[英]converting unsigned char > 255 to int
I'm programming with a PLC and I'm reading values out of it. 我正在用PLC编程,我正在读取它的价值。
It gives me the data in unsigned char
. 它为我提供了
unsigned char
的数据。 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. 这很好,但我的PLC中的值可能超过255.由于无符号字符不能给出超过255的值,我得到错误的信息。
The structure I get from the library: 我从图书馆得到的结构:
struct PlcVarValue
{
unsigned long ulTimeStamp ALIGNATTRIB;
unsigned char bQuality ALIGNATTRIB;
unsigned char byData[1] ALIGNATTRIB;
};
ulTimeStamp
gives the time ulTimeStamp
给出了时间 bQuality
gives true/false (be able to read it or not) bQuality
给出真/假(能否读取或不读) byData[1]
gives the data. byData[1]
给出了数据。 Anyways I'm trying this now: (where ppValues
is an object of PlcVarValue
) 反正我现在尝试此:(其中
ppValues
是一个对象PlcVarValue
)
unsigned char* variableValue = ppValues[0]->byData;
int iVariableValue = *variableValue;
This works fine... untill ppValues[0]->byData
is > 255; 这很好用......直到
ppValues[0]->byData
> 255;
When I try the following when the number is for example 257: 当我尝试以下数字时,例如257:
unsigned char testValue = ppValues[0]->byData[0];
unsigned char testValue2 = ppValues[0]->byData[1];
the output is testvalue = 1
and testvalue2 = 1
输出为
testvalue = 1
, testvalue2 = 1
that doesn't make sense to me. 这对我没有意义。
So my question is, how can I get this solved so it gives me the correct number? 所以我的问题是,我怎样才能解决这个问题,以便给出正确的数字?
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. 这实际上看起来像一个可变大小的结构,其中最后有一个大小为
1
的数组是获得它的常用方法。 See eg this tutorial about it . 参见例如本教程 。
In this case, both bytes being 1
for the value 257
is the correct values. 在这种情况下,值
257
两个字节都是1
是正确的值。 Think of the two bytes as a 16-bit value, and combine the bits. 将这两个字节视为16位值,并将这些位组合在一起。 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
. 一个字节将成为高字节,其中
1
对应于256
,然后添加低字节,即1
,你有256 + 1
,当然等于257
。 Simple binary arithmetic. 简单的二进制算术
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
. 哪个字节是高,哪个是我们不能说的低,但很容易检查是否可以强制包含值
258
的消息,因为那时一个字节仍然是1
但另一个将是2
。
How to combine it into a single unsigned 16-bit value is also easy if you know the bitwise shift and or operators: 如果您知道按位移位和/或运算符,如何将它组合成单个无符号16位值也很容易:
uint8_t high_byte = ...
uint8_t low_byte = ...
uint16_t word = high_byte << 8 | low_byte;
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