The man pages for htonl() seem to suggest that you can only use it for up to 32 bit values. (In reality, ntohl() is defined for unsigned long, which on my platform is 32 bits. I suppose if the unsigned long were 8 bytes, it would work for 64 bit ints).
My problem is that I need to convert 64 bit integers (in my case, this is an unsigned long long) from big endian to little endian. Right now, I need to do that specific conversion. But it would be even nicer if the function (like ntohl()) would NOT convert my 64 bit value if the target platform WAS big endian. (I'd rather avoid adding my own preprocessor magic to do this).
What can I use? I would like something that is standard if it exists, but I am open to implementation suggestions. I have seen this type of conversion done in the past using unions. I suppose I could have a union with an unsigned long long and a char[8]. Then swap the bytes around accordingly. (Obviously would break on platforms that were big endian).
Answer
Documentation: man htobe64 on Linux (glibc >= 2.9) or FreeBSD.
Unfortunately OpenBSD, FreeBSD and glibc (Linux) did not quite work together smoothly to create one (non-kernel-API) libc standard for this, during an attempt in 2009.
Currently, this short bit of preprocessor code:
#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
(tested on Linux and OpenBSD) should hide the differences. It gives you the Linux/FreeBSD-style macros on those 4 platforms.
Use example:
#include <stdint.h> // For 'uint64_t'
uint64_t host_int = 123;
uint64_t big_endian;
big_endian = htobe64( host_int );
host_int = be64toh( big_endian );
It's the most "standard C library"-ish approach available at the moment.