C : jansson json_decref - segmentation fault

Question

I have a for loop and while iterating the loop, if the index is even index the json object has to be freed and again a new object has to be constructed and again the process has to be repeated.

For the same, using the below script,

#include <stdio.h>
#include <string.h>
#include <jansson.h>

int main(void) {     
  char* s = NULL;      
  json_t *root = json_object();

  int myNum[10] = {10, 20, 10, 40, 10, 60, 10, 80, 10, 100};

  for(int i=0; i<10;i++)
  {
    if(i%2==0)
    {

      json_t *root = json_object();
    }

    char *key = (char*)malloc(2);
    snprintf(key, sizeof(key), "%d", myNum[i]);

    json_object_set_new( root, key, json_integer(i));      
    s = json_dumps(root, 0);
    puts(s);

    if(i%2==0){
    json_decref(root);        
    //free(s);
    }

  }
}

How to achieve the below result using jansson json object construction and clearing its memory when the index is even index?

{"10":0,"20":1}
{"10":2,"40":3}
{"10":4,"60":5}
{"10":6,"80":7}
{"10":8,"100":9}

Whereas now, the above script gives the below response,

{"10": 0}
Segmentation fault (core dumped)

Your `if(i%2==0)` body creates and then immediately leaks a 2nd, different pointer called `root`, which hides the previous/outer one. Why? That's not what you want, is it? Just assign to the existing, outer `root`. Also, you said you should free it, but you never try to. — underscore_d, Jun 18 '20 at 10:14
Anyway, the next step is to use a debugger to figure out _where_ that segfault occurs, then check the values of variables on the way to that, to determine what you did wrong. — underscore_d, Jun 18 '20 at 10:26

score 2 · Accepted Answer · answered Jun 18 '20 at 10:20

You have some memory issues:

int main(void) {     
  char* s = NULL;      
  json_t *root = json_object();

You request a new JSON object immediately in first iteration of your loop. The object created by this function call is not longer accessible then. This is a memory leak.


  int myNum[10] = {10, 20, 10, 40, 10, 60, 10, 80, 10, 100};

  for(int i=0; i<10;i++)
  {
    if(i%2==0)
    {
      json_t *root = json_object();
    }

    char *key = (char*)malloc(2);
    snprintf(key, sizeof(key), "%d", myNum[i]);

This writes 3 bytes into memory that can only hold 2 bytes.


    json_object_set_new( root, key, json_integer(i));      
    s = json_dumps(root, 0);
    puts(s);

    if(i%2==0){
    json_decref(root);

After this call you cannot use root any more. The object referenced is likely to be freed after decrementing the reference counter. If you try to add a new object in the next iteration, you don't have a valid object. This is likely to trigger your segmentation fault.

    //free(s);
    }
  }
}

Reading your description

if the index is even index the json object has to be freed and again a new object has to be constructed

That is true, but you don't do it like this.

To fix your problem, either

free only at the end of the iteration for odd numbers
request a new object immediately after json_decref. No need to wait for start of the second next iteration.

C : jansson json_decref - segmentation fault

1 Answers1