用C语言解释32位无符号长单精度IEEE-754浮点数

Question

I am using the XC32 compiler from Microchip, which is based on the standard C compiler.

I am reading a 32bit value from a device on a RS485 network and storing this in a unsigned long that I have typedef'ed as DWORD.

ie

typedef DWORD unsigned long;

As it stands, when I typecast this value to a float, the value I get is basically the floating point version of it's integer representation and not the proper IEEE-754 interpreted float.

ie

DWORD dword_value = readValueOnRS485();
float temp = (float)dword_value;

Here, dword_value would come through in HEX format as 0x4366C0C4 which as a decimal would be represented as 1130807492 so the typecasting of this to a float simply gives me 1.130807492*10^9 or 1130807492.0 which is not what I want.

I want the single precision IEEE-754 representation which would give me a float value of 230.75299072265625

So clearly typecasting to float doesn't work for me. I need a method that can convert this form me. I have looked all over in the XC32 library and I cannot find anything.

Does anyone know of a predefined method that does this interpretation properly for me? Or is there maybe some suggested method I can write? I am trying to avoid writing my own code for this specific task as I am worried I do not find an efficient solution if C already has a function for this.

The interesting thing is, if I do this to a char*, the value is represented on that char* correctly as 230.75:

sprintf(random_char_pointer, "%.2f, dword_value);

Here printing random_char_pointer to screen gives me 230.75 so the sprintf must be handling the interpreting for me correctly. Therefore I am assuming there is something in C already for me. Can anyone assist?

Answer 1

The recommended way to do stuff like this is to use a union:

union {
    DWORD w;
    float f;
} wordfloat;

wordfloat.w = dword_value;
temp = wordfloat.f;

This does what you want as per ISO 9899:2011 §6.5.2.3 ¶3 footnote 95:

A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member, ⁹⁵⁾ and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member.

95) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation.