Decoding negative integer values with a length of 3 byte (24bit) is not correct
Closed this issue · 2 comments
Eduard-G commented
A negative integer value of 3 bytes in length (for example -8336180) is incorrectly decoded from BACnet. That values are decoding in C# to an integer type (32 bit), but the highest 8 bits are not set to TRUE. That's necessary to get a negative 2's complement integer value.
Original code in ASN1.cs (starting at line 1176):
public static int decode_signed24(byte[] buffer, int offset, out int value)
{
value = (buffer[offset + 0] << 16) & 0x00ff0000;
value |= (buffer[offset + 1] << 8) & 0x0000ff00;
value |= buffer[offset + 2] & 0x000000ff;
return 3;
}Modified code that should be work:
public static int decode_signed24(byte[] buffer, int offset, out int value)
{
value = (buffer[offset + 0] << 16) & 0x00ff0000;
if ((value & 0x800000) != 0)
unchecked { value |= (int)0xff000000; }
value |= (buffer[offset + 1] << 8) & 0x0000ff00;
value |= buffer[offset + 2] & 0x000000ff;
return 3;
}