Thread: reallocating space for an array and returning the new array

Yes, you can return local variables - but you have to "catch" them at once, to ensure validity, because they go out of scope, and are thereafter, available for any other program to use that memory.


<< EDIT: perhaps more clear and to the point >>

Code:

int returnLocal(void); //prototype - specifies return will be an integer

int main(void) {  //main() also return an int in C
  int returnedInt;
   
  returnLocal();  //wrong nothing to catch the return value
  
  returnedInt=returnLocal();  //right! :)
  //do something with the value
  printf("\n Returned value: %d", returnedInt);
  //do something else with returnedInt

  return 0;
}
int returnLocal(void) {
  int a=3;
  int b=4;  

  return a+b; //add a and b, and return it.
}

I'm not sure how you plan on "making a new array", without using an allocation function (malloc(), calloc(), etc.). Can you explain that a bit more?

Overwrite one of the arrays passed with the result, and return that. That requires main() to allocate enough memory for the array that will be overwritten, so it can hold the result.

Alternately, statically allocate an array within the function large enough to hold the result (note there will be an upper limit on the size defined at compile time, and also limitations on how the function can be called).

Incidentally, you haven't said anything about how you detect the sizes of the arrays passed (or, for that matter, how main() would detect the size of the array returned).

Adak, it is possible to return local variables, but not the address of local variables - doing so causes the caller to exhibit undefined behaviour if it dereferences the returned address. This also means you cannot return local arrays, unless they are static.

As Grumpy stated, there HAS to be (Presumably) a limit to how large the numbers are. (how many digits can be stored in your static array). If so, your result array has to be 1 digit larger to account for a possible carry over. Alternatively you can just make it the same size and use a flag to remember whether you have an additional digit(1) at the beginning. Then, when you print the array you start by printing 1 or 0 depending on what the flag is and then move on to your array.