CODEWITHSUNDEEP
clinuxsegmentation-fault
user1944267
user1944267

Reputation: 1637

segmentation fault when freeing the buffer after sendto()

I get a segmentation fault when freeing the buffer 'pkt' after the function sendto()

u_char* create_pkt(u_char* pkt)
{
  ....
  pkt = (u_char *)malloc(40);
  ...
  return pkt
}


int main()
{
 ....
 u_char* pkt;
 create_pkt(pkt);
 if (sendto(sd, pkt, 40, 0, (struct sockaddr *)&sin, sizeof(struct sockaddr)) < 0)
 free(pkt);
 }

the debugging information shows:

Program received signal SIGSEGV, Segmentation fault.
0x0000003897482864 in __GI___libc_free (mem=0x7fffffffe010) at malloc.c:2986

what is wrong with this? thanks!

2986      ar_ptr = arena_for_chunk(p);
2986      ar_ptr = arena_for_chunk(p);

Upvotes: 1

Views: 425

Answers (3)

Mr Lister
Mr Lister

Reputation: 46579

The create_pkt function returns the newly allocated value, so you'll need to use that in the calling function.

pkt =create_pkt(pkt);

Otherwise the program will just ignore the pointer to the allocated memory and use the original (unassigned) value of pkt.

Edit: if you want to use the argument as something to assign the value to, you can write something like this

void create_pkt(u_char** pkt)
{
  ....
  *pkt = (u_char *)malloc(40);
  ...
}

and call it with

create_pkt(&pkt);

but I can't really recomment that.

Upvotes: 2

Valeri Atamaniouk
Valeri Atamaniouk

Reputation: 5163

You are trying to allocate the memory and loosing the allocated reference. So the garbage is sent and then attempt to release kills it.

u_char* create_pkt()
{
   u_char* pkt;
   ....
   pkt = (u_char *)malloc(40);
  ...
   return pkt;
}


int main()
{
   ....
   u_char* pkt;
   pkt = create_pkt();
   if (sendto(sd, pkt, 40, 0, (struct sockaddr *)&sin, sizeof(struct sockaddr)) < 0)
   free(pkt);
}

Upvotes: 0

Tony The Lion
Tony The Lion

Reputation: 63200

u_char* create_pkt(u_char* pkt) copies your pointer and then allocates it inside, but only allocates the copy. When the function returns your original pointer is still as it was, unallocated.

Now you can either return a pointer from this function or pass in a double pointer u_char** pkt and assign the address of pkt to it.

pkt = create_ptk(pkt); now you'll have allocated pkt.

for double pointer version this is how you'd call it:

create_pkt(&pkt);

Upvotes: 0

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