How to write negative binary number?

num = (num << 9) | (-7 & 0x1ff)

11111111111111111111111111111101×11111111111111111111111111111101 =
 1111111111111111111111111111101000000000000000000000000000001001
        (32 msb)                         (32 lsb)

void decTobin(int dec, int s) {
  //  while loop does not set all `b`,but following for loop uses all `b`
  // int b[s], i = 0;  
  int b[s] = { 0 }; // or int b[s]; memset(b, 0, sizeof b);
  int i = 0; 
}

void ndecTobin(int dec, int s) {
  int b[s], i = 0, a[s], decimal, decimalvalue = 0, g;
  decimal = 0;
  ...
  decimal += decimalvalue;
}

void decTobin(unsigned number, unsigned width) {
  int digit[width];
  for (unsigned i = width; i-- > 0; ) {
    digit[i] = number % 2;
    number /= 2;
  }

  printf("%u ", number);  // assume this is for debug

  for (unsigned i = 0; i<width; i++) {
    printf("%u", digit[i]);
  }
}