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cendiannessmodbusmodbus-tcp

Can I specify endianness when reinterpreting as a float?

I am writing a C program that communicates over modbus TCP. I send the float 3.14 on my modbus client, with a hexadecimal representation of 0x4048F5C3. My C code is correctly receiving the modbus packet, and now I just need to reinterpret the unsigned char *buffer as a float so I can get the value 3.14. Here is my code

int main()
{
    setup_modbus_tcp();
    unsigned char buffer[4] = { 0x00, 0x00, 0x00, 0x00 };
    get_modbus_data(buffer, 4);
        //Buffer now holds { 0x40, 0x48, 0xF5, 0xC3 };
    float value = *(float *)buffer;
        //value is now -490.564453
}

Now, I can tell that this problem is because of endianness, because if I swap endianness so that the buffer looks like this:

{ 0xC3, 0xF5, 0x48, 0x40 };

then everything works fine. But this feels a little bit messy. Do I have to swap endianness? Or is there a way that I can specify endianness when I do:

float value = *(float *)buffer;
Solution

  • The buffer must be handled manually, this includes changes to endianness. A function can always be used that converts the buffer to a floating point format.

    float value = Convert( buffer );

    To add, you way of converting the buffer is incorrect:

    float value = *(float *)buffer;

    An array of unsigned chars cannot be simply to a float type, as it causes undefined behavior. The memory must be copied to an object of type float:

    float value;
memcpy( &value , buffer , sizeof( value ) );  //assumes endianness has already been handled