C Realloc causing segmentation fault

Question

I am using following function to allocate memory:

int qmem_alloc(unsigned int num_bytes, void ** rslt){

void** temp;
if(rslt == NULL)
    return -1;
temp = (void **)malloc(num_bytes);
if(temp == NULL)
    return -2;
else
    rslt = temp;
    return 0;
}

And following function to reallocate memory:

int  qmem_allocz(unsigned num_bytes, void ** rslt){
void** temp;
void *test = (void *)malloc(10);
if(rslt == NULL)
    return -1;
temp = (void **)realloc(rslt, num_bytes);
printf("here");
if(temp == NULL)
    return -2;
else
    // free(rslt)

    return 0;
  }

Here is my main function:

struct qbuf { int idx; char data[256]; };
void main(){
struct qbuf * p = NULL;
printf("%d\n",qmem_alloc(sizeof(struct qbuf), (void **)&p));
printf("%d\n",qmem_allocz(100*sizeof(struct qbuf), (void **)&p));
}

The program can get memory allocated but it crashes when reallocation is done. Here is the error:

malloc.c:2868: mremap_chunk: Assertion `((size + offset) & (GLRO (dl_pagesize) - 1)) == 0' failed.

Why is this happening? How can i fix it?

Fix your code with *rlst = temp; before return in the alloc function. rlst = temp; assigns the pointer value to temp, you need to assign the value to which the pointer points to temp. — KamilCuk
– KamilCuk, Commented Oct 2, 2018 at 18:27
Curiosus, who or what text suggested casting, like (void **), the result of malloc().? — chux
– chux, Commented Oct 2, 2018 at 19:08
What is the reason for void *test = (void *)malloc(10);? It allocates memory and then code forgets about it. — chux
– chux, Commented Oct 2, 2018 at 21:15

kiran Biradar · Accepted Answer · 2018-10-02 19:00:20Z

Your allocation in qmem_alloc is wrong.

temp = (void **)malloc(num_bytes); //You are wrongly typecasting, don't typecast the malloc return.
rslt = temp; // This makes rslt points to object where temp is pointing

You simply need to do as below.

int qmem_alloc(unsigned int num_bytes, void ** rslt){
  if(rslt == NULL)
    return -1;

   *rslt = malloc(num_bytes);
   if(*rslt == NULL && num_bytes > 0)
      return -2;
   else
      return 0;
}

And your reallocation is wrong

temp = (void **)realloc(rslt, num_bytes); //You need to pass the object where rslt is pointing.

Sample code for reallocation:

int  qmem_allocz(unsigned num_bytes, void ** rslt){
   void* temp; // No pointer to pointer is needed

   void *test = (void *)malloc(10);
   if (test == NULL) return -3;

   if(rslt == NULL)
      return -1;

   temp = realloc(*rslt, num_bytes); //use *rslt to pass the address of object where rslt is pointing.

   if(temp == NULL && num_bytes > 0){
      return -2;
    }
    else{
     *rslt = temp;
      return 0;
    }
}

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