Hello,
I have been trying to solve this problem for a couple of days by now. I am pretty sure that this is a simple pointer arithmetics error on my part, but still, this is driving me crazy and I hope you don't mind asking me this question here.

I am writing a simple external which adds all the inlet signals. I expect to allow any number of inlets, so I am using the dsp_addv method. Here, for debugging purposes I am only using two inlets. When I access the inlets like in the example above everything works as expected.

t_int *ak_addthese_tilde_perform(t_int *w) {
    int i, j;  
   t_ak_addthese_tilde *x = (t_ak_addthese_tilde *)(w[1]);  
   int n = (int)(w[2]);  
   t_sample * in1 = (t_sample *)(w[3]);  
   t_sample * in2 = (t_sample *)(w[4]);  
   t_sample * out = (t_sample *)(w[5]); 

   while(n--) { 
     *out++ = *in1++ + *in2++; 
   } 

   return (w + 6); 
}

When I access all input signals with an array of pointers, I cannot hear the rightmost signal in the output. So in this code above I only hear the signal from the first inlet. Like so,

t_int *ak_addthese_tilde_perform(t_int *w) {
  int i, j;
  t_ak_addthese_tilde *x = (t_ak_addthese_tilde *)(w[1]);
  t_int n = (t_int)(w[2]);

  t_sample ** ins = getbytes(sizeof(t_sample *) * 2);
  t_sample * out = (t_sample *)(w[5]);

  for(i = 0; i < 2; i++) {
    ins[i] = (t_sample *)(w[3 + i]);
  }

  while(n--) {
    *out = 0.0;
    for(j = 0; j < 2; j++) {

      *out += *ins[j]++;
    }
    out++;
  }

  return (w + 6);
}
I would be grateful if you could point out where I am mistaken.
Best[/i]

I asked this question on the pd-list and IOhannes gave an answer that worked.
He explained that since signals can share memory, the code might overwrite the data in the last inlet.
So changing the code like shown below solved the problem.

t_sample temp;
while(n--) {
   temp = 0;
   for(j = 0; j < 2; i++) {
       temp += *ins[j]++;
   }
   *out++ = temp;
}

Best