CODEWITHSUNDEEP
c++cmemory
BeeBand
BeeBand

Reputation: 11513

Does endianness have an effect when copying bytes in memory?

Am I right in thinking that endianess is only relevant when we're talking about how to store a value and not relevant when copying memory?

For example

if I have a value 0xf2fe0000 and store it on a little endian system - the bytes get stored in the order 00, 00, fe and f2. But on a big endian system the bytes get stored f2, fe, 00 and 00.

Now - if I simply want to copy these 4 bytes to another 4 bytes (on the same system), on a little endian system am I going to end up with another 4 bytes containing 00, 00, fe and f2 in that order?

Or does endianness have an effect when copying these bytes in memory?

Upvotes: 4

Views: 2065

Answers (7)

Jyotishkar Dey
Jyotishkar Dey

Reputation: 1

Memcpy just copies bytes and doesn't care about endianness. So if you want to copy one network stream to another use memcpy.

Upvotes: 0

MattBianco
MattBianco

Reputation: 1531

Yes, you are correct thinking that you should be endianness-aware when storing or communicating binary values outside your current "scope". Generally you dont need to worry as long as everything is just inside your own program. If you copy memory, have in mind what you are copying. (You could get in trouble if you store long values and read ints).
Have a look at htonl(3) or books about network programming for some good explanations.

Upvotes: 0

Chubsdad
Chubsdad

Reputation: 25537

Answering the question title.

Assume 'int' to be of 4 bytes

union{
   unsigned int i;
   char a[4];
};

// elsewhere
i = 0x12345678;
cout << a[0];   // output depends on endianness. This is relevant during porting code
                // to different architectures

So, it is not about copying (alone)? It's about how you access?

It is also of significance while transferring raw bytes over a network!.

Here's the info on finding endianness programatically

Upvotes: 4

MSalters
MSalters

Reputation: 180245

memcpy doesn't know what it is copying. If it has to copy 43 61 74 00, it doesn't know whether it is copying 0x00746143 or 0x43617400 or a float or "Cat"

Upvotes: 3

sbi
sbi

Reputation: 224179

Basically, you have to worry about endianess only when you need to transfer binary data between architectures which differ in endianess.

However, when you transfer binary data between architectures, you will also have to worry about other things, like the size of integer types, the format of floating numbers and other nasty headaches.

Upvotes: 2

Bart van Ingen Schenau
Bart van Ingen Schenau

Reputation: 15768

Endianness is only relevant in two scenarios

  1. When manually inspecting a byte-dump of a multibyte object, you need to know if the bytes are ordered in little endian or big endian order to be able to correctly interpret the bytes.
  2. When the program is communicating multibyte values with the outside world, e.g. over a network connection or a file. Then both parties need to agree on the endianness used in the communication and, if needed, convert between the internal and external byte orders.

Upvotes: 7

Tristan
Tristan

Reputation: 3885

no when working on the same machine you don't have to worry about endianess, only when transferring binary data between little and big endian machines

Upvotes: 3

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