How to Implement a Portable 64-Bit Integer Byte Ordering Function in C ?

Portable 64-Bit Integer Byte Ordering Function in C

Question:

Can we obtain a standard function like htonl that works with 64-bit integers in C , allowing for conversion to network byte order?

Answer:

While there is no explicitly defined function like htonll in the C standard library, it is possible to create a portable implementation. Here are two approaches:

Using Dynamic Endianness Detection:

<code class="cpp">#define htonll(x) ((1 == htonl(1)) ? (x) : ((uint64_t)htonl((x) & 0xFFFFFFFF) << 32) | htonl((x) >> 32))
#define ntohll(x) ((1 == ntohl(1)) ? (x) : ((uint64_t)ntohl((x) & 0xFFFFFFFF) << 32) | ntohl((x) >> 32))</code>

• The macro htonll converts a 64-bit integer x to network byte order by swapping 32-bit words if the target architecture is little-endian.
• ntohll performs the reverse operation, converting from network byte order to native byte order.

Using Preprocessor Macros:

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

• This approach defines htonll and ntohll using preprocessor macros based on the __BIG_ENDIAN__ or __LITTLE_ENDIAN__ preprocessor symbols provided by most compilers.
• If the architecture is big-endian (symbol defined), the functions simply return the input value since no byte swapping is needed.
• For little-endian architectures, the functions perform byte swapping as in the previous example.

The above is the detailed content of How to Implement a Portable 64-Bit Integer Byte Ordering Function in C ?. For more information, please follow other related articles on the PHP Chinese website!