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Passing multi parameter array to a C++ DLL

  • #41
  • Edited 3:32am May 31, 2019 3:16am | Edited 3:32am
Quoting skinner36
Hi, Sometimes it would be very useful to use the same data between mql programs. I know that we can use global variables but that is only for a single double value. What if we wanted to access an array of double values. Is there any way that we could create an array in an EA and pass it to a dll, have the dll procees the array and then an indicator pick up the address of the pointer and access the same array? I am just trying to think outside the box here. Cheers, John

  1. You can send messages between MQL programs - in the same or in different terminals - with QuickChannel. You can send whatever you want as long as it's serializable. For example a pointer. The MQL framework uses this technique.
  2. You can store integer values in the window object of a chart. This way MQL programs in the same chart can exchange data. For example a pointer. The MQL framework uses this technique.
  3. You can store stringified values of all types directly in the chart. Another way for MQL programs in the same chart to exchange data. For example a pointer. The MQL framework uses this technique.
  4. You can store data sent by one MQL program in the DLL statically, and access it from another program in the same terminal. Tricky as you have to handle possible synchronization issues but possible. The MQL Expander uses this technique.
  5. Finally you can use shared memory. I don't use this technique.

PS: QuickChannel is the recommended and preferred approach to handle all kind of MQL communication. Easy to setup and super fast.

It takes blood, sweat and enough tears to wash away the blood.
  • #42
  • May 31, 2019 4:09am May 31, 2019 4:09am
Quoting emmzett
{quote} You can send messages between MQL programs - in the same or in different terminals - with QuickChannel. You can send whatever you want as long as it's serializable. For example a pointer. The MQL framework uses this technique. You can store integer values in the window object of a chart. This way MQL programs in the same chart can exchange data. For example a pointer. The MQL framework uses this technique. You can store stringified values of all types directly in the chart. Another way for MQL programs in the same chart to exchange data....
Hi Emmzett

I was just looking at the QuickChannel documentation and it looks very interesting. Have you used QuickChannel for passing a reference to a an array between MQL programs?
  • #43
  • Last Post: May 31, 2019 7:15am May 31, 2019 7:15am
Quoting skinner36
{quote} Hi Emmzett I was just looking at the QuickChannel documentation and it looks very interesting. Have you used QuickChannel for passing a reference to a an array between MQL programs?
No, not a pointer to an array allocated by another MQL program, and I don't recommend of doing so. In the other program (in any other program) you don't know if/when the memory your pointer is pointing to is released by the terminal. If you continue to access that memory once it is released, you will - more or less - immediately crash. As you have no way of knowing when/if the other program releases its memory, you are not allowed to access it all. Not even reading.

As Mladen said, there is no reason to not process your array in the MQL program which allocated the memory. You can only pass pointers around for addresses where you control allocation/deallocation.

PS: I use QuickChannel a lot for sending commands and state from one program/terminal to another. Trade commands, P/L stats, limits... anything. Do your calculations and if you really have to, send the results to somewhere else.
It takes blood, sweat and enough tears to wash away the blood.
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