BSD TCP/IP for Kyu - write and sendmsg

Interestingly there are several ways to write to a socket on a BSD system and it is important for me to understand those to be able to adapt the code to Kyu.

It turns out that write() and send() are the same, except that send() gives the ability to add flags (which are all zero for write(). The only one that I can imagine using is the one to send OOB (out of bounds) messages.

There is also sendto(), which is identical to send() if you have a connection. It allows an address to be specified (which might be useful for UDP, but seems of no use for TCP).

Finally there is sendmsg(). Here instead of giving "buf" and "len" you pass a msghdr structure which looks like this:

      struct msghdr {
               void         *msg_name;       /* Optional address */
               socklen_t     msg_namelen;    /* Size of address */
               struct iovec *msg_iov;        /* Scatter/gather array */
               size_t        msg_iovlen;     /* # elements in msg_iov */
               void         *msg_control;    /* Ancillary data, see below */
               size_t        msg_controllen; /* Ancillary data buffer len */
               int           msg_flags;      /* Flags (ignored) */
           };

In the code (kern/uipc_syscalls.c) this gets packaged up in a "uio" structure like this:

struct iovec {
        char    *iov_base;      /* Base address. */
        size_t   iov_len;       /* Length. */
};

struct uio {
        struct  iovec *uio_iov;
        int     uio_iovcnt;
        off_t   uio_offset;
        int     uio_resid;
        enum    uio_seg uio_segflg;
        enum    uio_rw uio_rw;
};

	auio.uio_iov = mp->msg_iov;
        auio.uio_iovcnt = mp->msg_iovlen;
        auio.uio_segflg = UIO_USERSPACE;
        auio.uio_rw = UIO_WRITE;
        auio.uio_offset = 0;
        auio.uio_resid = 0;

sosend ( so, addr, uio, top, control, flags)
sosend ( so, 0, uio, 0, 0, 0 )

error = uiomove(mtod(m, caddr_t), (int)len, uio);