free(ptr) not working

Projects Programming
1

Hi all,

Anyone know why my function freeMsg() isn't actually working? If I put a line if(msg==NULL) it returns false, after the free(msg) command.

Cheers

template <typename mavlink_message_type>
class storage {
public:
  storage(){
    msg = static_cast<mavlink_message_type*>(malloc(sizeof(mavlink_message_type)));
  }

  mavlink_message_type* msg;

  bool newData(){
    if(this->msg == NULL){
      this->msg = static_cast<mavlink_message_type*>(malloc (sizeof(mavlink_message_type)));
    }
    return true;
  }

  void freeMsg(){
    if(msg != NULL){
      free(msg);
    }
  }
};
2

char* ptr;

free(ptr) can only free the memory pointed by ptr, but cannot modify ptr in any way. If it could do that, the signature would be free(char**), and you'd passit &ptr, not ptr.

3

Ok, that's confused me a little.

ptr is pointing to mavlink_message_type*, which I should explain is an instance of a structure.

by calling free(ptr), I effectively want to just delete the existence of that structure.

I have tried free(&msg), and that didnt work.

I have tidied the code up a bit:

void loop(){
  Serial.print("Loop ");
  Serial.print(counter++);
  Serial.println(1+counter);

  createMe();
  freeMe();
}

bool createMe(){
    Serial.println("Allocating memory");
    msg = (mavlink_message_type*)(malloc(sizeof(mavlink_message_type)));
    if (msg != NULL){
      return true;
    }
    else{
      Serial.println("Error in creating msg");
      return false;
    }
  }

  bool freeMe(){
    Serial.println("Freeing memory");
    free(&msg);
    if (msg == NULL){
      return true;
    }
    else{
      Serial.println("Error in freeing msg");
      return false;
    }
  }

The output I get is the following:

Allocating memory
Loop 0
1
Loop 1
2
Freeing memory
Error in freeing msg
Loop 2
101
Loop 3
4
Frng memory
Error in freeing msg
Loop 4
4
Loop 5
6
4

After you free the memory pointed to, YOU must set the pointer to NULL. That is not done for you.

free(ptr);
ptr = NULL;
5

Gotcha,

Cheers Paul, didn't know that!

Si