Memory leak on peridical json_loads call

[lotan]

Hi,

I've search through the group an found similar issues to mine but wasn't able to find a solution.

I've reduced my code until I've ended up with this snippet:

int parse_json(const char *text) {

   json_t            *root;
   json_error_t       error;

   if (!json_is_object((root = json_loads(text, 0, &error)))) return 1;
   json_decref(root);

   return 0;
}

parse_json is called periodically and valgrind reports this:

==7271== 95,066 (144 direct, 94,922 indirect) bytes in 2 blocks are definitely l
ost in loss record 102 of 102
==7271==    at 0x4A090ED: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd6
4-linux.so)
==7271==    by 0x4C4CD11: json_object (in /usr/lib64/libjansson.so.4.3.1)
==7271==    by 0x4C4A684: ??? (in /usr/lib64/libjansson.so.4.3.1)
==7271==    by 0x4C4AA76: ??? (in /usr/lib64/libjansson.so.4.3.1)
==7271==    by 0x4C4ABF5: json_loads (in /usr/lib64/libjansson.so.4.3.1)
==7271==    by 0x4016F7: parse_json

For every iteration it seems to lose 72 direct and 47461 indirect bytes.

If I comment out the call to parse_json(), I don't lose any bytes.

There's not much to my example.
Will I have to free some additional json context if I call json_loads() periodically?

[Petri Lehtinen]

No you don't. Why this happens becomes clear if you look at the
definition of the json_is_object macro:

#define json_typeof(json) ((json)->type)
#define json_is_object(json) (json && json_typeof(json) == JSON_OBJECT)

When the macros are expanded, the if statement in parse_json()
becomes:

if ((root = json_loads(text, 0, &error)) && ((root = json_loads(text, 0, &error))->type) == JSON_OBJECT) return 1;

So json_loads() is called two times and the first result is thrown
away. Consequently, json_decref() only frees half of the memory that
is allocated.

You can confirm this by changing your code to:

int parse_json(const char *text) {

json_t *root;
json_error_t error;

root = json_loads(text, 0, &error);
if (!json_is_object(root)) return 1;
json_decref(root);

return 0;
}

This way, no memory is leaked.

Petri

[Jeff]

json_loads() could also return an array, right?  In that case json_is_object() would be false and you'd miss your json_decref().  Seems like you'd always want to call json_decref().  It will do the right thing even if passed NULL.  Maybe:

root = json_loads(...);

answer = json_is_object(root);

json_decref(root);

return answer;

[lotan]

Thank you very much.
It would indeed have helped to look at the code (the include would have even been enough).
Wrong assumption on my side. Sorry for the hassle.