converting unsigned char > 255 to int

Question

I'm programming with a PLC and I'm reading values out of it.

It gives me the data in unsigned char . That's fine, but the values in my PLC can be over 255. And since unsigned chars can't give a value over 255 I get the wrong information.

The structure I get from the library:

struct PlcVarValue
{
    unsigned long   ulTimeStamp         ALIGNATTRIB;
    unsigned char   bQuality            ALIGNATTRIB;
    unsigned char   byData[1]           ALIGNATTRIB;
};

• ulTimeStamp gives the time
• bQuality gives true/false (be able to read it or not)
• byData[1] gives the data.

Anyways I'm trying this now: (where ppValues is an object of PlcVarValue )

    unsigned char* variableValue =  ppValues[0]->byData;            
    int iVariableValue = *variableValue;

This works fine... untill ppValues[0]->byData is > 255;

When I try the following when the number is for example 257:

unsigned char testValue = ppValues[0]->byData[0];
unsigned char testValue2 = ppValues[0]->byData[1];

the output is testvalue = 1 and testvalue2 = 1

that doesn't make sense to me.

So my question is, how can I get this solved so it gives me the correct number?

Answer 1

That actually looks like a variable-sized structure, where having an array of size 1 at the end being a common way to have it. See eg this tutorial about it .

In this case, both bytes being 1 for the value 257 is the correct values. Think of the two bytes as a 16-bit value, and combine the bits. One byte will become the hight byte, where 1 corresponds to 256 , and then add the low bytes which is 1 and you have 256 + 1 which of course is equal to 257 . Simple binary arithmetic.

Which byte is the high, and which is the low we can't say, but it's easy to check if you can force a message that contains the value 258 instead, as then one byte will still be 1 but the other will be 2 .

How to combine it into a single unsigned 16-bit value is also easy if you know the bitwise shift and or operators:

uint8_t high_byte = ...
uint8_t low_byte = ...
uint16_t word = high_byte << 8 | low_byte;