How to Convert 64-Bit Integers in C Beyond ntohl()?

64-Bit Integer Conversion in C : Beyond ntohl()

The htonl() function, as suggested by its documentation, is limited to 32-bit integer conversion. However, in situations where you require 64-bit conversion, there are several options available.

Standard Library Functions

For Linux (glibc >= 2.9) and FreeBSD, the htobe64() function can be used to convert 64-bit integers from big endian to little endian. This function is part of the standard C library.

Union-Based Approach

Alternatively, you can use a union to convert between 64-bit integers and 8-byte character arrays. This approach involves manually swapping the bytes around for big endian platforms.

Preprocessor Macros

To hide platform-specific differences and provide a unified approach, you can use the following preprocessor code:

<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>

This code provides Linux/FreeBSD-style macros on Linux, OpenBSD, FreeBSD, and NetBSD.

Example Usage

To demonstrate the usage of the recommended approach, consider the following code:

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

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

  big_endian = htobe64(host_int);
  host_int = be64toh(big_endian);

  return 0;
}</code>

This code converts a 64-bit integer, host_int, from little endian to big endian and back, effectively preserving its value.

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