Question

I want to compare two Byte arrays.
byInputBuffer is an array which I filled by external device. 256 bytes comes in every main loop iteration and is always put in the beginning of byInputBuffer. It is declared like here:

byInputBuffer: array [0..1023] of byte;

DataArray is an array with dynamically declared size. So array is declared like here:

DataArray  : Array of Byte;

and then in code size is declared like here:

 SetLength(DataArray, DataLengthInt);

where DataLengthInt is 130952.

Main loop is iterated from 0 to 511, but when i=0 I get BoolToStr(verify)=-1 in code presented below:

leftBytes := 256;

verify := CompareMem(@byInputBuffer, @DataArray[(i*256)], leftBytes);

But I checked and contents of both arrays when i=0 is equal for the first 256 elements.

My question is why CompareMem returns -1?

Other used declarations:

  leftBytes : Integer;
  verify : Boolean;

Answer 1

You are calling CompareMem to compare the first 256 bytes of byInputBuffer against the first 256 bytes of DataArray.

You are then taking the return value of CompareMem and passing it through BoolToStr. That maps False to '0' and True to '-1'.

The conclusion therefore is that the first 256 bytes of the two arrays are equal. Because CompareMem returned True.

Note that your statement in the question that CompareMem returned -1 is obviously incorrect. That is a big part of our confusion. Precision and accuracy are crucial when programming.