Most performant way to compare first 2 bytes (given a pointer) to fixed values?

Question

Given a pointer I want to compare the first two bytes to fixed values. data is a void pointer. Is there a "better" way than this:

unsigned char foo[] = {0xFF, 0x3B};
memcmp(data, foo, 2);

Maybe where I dont have to create a new char array? Thanks!

Have you profiled to assume that that is the bottleneck in your application? — RedX, Oct 02 '13 at 09:43

LS_ᴅᴇᴠ · Answer 1 · 2013-10-02T10:01:37.210

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EDIT: Due to some concerns regarding sizeof(char), memory alignment and compiler/library optimizations,

As a PLATFORM DEPENDENT alternative, this answer MAY HAVE better performance then memcmp:

Little endian (Intel byte-order):

if (*(short*)foo == 0x3bff) ...

Big endian (Network byte order):

if (*(short*)foo == 0xff3b) ...

edited Oct 02 '13 at 10:01

answered Oct 02 '13 at 09:41

LS_ᴅᴇᴠ

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@redx: Nop, but avoiding a function call seams a big gain to me! – LS_ᴅᴇᴠ Oct 02 '13 at 09:44
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memcmp should be inlined with optimizations on. – RedX Oct 02 '13 at 09:44
I suppose so. But compiler should also optimize this comparison. – LS_ᴅᴇᴠ Oct 02 '13 at 09:48
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`memcmp` can be used to compare smallest possible types (`char`), so it must do comparison in a manner where alignment is not an issue. – user694733 Oct 02 '13 at 09:54
No, memcmp will not have this problem. You have to maintain 2 lines of code depending on the architecture you're running on. And this code is not portable at all: you're assuming `char` is 8 bits which is not guaranteed. – fjardon Oct 02 '13 at 09:56
@fjardon Also a good point. Added a advice clause in answer! – LS_ᴅᴇᴠ Oct 02 '13 at 10:03

score 1 · Accepted Answer · edited May 23 '17 at 12:03

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You should certainly try doing it using memcmp(), but if that creates overhead you can do it like @LS_dev suggested, although I would suggest doing explicit character accesses to avoid the endianness issue:

if(((unsigned char *) data)[0] == 0xff && ((unsigned char *) data)[1] == 0x3b)
{
}

of course, it would make sense to factor out the casting for a major clarity boost:

const unsigned char *chardata = data;
if(chardata[0] == 0xff && chardata[1] == 0x3b)
{
}

edited May 23 '17 at 12:03

Community

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answered Oct 02 '13 at 09:58

unwind

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+1 I thought about this also, but although safer, implies more operations. – LS_ᴅᴇᴠ Oct 02 '13 at 10:04

Most performant way to compare first 2 bytes (given a pointer) to fixed values?

2 Answers2