Free dynamically alloted memory to struct array containing pointer

Question

I have three files

struct.h  struct.c main.c

struct.h contains declaration of structs and some functions
struct.c contains global variable bglobal an instance of struct b and function implementations which use bglobal. It includes .h file
main.c call some of the functions declared in struct.h. It also includes .h file
struct.h contains two struct

struct a{
int *s
}
struct b{
struct a* arr
}
void init();
void more();

struct.c file

#include"struct.h"
struct b bglobal;
void init(){
   bglobal.arr = malloc(sizeof(struct a)*5);
}
void more(){
   *bglobal.arr[0].name = 'I';
}

main.c file

#include "main.h"
int main(){
init();
more();
}

I want that at end of program memory allocated to bglobal.arr get freed up. Using valgrind it says some bytes still reachable. How to achieve this?

Do reversed steps of the allocation or [just exit your program and have the OS free the memory](https://stackoverflow.com/questions/654754/what-really-happens-when-you-dont-free-after-malloc). — MikeCAT, Aug 04 '21 at 11:35
there is no such thing as "memory allocated to bglobal.arr" by the way. There is just memory allocated. — user253751, Aug 04 '21 at 11:37
free(bglobal.arr) works but I cant call it from main.c as bglobal is declared in struct.c — aquib nawaz, Aug 04 '21 at 11:44
@MikeCAT I can do reverse but ho to do at the end of program. I mean is there something like destructor in c. — aquib nawaz, Aug 04 '21 at 11:48
Register an exit handler in `init()` that frees the allocated memory when the program exits. Or provide an uninit() function that the client code can call themselves to free the resources. — moooeeeep, Aug 04 '21 at 11:49
I can't add anything to struct.h. Can u give me some link about exit handler. — aquib nawaz, Aug 04 '21 at 11:50
where should I register this . If I do in main.c I cant free bglobal as it is declared in struct.c. There is no main in struct.c — aquib nawaz, Aug 04 '21 at 12:04
Although it is cleaner to free everything that was allocated, it is usually only the *chronic* memory leaks that cause problems. — Ian Abbott, Aug 04 '21 at 12:16

chux - Reinstate Monica · Answer 1 · 2021-08-04T12:55:08.660

3

I want that at end of program memory allocated to bglobal.arr get freed up.

Add a complement function to init()

int main(){
  init();
  // do stuff
  uninit();  // this functions frees resources.
}

If struct.h does not allow improvement, make a local function in main.c.

void uninit(void) {
  extern struct b bglobal; // Take advantage that bglobal is not static
  free(bgolbal.arr);
}

bgolbal.arr = malloc(sizeof(struct b)*5); allocates to the wrong type. .arr is a struct a*. Allocate to the referenced object instead as it is easier to code right, review and maintain.

bgolbal.arr = malloc(sizeof *bgolbal.arr * 5);

edited Aug 04 '21 at 12:55

answered Aug 04 '21 at 12:15

chux - Reinstate Monica

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I cant add anything to str.h. So this method will not work as uninit() will be implemented in str.c but cant be referenced in main.c. – aquib nawaz Aug 04 '21 at 12:36
@aquib nawaz In `main.c` , `void uninit(void) { extern struct b bglobal; free(bgolbal.arr); }`. Take advantage that `bglobal` is not `static`. or better fight the questionable requirement "I can't add anything to struct.h." as `struct.c` is poorly written – chux - Reinstate Monica Aug 04 '21 at 12:53
This is annoying but this is assignment. – aquib nawaz Aug 04 '21 at 12:59
@aquib nawaz, go with `atexit()` approach then. As your coding experience grows, you will see that requirements are negotiable. Do not get stuck is a weak design. – chux - Reinstate Monica Aug 04 '21 at 13:02

score 2 · Accepted Answer · answered Aug 04 '21 at 12:49

2

You'll need to add a cleanup function to struct.c to free the memory. And since you can't reference that function from outside the module, you can set it up with atexit from init to be called when the program exits.

static void cleanup() {
    free(bglobal.arr);
}

void init(){
   bglobal.arr = malloc(sizeof(struct a)*5);
   atexit(cleanup);
}

answered Aug 04 '21 at 12:49

dbush

205,898
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Given OP's limitation, this is good - aside from the side issue of the mis-type in `sizeof(struct a)`. I'd also `free(bglobal.arr);` --> `free(bglobal.arr); bglobal.arr = 0;` to cope with other potential `atexit()` using `bglobal.arr`. – chux - Reinstate Monica Aug 04 '21 at 13:05

Free dynamically alloted memory to struct array containing pointer

2 Answers2