Detecting endianness programmatically in a C++ pro

Question

Is there a programmatic way to detect whether or not you are on a big-endian or little-endian architecture? I need to be able to write code that will execute on an Intel or PPC system and use exactly the same code (i.e. no conditional compilation).

Answer 1

Unless the endian header is GCC-only, it provides macros you can use.

#include "endian.h"
...
if (__BYTE_ORDER == __LITTLE_ENDIAN) { ... }
else if (__BYTE_ORDER == __BIG_ENDIAN) { ... }
else { throw std::runtime_error("Sorry, this version does not support PDP Endian!");
...

Answer 2

This is normally done at compile time (specially for performance reason) by using the header files available from the compiler or create your own. On linux you have the header file "/usr/include/endian.h"

Answer 3

For further details, you may want to check out this codeproject article Basic concepts on Endianness:

How to dynamically test for the Endian type at run time?

As explained in Computer Animation FAQ, you can use the following function to see if your code is running on a Little- or Big-Endian system: Collapse

#define BIG_ENDIAN      0
#define LITTLE_ENDIAN   1

int TestByteOrder()
{
   short int word = 0x0001;
   char *byte = (char *) &word;
   return(byte[0] ? LITTLE_ENDIAN : BIG_ENDIAN);
}

This code assigns the value 0001h to a 16-bit integer. A char pointer is then assigned to point at the first (least-significant) byte of the integer value. If the first byte of the integer is 0x01h, then the system is Little-Endian (the 0x01h is in the lowest, or least-significant, address). If it is 0x00h then the system is Big-Endian.

Answer 4

I was going through the textbook:Computer System: a programmer's perspective, and there is a problem to determine which endian is this by C program.

I used the feature of the pointer to do that as following:

#include <stdio.h>

int main(void){
    int i=1;
    unsigned char* ii = &i;

    printf("This computer is %s endian.\n", ((ii[0]==1) ? "little" : "big"));
    return 0;
}

As the int takes up 4 bytes, and char takes up only 1 bytes. We could use a char pointer to point to the int with value 1. Thus if the computer is little endian, the char that char pointer points to is with value 1, otherwise, its value should be 0.

Answer 5

You can use std::endian if you have access to C++20 compiler such as GCC 8+ or Clang 7+:

#include <type_traits>

if constexpr (std::endian::native == std::endian::big)
{
    // Big endian system
}
else if constexpr (std::endian::native == std::endian::little)
{
    // Little endian system
}
else
{
    // Something else
}

Answer 6

Ehm... It surprises me that noone has realized that the compiler will simply optimize the test out, and will put a fixed result as return value. This renders all code examples above, effectively useless. The only thing that would be returned is the endianness at compile-time! And yes, I tested all of the above examples. Here's an example with MSVC 9.0 (Visual Studio 2008).

Pure C code

int32 DNA_GetEndianness(void)
{
    union 
    {
        uint8  c[4];
        uint32 i;
    } u;

    u.i = 0x01020304;

    if (0x04 == u.c[0])
        return DNA_ENDIAN_LITTLE;
    else if (0x01 == u.c[0])
        return DNA_ENDIAN_BIG;
    else
        return DNA_ENDIAN_UNKNOWN;
}

Disassembly

PUBLIC  _DNA_GetEndianness
; Function compile flags: /Ogtpy
; File c:\development\dna\source\libraries\dna\endian.c
;   COMDAT _DNA_GetEndianness
_TEXT   SEGMENT
_DNA_GetEndianness PROC                 ; COMDAT

; 11   :     union 
; 12   :     {
; 13   :         uint8  c[4];
; 14   :         uint32 i;
; 15   :     } u;
; 16   : 
; 17   :     u.i = 1;
; 18   : 
; 19   :     if (1 == u.c[0])
; 20   :         return DNA_ENDIAN_LITTLE;

    mov eax, 1

; 21   :     else if (1 == u.c[3])
; 22   :         return DNA_ENDIAN_BIG;
; 23   :     else
; 24   :        return DNA_ENDIAN_UNKNOWN;
; 25   : }

    ret
_DNA_GetEndianness ENDP
END

Perhaps it is possible to turn off ANY compile-time optimization for just this function, but I don't know. Otherwise it's maybe possible to hardcode it in assembly, although that's not portable. And even then even that might get optimized out. It makes me think I need some really crappy assembler, implement the same code for all existing CPUs/instruction sets, and well.... never mind.

Also, someone here said that endianness does not change during run-time. WRONG. There are bi-endian machines out there. Their endianness can vary durng execution. ALSO, there's not only Little Endian and Big Endian, but also other endiannesses (what a word).

I hate and love coding at the same time...