How to check if a cJSON pointer was initialized/set earlier?

Question

I have a struct with cJSON pointers as in:

  struct { 

      cJSON *myJSON1;
      cJSON *myJSON2;
      ...
  } myStruct;

Somewhere in my code I create cJSON items as in myStruct.myJSON1 = cJSON_CreateObject()

At the end of the code, I want to call cJSON_Delete() on those pointers which were assigned. I presume this is the classic C case where there is no way to find out if the pointer was malloced in some way. Of course, I can keep a flag to keep track but I want a simple way. I read...

The cJSON struct is as:

  /* The cJSON structure: */
  typedef struct cJSON
  {
      struct cJSON *next;
      struct cJSON *prev;
      struct cJSON *child;
      int type;
      char *valuestring;
      /* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */
      int valueint;
      double valuedouble;
      char *string;
  } cJSON;

The function cJSON_Invalid() is available. The documentation states "(check with cJSON_IsInvalid): Represents an invalid item that doesn't contain any value. You automatically have this type if you set the item to all zero bytes." Do I have to memset the structure to 0 or just type?

In other words, my question is: What would be the easiest way to check if a cJSON pointer is malloced without creating an additional variable? Perhaps set "type" to zero? I can try such options but I want a definite answer which works in all situations.

score 1 · Accepted Answer · answered Jan 22 '20 at 02:14

If you just have an uninitialized pointer to a cJSON data structure the behavior is undefined. I.e. for cJSON *myJSON1;

cJSON_Invalid(myJSON1) might return 0 or 1.

And cJSON_Delete(myJSON1) will probably segfault. Or do nothing if you get lucky.

The solution is to always initialize cJSON data structures to NULL. Then you can just call cJSON_Delete(). If the pointer is still NULL the call will do nothing. If it was set up at some point in your program it will correctly release all the memory. Since you have a structure of multiple cJSON pointers you can set the entire data structure to 0 using memset.

You can take a look at the source code.

CJSON_PUBLIC(void) cJSON_Delete(cJSON *item)
{
    cJSON *next = NULL;
    while (item != NULL)
    {
    ...
    }
}

@hkp cJSON_Invalid() actually segfaults for me when not initialized. I think now that that makes sense because there is no way for the function to know if it was initialized. — asinix, Jan 22 '20 at 07:58

How to check if a cJSON pointer was initialized/set earlier?

1 Answers1