Check endianness programmatically

Question

I know that the most common method to test endianity programmatically is to cast to char* like this:

short temp = 0x1234;
char* tempChar = (char*)&temp;

But can it be done by casting to short* like this:

unsigned char test[2] = {1,0};  
if ( *(short *)test == 1)
    //Little-Endian
else
    //Big-Endian

Am I right that the "test" buffer will be saved (on x86 platforms) in the memory using Little-Endian convention (from right-to-left: "0" at lower address, "1" at higher) just like in case with the "temp" var?

And more generally if I have a string: char tab[] = "abcdef"; How would it be stored in the memory? Will it be reversed like: "fedcba"?

Thx. in advance:-)

PS.

Is there any way to see how exactly the data of a program look like in the system memory?. I would like to see that byte-swap in Little-Endian in "real life".

Possible duplicate of http://stackoverflow.com/questions/1001307/detecting-endianness-programmatically-in-a-c-program — David Hammen, Jun 25 '12 at 21:23
For your PS, use a debugger to inspect the memory at the address of a variable with integral type. If it has a non-symmetric value, you should be able to look at the memory contents and see the difference if you compare a big endian machine with a little endian machine. — jxh, Jun 25 '12 at 21:28
1) Your second example could fail, even on the correctly-endian machine, due to alignment failure. 2) There are more than two endiannesses in the world. — Robᵩ, Jun 25 '12 at 21:47

score 2 · Answer 1 · answered Jun 25 '12 at 21:16

2

Your code would probably work in practice (you could have just tried it!). However, technically, it invokes undefined behaviour; the standard doesn't allow you to access a char array through a pointer of another type.

And more generally if I have a string: char tab[] = "abcdef"; How would it be stored in the memory? Will it be reversed like: "fedcba"?

No. Otherwise tab[0] would give you f.

answered Jun 25 '12 at 21:16

Oliver Charlesworth

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Yes, bu tell me why do I get "" when – Mulligan Jun 25 '12 at 21:40
As far as I can tell, if you want to avoid undefined behavior in ISO C++, then you cannot check for endianess. ISO C++ is free of any references to stroage-endianness. – j_kubik Jun 25 '12 at 22:44
@j_kubik: I'm distinguishing between undefined behaviour and implementation-defined behaviour... – Oliver Charlesworth Jun 25 '12 at 22:45

score 0 · Answer 2 · answered Jun 25 '12 at 21:38

0

Your alternative method for checking endianness would work.

char tab[] = "abcdef" would be stored in that same order: abcdef

Endianness comes into play when you access multiple bytes (short, int, and so on). When you try to access tab[] as a short array using a little endian machine, you'd read it as ba, dc, fe (whatever their actual byte equivalents are, this is the order the chars are "evaluated" in the short).

answered Jun 25 '12 at 21:38

Pochi

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So i'm confused:-( In such a case endianness determines the order the bytes are stored/write into memory or rather, as You suggesting get/read/accessed??? – Mulligan Jun 25 '12 at 21:52
1

Yes, you only need to care about endianness when you're reading/writing multiple bytes in some data structure (if you read just one byte, it's the same in either little or big endian). – Pochi Jun 25 '12 at 22:12

eidolon · Answer 3 · 2012-06-26T00:31:48.797

Both ways are not guaranteed to work, furthermore, latter invokes undefined behavior.

First fails if sizeof(char) == sizeof(short).

Second may fail for the same reason, and is also unsafe: result of the pointer cast may have wrong alignment for short, and accessing the (short) value invokes undefined behavior (3.10.15).

But yes, the char buffer is stored sequentially into memory so that &test[0] < &test[1], and more generally, as others have already said, char tab[] = "abcdef" is not reversed or otherwise permuted regardless of endianness.

score 0 · Answer 4 · answered Jun 25 '12 at 22:58

0

It would be safer, i.e. standards-compliant, to use a union.

answered Jun 25 '12 at 22:58

user207421

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Check endianness programmatically

4 Answers4