套接字之recvfrom系统调用

recvfrom系统调用通过用户传入的接收空间构造msghdr，并且调用sock_recvmsg，该函数调用socket操作的recvmsg函数sock->ops->recvmsg，ipv4对应的是inet_recvmsg，该函数调用传输层的sk->sk_prot->recvmsg来接收数据，如tcp则调用tcp_recvmsg，接收完成之后记录地址结构长度信息；

/*
 *    Receive a frame from the socket and optionally record the address of the
 *    sender. We verify the buffers are writable and if needed move the
 *    sender address from kernel to user space.
 */

SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
        unsigned int, flags, struct sockaddr __user *, addr,
        int __user *, addr_len)
{
    struct socket *sock;
    struct iovec iov;
    struct msghdr msg;
    struct sockaddr_storage address;
    int err, err2;
    int fput_needed;

    /* 构造msghdr */
    err = import_single_range(READ, ubuf, size, &iov, &msg.msg_iter);
    if (unlikely(err))
        return err;

    /* 查找消息控制块 */
    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (!sock)
        goto out;

    msg.msg_control = NULL;
    msg.msg_controllen = 0;
    /* Save some cycles and don't copy the address if not needed */
    /* 地址结构信息 */
    msg.msg_name = addr ? (struct sockaddr *)&address : NULL;
    /* We assume all kernel code knows the size of sockaddr_storage */
    msg.msg_namelen = 0;
    msg.msg_iocb = NULL;
    msg.msg_flags = 0;

    /* 非阻塞标记 */
    if (sock->file->f_flags & O_NONBLOCK)
        flags |= MSG_DONTWAIT;

    /* 接收消息 */
    err = sock_recvmsg(sock, &msg, flags);

    /* 拷贝地址到用户空间 */
    if (err >= 0 && addr != NULL) {
        err2 = move_addr_to_user(&address,
                     msg.msg_namelen, addr, addr_len);
        if (err2 < 0)
            err = err2;
    }

    fput_light(sock->file, fput_needed);
out:
    return err;
}

 

tcp_rcvmsg的实现，请移步<TCP层recvmsg系统调用的实现分析>；