Getting incorrect return in a function

[ThiagoCavalcante]

Hi guys, I'm not really good at C/C++ and I'm trying to get my code work for a few days but I couldn't.

MAT2 _m_pow2(const MAT2 A, int p)
{
  MAT2 out, aux, aux2, z, tmp;
  int it_cnt, k, max_bit;

#define	Z(k) (((k) & 1) ? &tmp : &out)

  out = m_get2(A.m, A.n);
  tmp = m_get2(A.m, A.n);

  if(p == 0)
    out = m_ident2(out);
  else if(p > 0)
  {
    it_cnt = 1;
    for(max_bit = 0; ; max_bit++)
      if((p >> (max_bit+1)) == 0)
        break;
      tmp = m_copy2(A);
      for(k = 0; k < max_bit; k++)
      {
    	printf("k=%d\n", k); // This printf makes it works.
        *Z(it_cnt+1) = m_mlt2(*Z(it_cnt), *Z(it_cnt));
        it_cnt++;
        if(p & (1 << (max_bit-1)))
        {
          *Z(it_cnt+1) = m_mlt2(A, *Z(it_cnt));
          it_cnt++;
        }
        *Z(it_cnt+1) = m_zero2(*Z(it_cnt+1));
        p <<= 1;
      }
      if(it_cnt & 1)
      {
        out = m_copy2(*Z(it_cnt));
      }
  }

  return out;

#undef Z
}

When I insert a printf the result return the correct value. Is there anything wrong with this code, pointer or something else? What I am doing wrong? Thanks in advance!

[ne555]

post a minimal code example that does reproduce your issue.
we need to be able to compile and run your code.

your code is missing the headers of who-knows-what library you're using for matrix operations.
post a link to the library and its documentation


> *Z(it_cnt+1) = m_mlt2(A, *Z(it_cnt));
it would depend on the library, but if it's written in C, then that's quite likely a memory leak.
also, ¿is `A' a square matrix?

[ThiagoCavalcante]

Hi, firstly thanks for your reply. So, I'm not using a library, all the functions are implemented. There are a lot of code and I couldn't post the rest. The struct MAT2 is:

typedef	struct
{
  unsigned int m, n;
  unsigned int max_m, max_n, max_size;
  double me[MAX_SIZE][MAX_SIZE];   /* base is base of alloc'd mem */
}
MAT2;

The function m_get() simply initialize the matrix with zero, the function m_ident2() returns an identity matrix, the function m_copy2() simply copy the contenty of a matrix into a new matrix (not using pointer, I'm trying to not use pointers and dynamic allocation for a reason), the function m_mlt2() multiply two matrices.

It's written in C and I also think is a memory leak, but I don't knoow how to fix it. Yes, thematrix A is square. All the matrices by default is a 10x10 matrix, for example, if I need a 5x5 matrix, I can do it, but the rest elements will be zero.

[ne555]

upload to github or similar.
perhaps the mistake is in one of those other functions.

> When I insert a printf the result return the correct value.
so, ¿what does happen without the printf()? ¿crash, random values, incorrect but always the same?
i suspect out of bounds access, but i'm blind here.

[ThiagoCavalcante]

the result is a random value. I've tested the functions separately and it worked as expect, so, returned the correct value. I already implemented all that using pointers and dynamic allocation and it worked normally. Howerver, I tried to use static structures and avoid pointers, and the results changed. Seems that it's something about the position of printf in the stack.