How to Convert 64-Bit Integers from Big Endian to Little Endian in C ?

64-Bit Integer Conversion from Big Endian to Little Endian in C

The standard C function ntohl() converts 32-bit integers from network byte order (big-endian) to host byte order (little-endian) and vice versa. However, there is a need to convert 64-bit integers (specifically unsigned long long values) on some platforms.

Unfortunately, the C standard does not provide a dedicated function for 64-bit integer conversion. However, platform-specific implementations exist.

Platform-Specific Solutions

• Linux and FreeBSD: htobe64 and be64toh are defined in for 64-bit integer conversion.
• OpenBSD: Custom macros be16toh, be32toh, and be64toh are available in for byte order conversion.

Code Example

The following code can be used to convert 64-bit integers on various platforms:

<code class="cpp">#include <stdint.h> // For 'uint64_t'

#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

int main() {
  uint64_t host_int = 123;
  uint64_t big_endian;

  // Convert to big-endian
  big_endian = htobe64(host_int);

  // Convert back to host byte order
  host_int = be64toh(big_endian);

  return 0;
}</code>

This example will perform 64-bit integer conversion on Linux, FreeBSD, and OpenBSD using the appropriate platform-specific functions.

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