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[Cyan~Fire]

I'm writing some code for an app that reads mac datafiles (Macs are big-endian), and here's the endian swap func I just wrote for it. Of course, I want this to be lightning fast since it's used so much, so I was wondering if any of you could see any way to improve this. Thanks.

Code: Show/Hide void SwapEndian(void *pData, int iBytes) {    if (iBytes % 2)   //even check       return;    char *pSwapped = (char*)malloc(iBytes);    char *source = (char*)pData + iBytes;    char *dest = pSwapped;    while (iBytes--)       *dest++ = *source--;    memcpy(pData, pSwapped, iBytes);    free(pSwapped); }

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[Mr Ekted]

OMG! That is the single most inefficient way to do it. You really couldn't make it any worse without Qndre's help...

Code: Show/Hide void SwapEndian (void *pData, int iBytes) { char *p = (char *)pData; char *q = p + iBytes - 1; // don't even check for odd while (iBytes--)    *p++ = *q--; }

If you want to be even more efficient, write SWAP2(), SWAP4(), SWAP8() macros that do it all inline...

Code: Show/Hide #define SWAP2(x) ((x >> 8) | ((x & 0xff) << 8)) etc

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4,691 irradiated haggis!

[Cyan~Fire]

Umm wait is it just me or will that start overwriting data in pData as it copies? There has to be a temporary buffer or you get data loss.

And that's not the most inefficient way to do it, take a look at this code someone else wrote:

Code: Show/Hide inline void SwapEndian64(char *pData) {    char cTemp;    cTemp = pData[0];    pData[0] = pData[7];    pData[7] = cTemp;    cTemp = pData[1];    pData[1] = pData[6];    pData[6] = cTemp;    cTemp = pData[2];    pData[2] = pData[5];    pData[5] = cTemp;    cTemp = pData[3];    pData[3] = pData[4];    pData[4] = cTemp; }

Edit: Yep, just tested it.

Code: Show/Hide void SwapEndian (void *pData, int iBytes) { char *p = (char *)pData; char *q = p + iBytes - 1; // don't even check for odd while (iBytes--)    *p++ = *q--; } int main(int argv, char **argc) {    char test[8] = { 1, 2, 3, 4, 5, 6, 7, 8 };    SwapEndian(test, 8);    return 0; }

In the memory debug window, test ended up being: 08 07 06 05 05 06 07 08. Cool little relfection thingy though

[Cyan~Fire]

Here, since this function shouldn't ever be used for anything above 10 bytes, I'll use a constant size.

Code: Show/Hide void SwapEndian(void *pData, int iBytes) {    char swapped[10];   //should not be used above 80 bits    if (iBytes % 2)   //even check       return;    char *source = (char*)pData + iBytes;    char *dest = swapped;    while (iBytes--)       *dest++ = *source--;    memcpy(pData, swapped, iBytes); }

[Mr Ekted]

You are right, but this is best...

Code: Show/Hide void SwapEndian (void *pData, int iBytes) { char *p = (char *)pData; char *q = p + iBytes - 1; char  ch; // don't even check for odd iBytes >>= 1; while (iBytes--)    {    ch = *p;    *p++ = *q;    *q-- = ch;    } }

[SuSE]

Code: Show/Hide void switchtomac, it's cool { la la la smurfs are fun }

[Dustpuppy]

Code: Show/Hide void switchtomac, it's cool { la la la smurfs are fun. }

You forgot a period, that wouldn't compile
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[SuSE]

LIES!! I HAVE CODED A PROGRAM IN ASM THAT WILL EXTRACT YOUR BRAIN!!

[CypherJF]

*confused*
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Performance is often the art of cheating carefully. - James Gosling

[D1st0rt]

Ekted lol, I was at a bookstore and I saw a book you could've written. "How NOT to program in C++ (119 Programs that don't work)"
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[Mr Ekted]

LOL!

[Dustpuppy]

[Mr Ekted]

The one with the 536,189 bit key? It's hopeless...

[Dustpuppy]

[SuSE]

lol, god bless qndre