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

64-bit Endian Conversion in C

The htonl() function is typically used for converting 32-bit integers from host to network byte order. However, this may not be sufficient for 64-bit integers, which require a different approach.

Cross-platform Solution

For a standard cross-platform solution, consider using the htobe64() function. This function is available on Linux and FreeBSD, and can be used as follows:

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

uint64_t host_int = 123;
uint64_t big_endian;

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

Preprocessor Macros

If htobe64() is not available, you can use preprocessor macros to hide platform differences. The following code does this for 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>

Union-based Approach

Another option is to use a union to represent 64-bit integers in both big endian and little endian format. You can then swap the bytes accordingly. However, this approach may break on big endian platforms.

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