When you synthesize input with SendInput, you are also synthesizing the timestamp

Raymond Chen


A customer was reporting a problem when they used the
Send­Input function to simulate a drag/drop
operation for automated testing purposes.

I see the mouse move from one location to another,
and the starting and stopping locations are correct on the screen,
but the mouse moves instantaneously rather than waiting 500ms between
Here’s how I’m sending the input.

INPUT input[3] = { 0 };
// Click
input[0].type = INPUT_MOUSE;
input[0].mi.dwFlags = MOUSEEVENTF_LEFTDOWN;
input[0].mi.time = 500;
// Drag
input[1].type = INPUT_MOUSE;
input[1].mi.dwFlags = MOUSEEVENTF_MOVE;
input[1].mi.dx = 100;
input[1].mi.dy = 100;
input[1].mi.time = 1000;
// Release
input[2].type = INPUT_MOUSE;
input[2].mi.dwFlags = MOUSEEVENTF_LEFTUP;
input[2].mi.time = 500;
SendInput(3, input, sizeof(INPUT));

Well, yeah, all the events occur immediately
because you submitted them all at once.

The time field in the
MOUSE­INPUT structure is not for
introducing delays in playback.
Though I’m not sure what the customer thought
the time field was.
They say that they want a 500ms delay between operations.
At first, I thought that they may have misinterpreted
it as a delay relative to the time the
Send­Input call is made,
since they set
input[0].mi.time to 500 and
input[1].mi.time to 1000.
But if thay were the case, then setting
input[2].mi.time to 500
would end up going backward in time.
But looking at the big picture,
it’s probably not worth trying to figure out what they
were thinking,
since that code will have to be scrapped anyway.

The time field is for letting an input source
(typically a hardware device)
um, the mouse left button went down at 9:30 this morning.
Yes, I know it’s already 10am.
The PCI bus got a flat tire,
and then the spare was also flat,
and really there’s no point going into the details.
Sorry this message arrived late.”
The window manager (and anybody else who bothers to check
the time member of the MSG structure)
uses this information to do things like
detect double-clicks.
If the input source later reports,
“Hi, um, the mouse left button went up at 9:30:00.100 this morning,
sorry for the late report,”
the window manager says,
“Well, that was only 100 milliseconds after the button went down
thirty minutes ago,
so I guess that’s a double-click after all.
Could you try to be a bit more prompt with this information
in the future?

(Sarcasm added.)

In other words, the time member of the
MOUSE­INPUT structure is for backdating input
They still get delivered immediately,
but the timestamp allows the window manager
(and other code which looks at the timestamp)
to make decisions about how they should respond.

Note that post-dating the timestamp does not cause the
input delivery to be delayed,
and back-dating the timestamp does not cause the input to be
inserted into the input stream ahead of other input.
The input is merely delivered with a timestamp in the future
or in the past.
(And who knows what sort of havoc that will create
if a program checks the timestamps and notices that they
are either from the future or have traveled back in time.
Maybe you’ll get

a call from Microsoft Research asking for
more information about your time machine

If you want three input events to take place with a 500ms
delay between them,
then you need to call
Send­Input three times,
with a 500ms delay between the calls.

Raymond Chen
Raymond Chen

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