Endianness

Endianness

DanielH

Member #934

January 2001

Does Allegro have a function to check endianness?

Can someone post a snippet of a) how to tell what endianness the machine has b) how they would read/write an 32bit int?

You could read/write little e regardless of machine type then convert to big e if required. Sound about correct?

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Niunio

Member #1,975

March 2002

IIRC there's a define somewhere in the header files (maybe type.h?) so you can use #IFDEF to know the endianess. Also, there are defines for the target system (CPU and operating system).

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Current projects: Allegro.pas | MinGRo

DanielH

Member #934

January 2001

FYI, Allegro defines endianness with ALLEGRO_LITTLE_ENDIAN or ALLEGRO_BIG_ENDIAN defined

What about this pseudocode?
'type' would be a primitive like int, float, etc.

#SelectExpand
  1// returns number of bytes written or EOF if error
  2int write(ALLEGRO_FILE *file, type value)
  3{
  4    // if big endian, swap bytes
  5#ifdef ALLEGRO_BIG_ENDIAN
  6    value = swap_bytes(&value, sizeof(type));
  7#endif
  8
  9    // write little endian
 10    return write_bytes(file, &value, sizeof(type));
 11}
 12
 13// returns number of bytes read or EOF if error
 14int read(ALLEGRO_FILE *file, type&value)
 15{
 16    // read little endian
 17    int sz = read_bytes(file, &value, sizeof(type));
 18
 19    if (sz == EOF) return EOF:
 20
 21    // if big endian, swap bytes
 22#ifdef ALLEGRO_BIG_ENDIAN
 23    value = swap_bytes(&value, sizeof(type));
 24#endif
 25
 26    return sz;
 27}

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GullRaDriel

Member #3,861

September 2003

Allegro aside, here are some macros I'm using in my lib, mainly a scap of a lot of detection macros that I took here and there.

#SelectExpand
  1/*! Little endian macro value */
  2#define BYTEORDER_LITTLE_ENDIAN 0 // Little endian machine.
  3/*! Big endian macro value */
  4#define BYTEORDER_BIG_ENDIAN 1 // Big endian machine.
  5
  6
  7#ifndef BYTEORDER_ENDIAN
  8// Detect with GCC 4.6's macro.
  9#if defined(__BYTE_ORDER__)
 10#if (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
 11#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 12#elif (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
 13#define BYTEORDER_ENDIAN BYTEORDER_BIG_ENDIAN
 14#else
 15#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 16#warning "Unknown machine byteorder endianness detected. User needs to define BYTEORDER_ENDIAN."
 17#warning "Setting default to BYTEORDER_LITTLE_ENDIAN"
 18#endif
 19// Detect with GLIBC's endian.h.
 20#elif defined(__GLIBC__)
 21#include <endian.h>
 22#if (__BYTE_ORDER == __LITTLE_ENDIAN)
 23#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 24#elif (__BYTE_ORDER == __BIG_ENDIAN)
 25#define BYTEORDER_ENDIAN BYTEORDER_BIG_ENDIAN
 26#else
 27#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 28#warning "Unknown machine byteorder endianness detected. User needs to define BYTEORDER_ENDIAN."
 29#warning "Setting default to BYTEORDER_LITTLE_ENDIAN"
 30#endif
 31// Detect with _LITTLE_ENDIAN and _BIG_ENDIAN macro.
 32#elif defined(_LITTLE_ENDIAN) && !defined(_BIG_ENDIAN)
 33#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 34#elif defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
 35#define BYTEORDER_ENDIAN BYTEORDER_BIG_ENDIAN
 36// Detect with architecture macros.
 37#elif defined(__sparc) || defined(__sparc__) || defined(_POWER) || defined(__powerpc__) || defined(__ppc__) || defined(__hpux) || defined(__hppa) || defined(_MIPSEB) || defined(_POWER) || defined(__s390__)
 38#define BYTEORDER_ENDIAN BYTEORDER_BIG_ENDIAN
 39#elif defined(__i386__) || defined(__alpha__) || defined(__ia64) || defined(__ia64__) || defined(_M_IX86) || defined(_M_IA64) || defined(_M_ALPHA) || defined(__amd64) || defined(__amd64__) || defined(_M_AMD64) || defined(__x86_64) || defined(__x86_64__) || defined(_M_X64) || defined(__bfin__)
 40#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 41#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
 42#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 43#else
 44#define BYTEORDER_ENDIAN BYTEORDER_LITTLE_ENDIAN
 45#warning "Unknown machine byteorder endianness detected. User needs to define BYTEORDER_ENDIAN."
 46#warning "Setting default to BYTEORDER_LITTLE_ENDIAN"
 47#endif
 48#endif

Well. Old and new GCC specific code, maybe. I had no problem with it on Solaris, AIX, Linux, Windows MinGW.

BTW I also had to detect 32 or 64 bits, and that little piece covered all my needs:

#SelectExpand
  1#if defined( _WIN32 ) || defined( _WIN64 )
  2#if defined( _WIN64 )
  3#define __ENVBITS ENV_64BITS
  4#else
  5#define __ENVBITS ENV_32BITS
  6#endif
  7#endif
  8
  9#if !defined( __ENVBITS ) 
 10#if defined( __GNUC__ )
 11#if defined( __x86_64__ ) || defined( __ppc64__ )
 12#define __ENVBITS __ENV_64BITS
 13#else
 14#define  __ENVBITS __ENV_32BITS
 15#endif
 16#endif
 17#endif

Your mileage may vary. It may only be right on my own os targets.

Edit:
BTW endianness is only a problem if you are having datas or networks communications between systems that are not the same endian or bits. Problems may arise if i.e you write a datafile containing dumps on A and read it on B, and that B is not the same bits / endianness as A.
Commonly when I need to write something I'm rarely dumping a int64, as it's not portable across all my targets ( a mix of 32/64bits ), and anyway I always htonX before dumping, and ntohX when reading the dump. That way data is consistent.
I'm not sure if I'm clear, but my dear I'm tired today X-D

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