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)

Answer 1

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.