将浮点值从大端转换为小端

Question

Is it possible to convert float s from big to little endian? I have a big endian value from a PowerPC platform that I am sendING via TCP to a Windows process (little endian). This value is a float , but when I memcpy the value into a Win32 float type and then call _byteswap_ulong on that value, I always get 0.0000?

What am I doing wrong?

Answer 1

simply reverse the four bytes works

float ReverseFloat( const float inFloat )
{
   float retVal;
   char *floatToConvert = ( char* ) & inFloat;
   char *returnFloat = ( char* ) & retVal;

   // swap the bytes into a temporary buffer
   returnFloat[0] = floatToConvert[3];
   returnFloat[1] = floatToConvert[2];
   returnFloat[2] = floatToConvert[1];
   returnFloat[3] = floatToConvert[0];

   return retVal;
}

Answer 2

Here is a function can reverse byte order of any type.

template <typename T>
T bswap(T val) {
    T retVal;
    char *pVal = (char*) &val;
    char *pRetVal = (char*)&retVal;
    int size = sizeof(T);
    for(int i=0; i<size; i++) {
        pRetVal[size-1-i] = pVal[i];
    }

    return retVal;
}

Answer 3

I found something roughly like this a long time ago. It was good for a laugh , but ingest at your own peril. I've not even compiled it:

void * endian_swap(void * arg)
{
    unsigned int n = *((int*)arg);
    n = ((n >>  8) & 0x00ff00ff) | ((n <<  8) & 0xff00ff00);
    n = ((n >> 16) & 0x0000ffff) | ((n << 16) & 0xffff0000);
    *arg = n;   

    return arg;
}

Answer 4

An elegant way to do the byte exchange is to use a union:

float big2little (float f)
{
    union
    {
        float f;
        char b[4];
    } src, dst;

    src.f = f;
    dst.b[3] = src.b[0];
    dst.b[2] = src.b[1];
    dst.b[1] = src.b[2];
    dst.b[0] = src.b[3];
    return dst.f;
}

Following jjmerelo's recommendation to write a loop, a more generic solution could be:

typedef float number_t;
#define NUMBER_SIZE sizeof(number_t)

number_t big2little (number_t n)
{
    union
    {
        number_t n;
        char b[NUMBER_SIZE];
    } src, dst;

    src.n = n;
    for (size_t i=0; i<NUMBER_SIZE; i++)
        dst.b[i] = src.b[NUMBER_SIZE-1 - i];

    return dst.n;
}

Answer 5

Don't memcpy the data directly into a float type. Keep it as char data, swap the bytes and then treat it as a float.

Answer 6

It might be easier to use the ntoa and related functions to convert from network to host and from host to network..the advantage it would be portable. Here is a link to an article that explains how to do this.

Answer 7

From SDL_endian.h with slight changes:

std::uint32_t Swap32(std::uint32_t x)
{
    return static_cast<std::uint32_t>((x << 24) | ((x << 8) & 0x00FF0000) |
                                      ((x >> 8) & 0x0000FF00) | (x >> 24));
}

float SwapFloat(float x)
{
    union
    {
        float f;
        std::uint32_t ui32;
    } swapper;
    swapper.f = x;
    swapper.ui32 = Swap32(swapper.ui32);
    return swapper.f;
}

Answer 8

This value is a float, but when I "memcpy" the value into a win32 float type and then call _byteswap_ulong on that value, I always get 0.0000?

This should work. Can you post the code you have?

However, if you care for performance (perhaps you do not, in that case you can ignore the rest), it should be possible to avoid memcpy, either by directly loading it into the target location and swapping the bytes there, or using a swap which does the swapping while copying.

Answer 9

in some case, especially on modbus: network byte order for a float is:

nfloat[0] = float[1]
nfloat[1] = float[0]
nfloat[2] = float[3]
nfloat[3] = float[2]

Answer 10

A nice way to get to the value is to use struct.pack/unpack.
You can use the ">" character in the format to indicate that the bytes are in reverse (big endian).

from struct import unpack, pack
sebytes = '\xc8\x00\x00\x00'
print unpack('l', sebytes)
bebytes = '\x00\x00\x00\xc8'
print unpack('>l', bebytes)

output:

(200,)
(200,)