Declare a global array when its size is only available at run time

Projects Programming
1

Is it possible to declare a globally available array when the number of elements will only be available at run time?

Thanks
Paulo

2

Yes, you can use malloc() to get the necessary memory. Take care that your array doesn't get to big!

4

Ok, I am not that advanced.
Let's see:

I declare an global array on top of my sketch (before setup(){ ):

String imagePath[];

Then on setup() I will get the info that I have, say, 24 imagePaths to deal with (say "/image01.jpg" to "/image24.jpg")

After some research I found some info about the subject and got something like this:

int * heapArray =(int ) malloc(arrayLengthsizeof(int));

Now I see no correlation of how can I apply this to my imagePath array.

Help welcome.
Thanks

5

What type of Arduino are we talking about?

Don't use String on AVR Arduinos!

BTW, Delta_G's comment is absolutely true, use a dynamic array of C++ objects is quite tricky and should not be used small MCUs. In bigger C++ projects you'd use container classes (p.e. vector) for this.

String **imagePaths;

...

int numberOfPaths = 24;

imagePaths = (String **) malloc(numberOfPaths * sizeof(String *));

...

imagePaths[0] = new String("TestString");
7

Thanks Delta_G,

I thought about that, may be 128...
But then wouldn't I be wasting too much memory is the typical value was around 12?

Hi there pylon,
I believe it will do the job, it is a Mega but I am thinking about migrating the project to ESP32.
One doubt:
In the sizeof(String *));
Should I then put the expected size of, for instance: "/image01.jpg" = 12?
Would it be this: imagePaths = (String **) malloc(numberOfPaths * 12); ?
Thanks

9

sizeof (String*) . . . is a constant.
sizeof (String) . . . is also a constant.

Don't confuse the size of the class with the length of the string.

10

In the Arduino IDE, you have access to the Himem area of a ESP32 - WROVER, that would allow you to get about a 4MB array. I use Himem to store any buffers I may use, such as serial receive buffer.

The ESP32 has a built-in OS called freeRTOS. The freeRTOS API has an Espressiff addition called 'ring buffer' see API reference: (FreeRTOS (Supplemental Features) - ESP32 - — ESP-IDF Programming Guide latest documentation and freeRTOS has the stream buffer RTOS stream buffer API functions that might be of interest to you when moving around large bits.

11

Would it be this: imagePaths = (String **) malloc(numberOfPaths * 12); ?

No, the array is an array of String pointers, so one element has the size of a pointer, the compiler knows that while compiling. The size of the contained string value isn't needed at that time as the String class is allocating and deallocating it on the fly. That's the reason you shouldn't use it on an AVR Arduino (as the Mega2560 is!).

12

From where are you getting the data at run time?

It looks like you might be making a copy of an SD card directory listing. Since that data is on the SD card you should be able to read it from the SD card when you need it, without having to keep a copy in memory.