C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Portable endianess          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message hantheman Guest Posted: Sat Dec 20, 2003 10:07 am    Post subject: Portable endianess Is this a portable implementation? #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) #define htons(A) (A) #define htonl(A) (A) #define ntohs(A) (A) #define ntohl(A) (A) #elif defined(LITTLE_ENDIAN) && !defined(BIG_ENDIAN) #define htons(A) ((((uint16)(A) & 0xff00) >> | (((uint16)(A) & 0x00ff) << ) #define htonl(A) ((((uint32)(A) & 0xff000000) >> 24) | (((uint32)(A) & 0x00ff0000) >> | (((uint32)(A) & 0x0000ff00) << | (((uint32)(A) & 0x000000ff) << 24)) #define ntohs htons #define ntohl htohl #else #error "Either BIG_ENDIAN or LITTLE_ENDIAN must be #defined, but not both." #endif Back to top Jeff Schwab Guest Posted: Sat Dec 20, 2003 2:41 pm    Post subject: Re: Portable endianess hantheman wrote: Quote: Is this a portable implementation? #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) #define htons(A) (A) #define htonl(A) (A) #define ntohs(A) (A) #define ntohl(A) (A) #elif defined(LITTLE_ENDIAN) && !defined(BIG_ENDIAN) #define htons(A) ((((uint16)(A) & 0xff00) >> | (((uint16)(A) & 0x00ff) << ) #define htonl(A) ((((uint32)(A) & 0xff000000) >> 24) | (((uint32)(A) & 0x00ff0000) >> | (((uint32)(A) & 0x0000ff00) << | (((uint32)(A) & 0x000000ff) << 24)) #define ntohs htons #define ntohl htohl #else #error "Either BIG_ENDIAN or LITTLE_ENDIAN must be #defined, but not both." #endif Who said bytes had eight bits? Back to top Heinz Ozwirk Guest Posted: Sun Dec 21, 2003 10:51 am    Post subject: Re: Portable endianess "Jeff Schwab" <jeffplus (AT) comcast (DOT) net> schrieb im Newsbeitrag news:yM6dnc13PoxgwnmiRVn-hQ (AT) comcast (DOT) com... : hantheman wrote: : > Is this a portable implementation? : > : > #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) : > : > #define htons(A) (A) : > #define htonl(A) (A) : > #define ntohs(A) (A) : > #define ntohl(A) (A) : > : > #elif defined(LITTLE_ENDIAN) && !defined(BIG_ENDIAN) : > : > #define htons(A) ((((uint16)(A) & 0xff00) >> | : > (((uint16)(A) & 0x00ff) << ) : > #define htonl(A) ((((uint32)(A) & 0xff000000) >> 24) | : > (((uint32)(A) & 0x00ff0000) >> | : > (((uint32)(A) & 0x0000ff00) << | : > (((uint32)(A) & 0x000000ff) << 24)) : > #define ntohs htons : > #define ntohl htohl : > : > #else : > : > #error "Either BIG_ENDIAN or LITTLE_ENDIAN must be #defined, but not both." : > : > #endif : : Who said bytes had eight bits? Who's talking about bytes? The 8 bits you probaly assume to be the number of bits in a (C++-) "byte" are the number of bits in a network octet, which is defined to be 8 bits. Heinz Back to top Jeff Schwab Guest Posted: Sun Dec 21, 2003 4:09 pm    Post subject: Re: Portable endianess Heinz Ozwirk wrote: Quote: "Jeff Schwab" <jeffplus (AT) comcast (DOT) net> schrieb im Newsbeitrag news:yM6dnc13PoxgwnmiRVn-hQ (AT) comcast (DOT) com... : hantheman wrote: : > Is this a portable implementation? : : > #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) : : > #define htons(A) (A) : > #define htonl(A) (A) : > #define ntohs(A) (A) : > #define ntohl(A) (A) : : > #elif defined(LITTLE_ENDIAN) && !defined(BIG_ENDIAN) : : > #define htons(A) ((((uint16)(A) & 0xff00) >> | : > (((uint16)(A) & 0x00ff) << ) : > #define htonl(A) ((((uint32)(A) & 0xff000000) >> 24) | : > (((uint32)(A) & 0x00ff0000) >> | : > (((uint32)(A) & 0x0000ff00) << | : > (((uint32)(A) & 0x000000ff) << 24)) : > #define ntohs htons : > #define ntohl htohl : : > #else : : > #error "Either BIG_ENDIAN or LITTLE_ENDIAN