Endianness manipulation - is there a C library for this?

Question

Hi all,

With the sort of programs I write (working with raw file data) I often need functions to convert between big and little endian. Usually I write these myself (which is covered by many other posts here) but I'm not that keen on doing this for a number of reasons - the main one being lack of testing. I don't really want to spend ages testing my code in a big endian emulator, and often just omit the code for big endian machines altogether. I also would rather make use of faster functions provided by various compilers, while still keeping my programs cross-platform.

The only things I can find are socket calls like htons() but they require different #include files on each platform, and some GPL code like this, however that particular file, while comprehensive, seems to miss out on some of the high performance functions provided by some compilers.

So, does anyone know of a library (ideally just a .h file) that is well tested and provides a standard set of functions for dealing with endianness across many compilers and platforms?

Answer 1

There have been a number of proposals for a Boost class (for C++, at least) to do exactly that over the last decade, but none have ever come to fruition, unfortunately.

I'm not aware of any better generalized solution than the htons() function set.

Answer 2

It's easiest just to not write endian-dependent code. You never should care exactly what endianness is of the system you're running on; the only thing that should matter is what the mandated endianness is for the external data you're reading or writing. You shouldn't be asking about conversions between big- and little-endian values, but rather about conversions from a specific endianness to the host endianness, and you can write that code in an endian-agnostic way that's (almost) completely portable:

For example: suppose you're reading a 32-bit big-endian integer from a file stream:

/*
 * Note that callers should check feof(fp) afterward to verify that
 * there was enough data to read.
 */
uint32_t GetBE32(FILE* fp)
{
    uint32_t result;
    result  = fgetc(fp) << 24;
    result |= fgetc(fp) << 16;
    result |= fgetc(fp) <<  8;
    result |= fgetc(fp);
    return result;
}

uint32_t GetLE32(FILE* fp)
{
    uint32_t result;
    result  = fgetc(fp);
    result |= fgetc(fp) <<  8;
    result |= fgetc(fp) << 16;
    result |= fgetc(fp) << 24;
    return result;
}

(I say "(almost) completely portable" because it does assume that there are 8 bits per byte. But if you're on a system where that isn't true, you're probably going to have bigger issues when dealing with external data.)