Detecting Endianness

Question

I'm currently trying to create a C source code which properly handles I/O whatever the endianness of the target system.

I've selected "little endian" as my I/O convention, which means that, for big endian CPU, I need to convert data while writing or reading.

Conversion is not the issue. The problem I face is to detect endianness, preferably at compile time (since CPU do not change endianness in the middle of execution...).

Up to now, I've been using this :

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
...
#else
...
#endif

It's documented as a GCC pre-defined macro, and Visual seems to understand it too.

However, I've received report that the check fails for some big_endian systems (PowerPC).

So, I'm looking for a foolproof solution, which ensures that endianess is correctly detected, whatever the compiler and the target system. well, most of them at least...

[Edit] : Most of the solutions proposed rely on "run-time tests". These tests may sometimes be properly evaluated by compilers during compilation, and therefore cost no real runtime performance.

However, branching with some kind of << if (0) { ... } else { ... } >> is not enough. In the current code implementation, variable and functions declaration depend on big_endian detection. These cannot be changed with an if statement.

Well, obviously, there is fall back plan, which is to rewrite the code...

I would prefer to avoid that, but, well, it looks like a diminishing hope...

[Edit 2] : I have tested "run-time tests", by deeply modifying the code. Although they do their job correctly, these tests also impact performance.

I was expecting that, since the tests have predictable output, the compiler could eliminate bad branches. But unfortunately, it doesn't work all the time. MSVC is good compiler, and is successful in eliminating bad branches, but GCC has mixed results, depending on versions, kind of tests, and with greater impact on 64 bits than on 32 bits.

It's strange. And it also means that the run-time tests cannot be ensured to be dealt with by the compiler.

Edit 3 : These days, I'm using a compile-time constant union, expecting the compiler to solve it to a clear yes/no signal. And it works pretty well : https://godbolt.org/g/DAafKo

Answer 1

As stated earlier, the only "real" way to detect Big Endian is to use runtime tests.

However, sometimes, a macro might be preferred.

Unfortunately, I've not found a single "test" to detect this situation, rather a collection of them.

For example, GCC recommends : __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ . However, this only works with latest versions, and earlier versions (and other compilers) will give this test a false value "true", since NULL == NULL. So you need the more complete version : defined(__BYTE_ORDER__)&&(__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)

OK, now this works for newest GCC, but what about other compilers ?

You may try __BIG_ENDIAN__ or __BIG_ENDIAN or _BIG_ENDIAN which are often defined on big endian compilers.

This will improve detection. But if you specifically target PowerPC platforms, you can add a few more tests to improve even more detection. Try _ARCH_PPC or __PPC__ or __PPC or PPC or __powerpc__ or __powerpc or even powerpc. Bind all these defines together, and you have a pretty fair chance to detect big endian systems, and powerpc in particular, whatever the compiler and its version.

So, to summarize, there is no such thing as a "standard pre-defined macros" which guarantees to detect big-endian CPU on all platforms and compilers, but there are many such pre-defined macros which, collectively, give a high probability of correctly detecting big endian under most circumstances.

Answer 2

At compile time in C you can't do much more than trusting preprocessor #defines, and there are no standard solutions because the C standard isn't concerned with endianness.

Still, you could add an assertion that is done at runtime at the start of the program to make sure that the assumption done when compiling was true:

inline int IsBigEndian()
{
    int i=1;
    return ! *((char *)&i);
}

/* ... */

#ifdef COMPILED_FOR_BIG_ENDIAN
assert(IsBigEndian());
#elif COMPILED_FOR_LITTLE_ENDIAN
assert(!IsBigEndian());
#else
#error "No endianness macro defined"
#endif

(where COMPILED_FOR_BIG_ENDIAN and COMPILED_FOR_LITTLE_ENDIAN are macros #defined previously according to your preprocessor endianness checks)

Answer 3

Instead of looking for a compile-time check, why not just use big-endian order (which is considered the "network order" by many) and use the htons/htonl/ntohs/ntohl functions provided by most UNIX-systems and Windows. They're already defined to do the job you're trying to do. Why reinvent the wheel?

Answer 4

Try something like:

if(*(char *)(int[]){1}) {
    /* little endian code */
} else {
    /* big endian code */
}

and see if your compiler resolves it at compile-time. If not, you might have better luck doing the same with a union. Actually I like defining macros using unions that evaluate to 0,1 or 1,0 (respectively) so that I can just do things like accessing buf[HI] and buf[LO].

Answer 5

Notwithstanding compiler-defined macros, I don't think there's a compile-time way to detect this, since determining the endianness of an architecture involves analyzing the manner in which it stores data in memory.

Here's a function which does just that:

bool IsLittleEndian () {

    int i=1;

    return (int)*((unsigned char *)&i)==1;

}

Answer 6

As others have pointed out, there isn't a portable way to check for endianness at compile-time. However, one option would be to use the autoconf tool as part of your build script to detect whether the system is big-endian or little-endian, then to use the AC_C_BIGENDIAN macro, which holds this information. In a sense, this builds a program that detects at runtime whether the system is big-endian or little-endian, then has that program output information that can then be used statically by the main source code.

