext_buf > > Other questions: > > 1. When using ext. storage is the space allocated by M_databuf wasted? > > 2. How the system decides 256 bytes for each mbuf isn't enough and it > needs a mbuf cluster? Isn't chaining useful there? > > 3. How does changing MSIZE affects the whole thing? > > 4. What about MCLBYTES? > > Sorry to make so many questions at once but I find it very interesting > and I'm really willing to learn how the building blocks of the network > stack work. Perhaps my questions are out of reality.. it's risk. > > Thanks, > > -- > Giovanni P. Tirloni <gpt at tirloni.org> > Fingerprint: 8C3F BEC5 79BD 3E9B EDB8 72F4 16E8 BA5E D031 5C26 _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
