CODEWITHSUNDEEP
cpointerssegmentation-fault
phocean
phocean

Reputation: 31

Issue with C double pointer allocation

I read tons of tutos and snippets, but I still don't understand why I get a segfault with this:

int fun(char **p) {

  int i;

  *p = malloc(2);
  *p[0]=10;
  *p[1]=20; // segfault NULL pointer

  printf("fun()/n");
  for (i=0; i<2; i++)
   printf("%d ",*p[i]);
}

int main(int argc, const char *argv[])
{
  char* buffer;
  int i;

  fun(&buffer);

  printf("main()\n");

  for (i=0; i<2; i++)
   printf("%d ",buffer[i]);

  return 0;
}

In gdb, it gives:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x0000000100000dea in fun (p=0x7fff5fbffab0) at test.c:10
10    *p[1]=20;
(gdb) p *p[0]
$1 = 10 '\n'
(gdb) p *p[1]
Cannot access memory at address 0x0
(gdb)

I have seen a lot of similar snippets, but there is surely something I am deeply misunderstanding.

Upvotes: 2

Views: 215

Answers (2)

Nitish
Nitish

Reputation: 11

try returning the address of the same data type of the variable at the LHS. malloc() by default returns a void value. p = (char) malloc(2); this way compiler knows how many bytes it has to move the pointer to fetch new variable.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477040

You mean (*p)[1]. What you said is *(p[1]).

Upvotes: 4

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