How to get the length of a variable size array of structs?

Question

I'm breaking my head trying to sort out this problem, and I have not been able to sort it out. Given I have a struct:

struct person { String name, String city, int age }

I am reading a list of people from a certain file provided externally, and populating an array of this struct. This is done via a function, lets say

person * readFromFile(filename);

This function has the logic to read the file, create a variable size array of structs (adapted to the number of persons in the file), and return said array.

However, when I try to assign that outcome to a pointer, I'm not getting the elements of the array:

... person * myPeople; myPeople = readFromFile("/people.txt"); int n= ;// different things I-ve tried here Serial.println("n is" + n); for(int i=0; i<n; i++) { Serial.println(myPeople.name + " (" + String(myPeople.age) + "), "+myPeople.city }

I've tried several things to get the number of elements once the array is populated after researching how to do it, to know:

int n = sizeof(myPeople)/sizeof(myPeople[0]); int n = sizeof myPeople / sizeof *myPeople; int n = sizeof(myPeople) / sizeof(person); int n = (&myPeople)[1] - myPeople;

But to no avail: I have 5 elements in the file, but n does never show the expected value (and of course the for loop breaks).

Can I get some assistance? what am I be doing wrong?

Thanks

You need to store the length of the array explicitly, there is no way to retrieve it from a pointer — UnholySheep
– UnholySheep, Commented Oct 23, 2020 at 12:07
Does this answer your question? How to find the 'sizeof' (a pointer pointing to an array)? — UnholySheep
– UnholySheep, Commented Oct 23, 2020 at 12:07
Wrap it into a struct with more info: struct personArray { struct person *data; int totalelements; int nelements; };... every time you realloc() data (when nelements == totalelements and you need more space) adjust totalelements, everytime you add a person adjust nelements. — pmg
– pmg, Commented Oct 23, 2020 at 12:14
Or... person * readFromFile(filename, &nReadPersons) , where the latter is an out-argument, populated by your function and specifies the number of items in the returned sequence. All that's left it to actually write it now. Personally I prefer doing what @pmg suggests. it keeps codependent data together, nice and tidy. — WhozCraig
– WhozCraig, Commented Oct 23, 2020 at 12:18
Where do you actually allocate memory? And why on earth are you doing file handling on an Arduino? — Lundin
– Lundin, Commented Oct 23, 2020 at 13:34

paladin · Accepted Answer · 2020-10-24 07:23:15Z

1

Instead of using "String" (Whatever that is in your case.), use a fixed array of char for each string.

Like so:

struct person { char name[32], /* string name has a length of 31 + 1 for NULL-termination */ char city[32], /* string city has a length of 31 + 1 for NULL-termination */ int age }

answered Oct 24, 2020 at 7:23

paladin

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2 Comments

ggonmar Over a year ago

Despite my reluctancy to use char instead of String, I ended up using this as the pure only way of getting rid of my problems.... thanks!

paladin Over a year ago

No problem. You should also add the following modifier to your struct: __attribute__((__packed__)), it ensures that your struct will be always of the same byte-size, even on different CPU architectures. By the way, when working with pointers over structs, it's always a good idea to use the most basic data-types (if possible) within that struct. Or at least use other packed structs as data-types within that struct, which characteristics you exactly know.

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