Detecting endianness programmatically in a C++ program

I don't like the method based on type punning - it will often be warned against by compiler. That's exactly what unions are for!

bool is_big_endian(void)
{
    union {
        uint32_t i;
        char c[4];
    } bint = {0x01020304};

    return bint.c[0] == 1;
}

The principle is equivalent to the type case as suggested by others, but this is clearer - and according to C99, is guaranteed to be correct. GCC prefers this compared to the direct pointer cast.

This is also much better than fixing the endianness at compile time - for OSes which support multi-architecture (fat binary on Mac OS X for example), this will work for both ppc/i386, whereas it is very easy to mess things up otherwise.

You can do it by setting an int and masking off bits, but probably the easiest way is just to use the built in network byte conversion ops (since network byte order is always big endian).

if ( htonl(47) == 47 ) {
  // Big endian
} else {
  // Little endian.
}

Bit fiddling could be faster, but this way is simple, straightforward and pretty impossible to mess up.

Please see this article:

Here is some code to determine what is the type of your machine

int num = 1;
if(*(char *)&num == 1)
{
    printf("\nLittle-Endian\n");
}
else
{
    printf("Big-Endian\n");
}

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".

I surprised no one has mentioned the macros which the pre-processor defines by default. While these will vary depending on your platform; they are much cleaner than having to write your own endian-check.

For example; if we look at the built-in macros which GCC defines (on an x86-64 machine):

:| gcc -dM -E -x c - | grep -i endian

#define __LITTLE_ENDIAN__ 1

On a PPC machine I get:

:| gcc -dM -E -x c - | grep -i endian

#define __BIG_ENDIAN__ 1
#define _BIG_ENDIAN 1

(The :| gcc -dM -E -x c - magic prints out all built-in macros.)

Do not use a union!

C++ does not permit type punning via unions!
Reading from a union field that was not the last field written to is undefined behaviour!
Many compilers support doing so as an extension, but the language makes no guarantee.

See this answer for more details:

https://stackoverflow.com/a/11996970

There are only two valid answers that are guaranteed to be portable.

The first answer, if you have access to a system that supports C++20,
is to use std::endian from the <bit> header.

C++20 Onwards

constexpr bool is_little_endian = (std::endian::native == std::endian::little);

Prior to C++20, the only valid answer is to store an integer and then inspect its first byte through type punning. Unlike the use of unions, this is expressly allowed by C++'s type system.

It's also important to remember that for optimum portability static_cast should be used,
because reinterpret_cast is implementation defined.

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined: ... a char or unsigned char type.

C++11 Onwards

enum class endianness
{
    little = 0,
    big = 1,
};

inline endianness get_system_endianness()
{
    const int value { 0x01 };
    const void * address = static_cast<const void *>(&value);
    const unsigned char * least_significant_address = static_cast<const unsigned char *>(address);
    return (*least_significant_address == 0x01) ? endianness::little : endianness::big;
}

C++11 Onwards (without enum)

inline bool is_system_little_endian()
{
    const int value { 0x01 };
    const void * address = static_cast<const void *>(&value);
    const unsigned char * least_significant_address = static_cast<const unsigned char *>(address);
    return (*least_significant_address == 0x01);
}

C++98/C++03

inline bool is_system_little_endian()
{
    const int value = 0x01;
    const void * address = static_cast<const void *>(&value);
    const unsigned char * least_significant_address = static_cast<const unsigned char *>(address);
    return (*least_significant_address == 0x01);
}

Declare an int variable:

int variable = 0xFF;

Now use char* pointers to various parts of it and check what is in those parts.

char* startPart = reinterpret_cast<char*>( &variable );
char* endPart = reinterpret_cast<char*>( &variable ) + sizeof( int ) - 1;

Depending on which one points to 0xFF byte now you can detect endianness. This requires sizeof( int ) > sizeof( char ), but it's definitely true for the discussed platforms.

Unless you're using a framework that has been ported to PPC and Intel processors, you will have to do conditional compiles, since PPC and Intel platforms have completely different hardware architectures, pipelines, busses, etc. This renders the assembly code completely different between the two.

As for finding endianness, do the following:

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

You will either get tempChar to be 0x12 or 0x34, from which you will know the endianness.

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.

I would do something like this:

bool isBigEndian() {
    static unsigned long x(1);
    static bool result(reinterpret_cast<unsigned char*>(&x)[0] == 0);
    return result;
}

Along these lines, you would get a time efficient function that only does the calculation once.

You can also do this via the preprocessor using something like a Boost header file which can be found in Boost endian.

See Endianness - C-Level Code illustration.

// assuming target architecture is 32-bit = 4-Bytes
enum ENDIANNESS{ LITTLEENDIAN , BIGENDIAN , UNHANDLE };


ENDIANNESS CheckArchEndianalityV1( void )
{
    int Endian = 0x00000001; // assuming target architecture is 32-bit    

    // as Endian = 0x00000001 so MSB (Most Significant Byte) = 0x00 and LSB (Least     Significant Byte) = 0x01
    // casting down to a single byte value LSB discarding higher bytes    

    return (*(char *) &Endian == 0x01) ? LITTLEENDIAN : BIGENDIAN;
} 

int i=1;
char *c=(char*)&i;
bool littleendian=c;

Here's another C version. It defines a macro called wicked_cast() for inline type punning via C99 union literals and the non-standard __typeof__ operator.

#include <limits.h>

#if UCHAR_MAX == UINT_MAX
#error endianness irrelevant as sizeof(int) == 1
#endif

#define wicked_cast(TYPE, VALUE) \
    (((union { __typeof__(VALUE) src; TYPE dest; }){ .src = VALUE }).dest)

_Bool is_little_endian(void)
{
    return wicked_cast(unsigned char, 1u);
}

If integers are single-byte values, endianness makes no sense and a compile-time error will be generated.

a c++20 solution:

constexpr bool compare(auto const c, auto const ...a) noexcept
{
  return [&]<auto ...I>(std::index_sequence<I...>) noexcept
    {
      return ((std::uint8_t(c >> 8 * I) == a) && ...);
    }(std::make_index_sequence<sizeof...(a)>());
}

static constexpr auto is_big_endian_v{
  compare(std::uint32_t(0x01234567), 0x01, 0x23, 0x45, 0x67)
};

static constexpr auto is_little_endian_v{
  compare(std::uint32_t(0x01234567), 0x67, 0x45, 0x23, 0x01)
};

static constexpr auto is_pdp_endian_v{
  compare(std::uint32_t(0x01234567), 0x23, 0x01, 0x67, 0x45)
};

The task can be accomplished more easily, but for some reason the <bit> header file is not always present. Here's a demo.

How about this?

#include <cstdio>

int main()
{
    unsigned int n = 1;
    char *p = 0;

    p = (char*)&n;
    if (*p == 1)
        std::printf("Little Endian\n");
    else 
        if (*(p + sizeof(int) - 1) == 1)
            std::printf("Big Endian\n");
        else
            std::printf("What the crap?\n");
    return 0;
}