must be #defined, but not both." : : > #endif : : Who said bytes had eight bits? Who's talking about bytes? The 8 bits you probaly assume to be the number of bits in a (C++-) "byte" are the number of bits in a network octet, which is defined to be 8 bits. Heinz Thanks for clarifying. Poor assumption on my part. Another question: Why are you doing all this in the preprocessor? Back to top Gene Wirchenko Guest Posted: Mon Dec 22, 2003 5:52 pm    Post subject: Re: Portable endianess On Sun, 21 Dec 2003 11:51:46 +0100, "Heinz Ozwirk" <wansor42 (AT) gmx (DOT) de> wrote: Quote: "Jeff Schwab" <jeffplus (AT) comcast (DOT) net> schrieb im Newsbeitrag news:yM6dnc13PoxgwnmiRVn-hQ (AT) comcast (DOT) com... : hantheman wrote: : > Is this a portable implementation? #include <winsock.h> and #include <winsock2.h> are. Unless you are implementing Winsock, you should use your system's header. [snip] Sincerely, Gene Wirchenko Back to top Stewart Gordon Guest Posted: Sat Dec 27, 2003 10:30 pm    Post subject: Re: Portable endianess Jeff Schwab wrote: Quote: hantheman wrote: Is this a portable implementation? #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) snip Only if you can rely on the programmer's knowing which endianness his/her platform is. Moreover, there might not even be a programmer, but merely someone who's compiling a program delivered in source form. I recently did something with this. Basically, I was modifying rayshade to generate BMP output. I don't have the code over here, but it was something like: union EndianBytes { unsigned char b[4]; short s; long l; } short LEShort(short s) { EndianBytes e; e.b[0] = (unsigned char) s; e.b[1] = (unsigned char) (s >> ; return e.s; } LEShort converts a short from platform byte-order to little-endian or vice versa. It's straightforward to write BEShort, LELong and BELong on the same principle. Of course it still relies on some standard dimensions (byte = 8 bits, short = 2 bytes, long = 4 bytes), but it'll work on either BE or LE platforms without requiring any extra attention or knowledge on the side of the person writing/compiling a program that uses it. Stewart. -- My e-mail is valid but not my primary mailbox. Please keep replies on on the 'group where everyone may benefit. Back to top Nick Hounsome Guest Posted: Sun Dec 28, 2003 10:14 am    Post subject: Re: Portable endianess "Stewart Gordon" <smjg_1998 (AT) yahoo (DOT) com> wrote Quote: Jeff Schwab wrote: hantheman wrote: Is this a portable implementation? #if defined(BIG_ENDIAN) && !defined(LITTLE_ENDIAN) snip Only if you can rely on the programmer's knowing which endianness his/her platform is. I've recently been using QNX and they have a very clean and logical way of pushing the endianness issue into 'the implementation': They just provide macros such as ENDIAN_BE16(x), ENDIAN_LE16(x), ENDIAN_BE32(x) etc.. If you are dealing with an external format that you know to be Bigendian 16 bits then whenever you read it in you convert it with internal_format = ENDIAN_BE16(external_format) and when you want to send it out you use external_format = ENDIAN_BE16(internal_format) The implementation conditionally compiles the macros to be either a byte swap or no op depending on the endianness of the system. Obviously this doesn't actually answer the question of how to determine the endianness of a system (which cannot portably be done at compile time) but is a very clean way of keeping all (host) endianness issues hidden away in one header file and concentrates instead on interface endianness which is as it should be. 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