sendmsg系统调用，主要工作是将用户空间的消息头复制到内核空间中，对消息头进行检查。最后逐级调用发包接口发送数据。

https://www.cnblogs.com/wanpengcoder/p/11749313.html

send、sendto、sendmsg区别：

https://www.cnblogs.com/hnrainll/archive/2011/08/16/2141483.html

send只可用于基于连接的套接字，send 和 write唯一的不同点是标志的存在，当标志为0时，send等同于write。sendto 和 sendmsg既可用于无连接的套接字，也可用于基于连接的套接字。

SYSCALL_DEFINE3(sendmsg, int, fd, struct user_msghdr __user *, msg, unsigned int, flags)

E:\linux-4.1.45\linux-4.1.45
et\socket.c
SYSCALL_DEFINE3(sendmsg, int, fd, struct user_msghdr __user *, msg, unsigned int, flags)
{
	if (flags & MSG_CMSG_COMPAT)
		return -EINVAL;
	return __sys_sendmsg(fd, msg, flags);
}

__sys_sendmsg()

/*
 *	BSD sendmsg interface
 */

long __sys_sendmsg(int fd, struct user_msghdr __user *msg, unsigned flags)
{
	int fput_needed, err;
	struct msghdr msg_sys;
	struct socket *sock;

	sock = sockfd_lookup_light(fd, &err, &fput_needed);//根据fd找到对应socket
	if (!sock)
		goto out;

        //发送数据
	err = ___sys_sendmsg(sock, msg, &msg_sys, flags, NULL);

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

___sys_sendmsg()

static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
			 struct msghdr *msg_sys, unsigned int flags,
			 struct used_address *used_address)
{
	struct compat_msghdr __user *msg_compat =
	    (struct compat_msghdr __user *)msg;
	struct sockaddr_storage address;
	struct iovec iovstack[UIO_FASTIOV], *iov = iovstack;
	unsigned char ctl[sizeof(struct cmsghdr) + 20]
	    __attribute__ ((aligned(sizeof(__kernel_size_t))));
	/* 20 is size of ipv6_pktinfo */
	unsigned char *ctl_buf = ctl;
	int ctl_len;
	ssize_t err;

	msg_sys->msg_name = &address;

	if (MSG_CMSG_COMPAT & flags)
                 /* 从64位消息头拷贝数据到32位消息头 */
		err = get_compat_msghdr(msg_sys, msg_compat, NULL, &iov);
	else
		err = copy_msghdr_from_user(msg_sys, msg, NULL, &iov);
	if (err < 0)
		return err;

	err = -ENOBUFS;

        /* 控制信息长度错误 */
	if (msg_sys->msg_controllen > INT_MAX)
		goto out_freeiov;

        /* 拷贝控制信息 */
	ctl_len = msg_sys->msg_controllen;
	if ((MSG_CMSG_COMPAT & flags) && ctl_len) {
		err =
		    cmsghdr_from_user_compat_to_kern(msg_sys, sock->sk, ctl,
						     sizeof(ctl));
		if (err)
			goto out_freeiov;
		ctl_buf = msg_sys->msg_control;
		ctl_len = msg_sys->msg_controllen;
	} else if (ctl_len) {
		if (ctl_len > sizeof(ctl)) {
			ctl_buf = sock_kmalloc(sock->sk, ctl_len, GFP_KERNEL);
			if (ctl_buf == NULL)
				goto out_freeiov;
		}
		err = -EFAULT;
		/*
		 * Careful! Before this, msg_sys->msg_control contains a user pointer.
		 * Afterwards, it will be a kernel pointer. Thus the compiler-assisted
		 * checking falls down on this.
		 */
		if (copy_from_user(ctl_buf,
				   (void __user __force *)msg_sys->msg_control,
				   ctl_len))
			goto out_freectl;
		msg_sys->msg_control = ctl_buf;
	}

         /* 设置发送标记 */
	msg_sys->msg_flags = flags;

        /* 设置非阻塞标记 */
	if (sock->file->f_flags & O_NONBLOCK)
		msg_sys->msg_flags |= MSG_DONTWAIT;
	
        /* 如果这次发送的地址跟上次成功发送的一致 */
        /*
	 * If this is sendmmsg() and current destination address is same as
	 * previously succeeded address, omit asking LSM's decision.
	 * used_address->name_len is initialized to UINT_MAX so that the first
	 * destination address never matches.
	 */
	if (used_address && msg_sys->msg_name &&
	    used_address->name_len == msg_sys->msg_namelen &&
	    !memcmp(&used_address->name, msg_sys->msg_name,
		    used_address->name_len)) {
                /* 无需进行检查，直接发送 */
		err = sock_sendmsg_nosec(sock, msg_sys);
		goto out_freectl;
	}
        /* 进行安全模块检查后发送 */
	err = sock_sendmsg(sock, msg_sys);

        /* 发送成功需要更新成功地址记录 */
	/*
	 * If this is sendmmsg() and sending to current destination address was
	 * successful, remember it.
	 */
	if (used_address && err >= 0) {
		used_address->name_len = msg_sys->msg_namelen;
		if (msg_sys->msg_name)
			memcpy(&used_address->name, msg_sys->msg_name,
			       used_address->name_len);
	}

out_freectl:
	if (ctl_buf != ctl)
		sock_kfree_s(sock->sk, ctl_buf, ctl_len);
out_freeiov:
	kfree(iov);
	return err;
}

在项目中测试发现，没有定义CONFIG_COMPAT宏， 用的是#define compat_msghdr msghdr

E:\linux-4.1.45\linux-4.1.45\include
et\compat.h
#if defined(CONFIG_COMPAT)

#include 

struct compat_msghdr {
	compat_uptr_t	msg_name;	/* void * */
	compat_int_t	msg_namelen;
	compat_uptr_t	msg_iov;	/* struct compat_iovec * */
	compat_size_t	msg_iovlen;
	compat_uptr_t	msg_control;	/* void * */
	compat_size_t	msg_controllen;
	compat_uint_t	msg_flags;
};

...

#else /* defined(CONFIG_COMPAT) */
/*
 * To avoid compiler warnings:
 */
#define compat_msghdr	msghdr
#define compat_mmsghdr	mmsghdr
#endif /* defined(CONFIG_COMPAT) */

sock_sendmsg()

int sock_sendmsg(struct socket *sock, struct msghdr *msg)
{
	int err = security_socket_sendmsg(sock, msg,
					  msg_data_left(msg));

	return err ?: sock_sendmsg_nosec(sock, msg);
}

sock_sendmsg_nosec()

static inline int sock_sendmsg_nosec(struct socket *sock, struct msghdr *msg)
{
	int ret = sock->ops->sendmsg(sock, msg, msg_data_left(msg));
	BUG_ON(ret == -EIOCBQUEUED);
	return ret;
}

inet_sendmsg()

int inet_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
{
	struct sock *sk = sock->sk;

	sock_rps_record_flow(sk);

	/* We may need to bind the socket. */
	if (!inet_sk(sk)->inet_num && !sk->sk_prot->no_autobind &&
	    inet_autobind(sk))
		return -EAGAIN;

	return sk->sk_prot->sendmsg(sk, msg, size);
}