How to Convert 64-bit Values Between Network and Host Byte Order in C ?

ntohl() Extension for 64-bit Values

The C function ntohl() is typically used for converting 32-bit values from network byte order to host byte order. However, in certain scenarios, one might require a similar operation for 64-bit values.

Solution

The man pages for htonl() indicate its limitation to 32-bit values, as unsigned long on certain platforms is 32 bits in size. To address the need for 64-bit conversion, several approaches can be explored:

Standard Library:

• htobe64() and be64toh(): These functions are available on Linux (glibc >= 2.9) and FreeBSD for converting 64-bit values between big endian and little endian.

Implementation Suggestions:

• Union with unsigned long long and char[8]: A union can be used to manually swap bytes if the platform is not big endian. However, this method is not recommended as it relies on system-dependent assumptions.

Preprocessor Magic:

• Platform-specific headers: The following preprocessor code allows for portability across different platforms, including Linux, FreeBSD, and OpenBSD:

<code class="cpp">#if defined(__linux__)
#  include <endian.h>
#elif defined(__FreeBSD__) || defined(__NetBSD__)
#  include <sys/endian.h>
#elif defined(__OpenBSD__)
#  include <sys/types.h>
#  define be16toh(x) betoh16(x)
#  define be32toh(x) betoh32(x)
#  define be64toh(x) betoh64(x)
#endif</code>

Usage Example:

Using the preprocessor magic, the conversion can be performed using the following code snippet:

<code class="cpp">  #include <stdint.h>

  uint64_t  host_int = 123;
  uint64_t  big_endian;

  big_endian = htobe64( host_int );
  host_int = be64toh( big_endian );</code>

This approach provides a standard C library-like interface that is compatible across multiple platforms.

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