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Comments on What is CPU endianness?

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What is CPU endianness?

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I was fooling around with the following C code on my trusty old x86 PC:

#include <stdint.h>
#include <stdio.h>

int main (void)
{
  uint32_t u32 = 0xAABBCCDD;
  uint8_t* ptr = (uint8_t*)&u32;
  for(size_t i=0; i<sizeof(uint32_t); i++)
  {
    printf("%.2X", ptr[i]);
  }
}

To my surprise, this prints DDCCBBAA with all bytes backwards. Someone told me this was because of "endianness" and that my x86 is "little endian". What is the meaning of this?

big-endian litte-endian endianness
posted almost 4 years ago
4y ago
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Lundin‭
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1 comment thread

General comments (7 comments)
General comments
C. K. Young‭ wrote over 3 years ago · edited over 3 years ago
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The code snippet in the question violates the strict aliasing rule, and is undefined behaviour. :-( The safe C way to type-pun is to use a union. I'll post a suggested edit with this (feel free to reformat/edit to taste).

Lundin‭ wrote over 3 years ago · edited over 3 years ago
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@Chris Jester-Young‭ No, that's wrong. There is a special rule allowing us to inspect any type in C by using a character type (uint8_t is always a character type if supported). C17 6.3.2.3/7. "When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object. Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object."

Lundin‭ wrote over 3 years ago
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@Chris Jester-Young‭ Furthermore, if you read the actual "strict aliasing rule", it has an explicit exception for lvalue access through a character type. C17 6.5/7: "An object shall have its stored value accessed only by an lvalue expression that has one of the following types: ... - a character type."

dmckee‭ wrote over 3 years ago
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Anyone one who regularly crosses the line between c and c++ should keep in mind that c++’s strict aliasing rule is stricter than c’s. Punning in-place is possible a couple of ways in c and essentially forbidden in c++. Drives me bats because the committee could have made an exception for POD types, but there you have it.

jrh‭ wrote over 3 years ago · edited over 3 years ago
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@dmckee you can use attributes to make specific types exempt from strict aliasing, also note that MSVC has no strict aliasing. Lundin, would you add this attribute to your post behind a "not MSVC" guard? Or mention fno-strict-aliasing? I've found that very few people know about type punning and I've found some real messes over the years.

Skipping 1 deleted comment.

jrh‭ wrote over 3 years ago
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Ok, I'm not really 100% sure whether you avoided type punning UB, to be very pedantic about it uint8_t* is probably a char but might not be, you might not be completely off the hook in C either

Lundin‭ wrote over 3 years ago
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@jrh uint8_t will have to be a character type if supported, I've written answers explaining why on SO. But please don't derail comments further; this has nothing to do with endianess. If you have a question about strict aliasing, please ask a separate question.