C Macro definition to determine big endian or little endian machine ?

Question

Is there a one line macro definition to determine the endianness of the machine. I am using the following code but converting it to macro would be too long.

unsigned char test_endian( void )
{
    int test_var = 1;
    unsigned char test_endian* = (unsigned char*)&test_var;

    return (test_endian[0] == NULL);
}

Answer 1

Use an inline function rather than a macro. Besides, you need to store something in memory which is a not-so-nice side effect of a macro.

You could convert it to a short macro using a static or global variable, like this:

static int s_endianess = 0;
#define ENDIANESS() ((s_endianess = 1), (*(unsigned char*) &s_endianess) == 0)

Answer 2

There is no standard, but on many systems including <endian.h> will give you some defines to look for.

Answer 3

If you want to only rely on the preprocessor, you have to figure out the list of predefined symbols. Preprocessor arithmetics has no concept of addressing.

GCC defines __LITTLE_ENDIAN__ or __BIG_ENDIAN__

$ gcc -E -dM - < /dev/null |grep ENDIAN
#define __LITTLE_ENDIAN__ 1

Then, you can add more preprocessor conditional directives based on platform detection like #ifdef _WIN32 etc.

Answer 4

Whilst there is no portable #define or something to rely upon, platforms do provide standard functions for converting to and from your 'host' endian.

Generally, you do storage - to disk, or network - using 'network endian', which is BIG endian, and local computation using host endian (which on x86 is LITTLE endian). You use htons() and ntohs() and friends to convert between the two.

Answer 5

Try this:

#include<stdio.h>        
int x=1;
#define TEST (*(char*)&(x)==1)?printf("little endian"):printf("Big endian")
int main()
{

   TEST;
}

Answer 6

If you desperately want to use the preprocessor, you could abuse string literals:

#include <stdint.h>

#define IS_BIG_ENDIAN (*(uint16_t *)"\0\xff" < 0x100)

In general though, you should try to write code that does not depend on the endianness of the host platform.

Answer 7

Code supporting arbitrary byte orders, ready to be put into a file called order32.h:

#ifndef ORDER32_H
#define ORDER32_H

#include <limits.h>
#include <stdint.h>

#if CHAR_BIT != 8
#error "unsupported char size"
#endif

enum
{
    O32_LITTLE_ENDIAN = 0x03020100ul,
    O32_BIG_ENDIAN = 0x00010203ul,
    O32_PDP_ENDIAN = 0x01000302ul
};

static const union { unsigned char bytes[4]; uint32_t value; } o32_host_order =
    { { 0, 1, 2, 3 } };

#define O32_HOST_ORDER (o32_host_order.value)

#endif

You would check for little endian systems via

O32_HOST_ORDER == O32_LITTLE_ENDIAN