Problem in allocating memory and set value to a refrence variable in c++ file body (Not in function)

Question

I am new in c and c++. I want to allocate memory and set a value to a pointer in my c++ file body so it will execute only once.
Here is my code:
myFile.h:

struct SelectedRows_t {
  uint32_t rowsLen;
  SelectData_t* rows[];
};
extern SelectedRows_t* selectedRows;

myFile.cpp

SelectedRows_t* selectedRows = (SelectedRows_t*)malloc(sizeof(selectedRows->rowsLen));

// some functions which use selectedRows variable

But I can't find a way to also initialize my variable. I need to set rowsLen to 0 at the start of my program.

I don't have an init or main function as I am trying to write a library which can be used anywhere alongside other c++ codes.
I need to assign this 0 to this variable only once and in the start of my program.
I have to allocate memory to this variable myself because otherwise codes like this selectedRows->rowsLen will crash my program.
I can't realloc this variable in my functions because of rows variable inside this struct needs to be free before any memory reallocation.
I don't know it this matters or not but I am writing this program to be run on esp32 boards.

Thanks in advance.

Answer 1

I solved the problem by using a function to do all my works in it and return the pointer to the final SelectedRows_t:

SelectedRows_t* createSelectedRows() {
  SelectedRows_t* selectedRowsTemp = (SelectedRows_t*)malloc(sizeof(selectedRows->rowsLen));
  selectedRowsTemp->rowsLen=0;
  return selectedRowsTemp;
}

SelectedRows_t* selectedRows = createSelectedRows();