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

htonl for 64-bit Integers in C : A Comprehensive Guide

Introduction:

Many network protocols require storing 64-bit integers in "network byte order." This differs from the host byte order, potentially causing compatibility issues. Fortunately, for 32-bit integers, the htonl function is widely used for converting to network byte order. Developers often seek an analogous function for 64-bit integers. This article explores the availability of such a function and provides an implementation if not available.

Existence of a Standard Function:

Unfortunately, there is no standard htonll function in the C standard library or any widely adopted portable library.

Implementation:

In the absence of a standard function, we can implement our own portable version. Here are two approaches:

1. Runtime Byte-Swapping:

This approach determines if byte-swapping is necessary by checking the endianness at runtime using htonl(1). If byte-swapping is needed, we swap the 32-bit halves of the 64-bit integer using htonl. The code for this approach is:

<code class="c++">#define htonll(x) ((1==htonl(1)) ? (x) : ((uint64_t)htonl((x) &amp; 0xFFFFFFFF) << 32) | htonl((x) >> 32))</code>

2. Compile-Time Byte-Swapping:

If you have access to a compiler that defines preprocessor macros like __BIG_ENDIAN__, you can use them to optimize the byte-swapping process at compile time. Here's an example:

<code class="c++">#if __BIG_ENDIAN__
# define htonll(x) (x)
# define ntohll(x) (x)
#else
# define htonll(x) (((uint64_t)htonl((x) &amp; 0xFFFFFFFF) << 32) | htonl((x) >> 32))
# define ntohll(x) (((uint64_t)ntohl((x) &amp; 0xFFFFFFFF) << 32) | ntohl((x) >> 32))
#endif</code>

Conclusion:

In conclusion, there is no standard htonll function in C . However, we have provided portable implementations that determine endianness at runtime or compile-time, allowing for the conversion of 64-bit integers to network byte order.

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