Hope this helps!

Answer 7

This comes from p. 45 of Pointers in C:

#include <stdio.h>
#define BIG_ENDIAN 0
#define LITTLE_ENDIAN 1

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

int main(int argc, char* argv[])
{
   int value;
   value = endian();
   if (value == 1)
      printf("The machine is Little Endian\n");
   else
      printf("The machine is Big Endian\n");
   return 0;
}

Answer 8

Socket's ntohl function can be used for this purpose. Source

// Soner
#include <stdio.h>
#include <arpa/inet.h>


int main() {
    if (ntohl(0x12345678) == 0x12345678) {
        printf("big-endian\n");
    } else if (ntohl(0x12345678) == 0x78563412) {
        printf("little-endian\n");
    } else {
        printf("(stupid)-middle-endian\n");
    }
    return 0;
}

Answer 9

My GCC version is 9.3.0, it's configured to support powerpc64 platform, and I've tested it and verified that it supports the following macros logic:

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
......
#endif
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
.....
#endif

Answer 10

As of C++20, no more hacks or compiler extensions are necessary.

https://en.cppreference.com/w/cpp/types/endian

std::endian (Defined in header <bit>)

enum class endian
{
    little = /*implementation-defined*/,
    big    = /*implementation-defined*/,
    native = /*implementation-defined*/
};

• If all scalar types are little-endian, std::endian::native equals std::endian::little

• If all scalar types are big-endian, std::endian::native equals std::endian::big

Answer 11

You can't detect it at compile time to be portable across all compilers. Maybe you can change the code to do it at run-time - this is achievable.

Answer 12

It is not possible to detect endianness portably in C with preprocessor directives.

Answer 13

I took the liberty of reformatting the quoted text

As of 2017-07-18, I use union { unsigned u; unsigned char c[4]; }

If sizeof (unsigned) != 4 your test may fail.

It may be better to use

union { unsigned u; unsigned char c[sizeof (unsigned)]; }

Answer 14

As most have mentioned, compile time is your best bet. Assuming you do not do cross compilations and you use cmake (it will also work with other tools such as a configure script, of course) then you can use a pre-test which is a compiled .c or .cpp file and that gives you the actual verified endianness of the processor you're running on.

With cmake you use the TestBigEndian macro. It sets a variable which you can then pass to your software. Something like this (untested):

TestBigEndian(IS_BIG_ENDIAN)
...
set(CFLAGS ${CFLAGS} -DIS_BIG_ENDIAN=${IS_BIG_ENDIAN}) // C
set(CXXFLAGS ${CXXFLAGS} -DIS_BIG_ENDIAN=${IS_BIG_ENDIAN}) // C++

Then in your C/C++ code you can check that IS_BIG_ENDIAN define:

#if IS_BIG_ENDIAN
    ...do big endian stuff here...
#else
    ...do little endian stuff here...
#endif

So the main problem with such a test is cross compiling since you may be on a completely different CPU with a different endianness... but at least it gives you the endianness at time of compiling the rest of your code and will work for most projects.

Answer 15

I provided a general approach in C with no preprocessor, but only runtime that compute endianess for every C type.

the output if this on my Linux x86_64 architecture is:

fabrizio@toshibaSeb:~/git/pegaso/scripts$ gcc -o sizeof_endianess sizeof_endianess.c 
fabrizio@toshibaSeb:~/git/pegaso/scripts$ ./sizeof_endianess 
INTEGER TYPE  | signed  |  unsigned  | 0x010203...             | Endianess
--------------+---------+------------+-------------------------+--------------
int           |  4      |      4     | 04 03 02 01             | little
char          |  1      |      1     | -                       | -
short         |  2      |      2     | 02 01                   | little
long int      |  8      |      8     | 08 07 06 05 04 03 02 01 | little
long long int |  8      |      8     | 08 07 06 05 04 03 02 01 | little
--------------+---------+------------+-------------------------+--------------
FLOATING POINT| size    |
--------------+---------+
float         |  4
double        |  8
long double   | 16

Get source at: https://github.com/bzimage-it/pegaso/blob/master/scripts/sizeof_endianess.c

This is a more general approach is to not detect endianess at compilation time (not possibile) nor assume any endianess escludes another one. In fact is important to remark that endianess is not a concept of the architecture/processor but regards single type. As argued by @Christoph at https://stackoverflow.com/a/4712594/3280080 PDP-11 for example can have different endianess at the same time.

The approach consist to set an integer to be x = 0x010203... as long is it, then print them looking at casted-at-single-byte incrementing the address by one.

Can somebody test it please in a big endian and/or mixed endianess ?

Answer 16

I know I'm late to this party, but here is my take.

int is_big_endian() {
    return 1 & *(uint16_t*)"01";
}

This is based on the fact that '0' is 48 in decimal and '1' 49, so '1' has the LSB bit set, while '0' not. I could make them '\x00' and '\x01' but I think my version makes it more readable.

Answer 17

#define BIG_ENDIAN ((1 >> 1 == 0) ? 0 : 1)