SendMessage的大致流程

1、构造send_message_info结构体，然后调用send_message函数。 SendMessageA和SendMessageW的区别有两处：info.type一个是MSG_ASCII，一个是MSG_UNICODE；send_message函数的第三个参数一个为TRUE一个为FALSE。

LRESULT WINAPI SendMessageA( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam )
{
    DWORD_PTR res = 0;
    struct send_message_info info;

    info.type    = MSG_ASCII;
    info.hwnd    = hwnd;
    info.msg     = msg;
    info.wparam  = wparam;
    info.lparam  = lparam;
    info.flags   = SMTO_NORMAL;
    info.timeout = 0;
    info.wm_char  = WMCHAR_MAP_SENDMESSAGE;
    send_message( &info, &res, FALSE );
    return res;
}

2、send_message

首先判断是不是广播消息，从is_broadcast函数代码来看，目的窗口句柄为HWND_BROADCAST或HWND_TOPMOST的是广播消息。对于广播消息，它使用EnumWindows通过回调函数broadcast_message_callback，将消息发给每一个窗体。

GetWindowThreadProcessId函数，通过目的窗口句柄获取窗口所在进程和线程id

如果目的窗口线程和当前发送线程相同，它直接调用窗口的回调函数。

如果是不同的线程，最终会调用到send_inter_thread_message发送线程间消息。WINPROC_CallProcAtoW这里有一次转换，目前不清楚它是什么用，它的第一个回调参数是真正发送消息的地方。

static BOOL send_message( struct send_message_info *info, DWORD_PTR *res_ptr, BOOL unicode )
{
    DWORD dest_pid;
    BOOL ret;
    LRESULT result;

    if (is_broadcast(info->hwnd))
    {
        EnumWindows( broadcast_message_callback, (LPARAM)info );
        if (res_ptr) *res_ptr = 1;
        return TRUE;
    }

    if (!(info->dest_tid = GetWindowThreadProcessId( info->hwnd, &dest_pid ))) return FALSE;

    if (USER_IsExitingThread( info->dest_tid )) return FALSE;

    SPY_EnterMessage( SPY_SENDMESSAGE, info->hwnd, info->msg, info->wparam, info->lparam );

    if (info->dest_tid == GetCurrentThreadId())
    {
        result = call_window_proc( info->hwnd, info->msg, info->wparam, info->lparam,
                                   unicode, TRUE, info->wm_char );
        if (info->type == MSG_CALLBACK)
            call_sendmsg_callback( info->callback, info->hwnd, info->msg, info->data, result );
        ret = TRUE;
    }
    else
    {
        if (dest_pid != GetCurrentProcessId() && (info->type == MSG_ASCII || info->type == MSG_UNICODE))
            info->type = MSG_OTHER_PROCESS;

        /* MSG_ASCII can be sent unconverted except for WM_CHAR; everything else needs to be Unicode */
        if (!unicode && is_unicode_message( info->msg ) &&
            (info->type != MSG_ASCII || info->msg == WM_CHAR))
            ret = WINPROC_CallProcAtoW( send_inter_thread_callback, info->hwnd, info->msg,
                                        info->wparam, info->lparam, &result, info, info->wm_char );
        else
            ret = send_inter_thread_message( info, &result );
    }

    SPY_ExitMessage( SPY_RESULT_OK, info->hwnd, info->msg, result, info->wparam, info->lparam );
    if (ret && res_ptr) *res_ptr = result;
    return ret;
}

3、send_inter_thread_message

把消息放到目标线程的消息队列，并等待消息处理完成。

static LRESULT send_inter_thread_message( const struct send_message_info *info, LRESULT *res_ptr )
{
    size_t reply_size = 0;

    TRACE( "hwnd %p msg %x (%s) wp %lx lp %lx\n",
           info->hwnd, info->msg, SPY_GetMsgName(info->msg, info->hwnd), info->wparam, info->lparam );

    USER_CheckNotLock();

    if (!put_message_in_queue( info, &reply_size )) return 0;

    /* there's no reply to wait for on notify/callback messages */
    if (info->type == MSG_NOTIFY || info->type == MSG_CALLBACK) return 1;

    wait_message_reply( info->flags );
    return retrieve_reply( info, reply_size, res_ptr );
}

这里面的函数涉及到了更底层的调用，等以后理解深刻了再分析吧。