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

while there is no quick and standard way to determine it, this will output it:

#include <stdio.h> 
int main()  
{ 
   unsigned int i = 1; 
   char *c = (char*)&i; 
   if (*c)     
       printf("Little endian"); 
   else
       printf("Big endian"); 
   getchar(); 
   return 0; 
} 

Answer 2

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.

Answer 3

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.

Answer 4

untested, but in my mind, this should work? cause it'll be 0x01 on little endian, and 0x00 on big endian?

bool runtimeIsLittleEndian(void)
{
 volatile uint16_t i=1;
 return  ((uint8_t*)&i)[0]==0x01;//0x01=little, 0x00=big
}

Answer 5

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");
}

Answer 6

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;
}