How do I convert between big-endian and little-endian values in C++?

Question

How do I convert between big-endian and little-endian values in C++? I'm using VC++ 6.0.when I used _byteswap_ulong() function it requires the header file intrin.h. When I include the header it reports an error saying incompatible compiler and that intrin.h is for the gcc compiler. So is there any other functions to convert between big-endian and little-endian values in VC++ other than this function?

Answer 1

The Docs seem to say these functions live in stdlib.h.

Answer 2

In POSIX-compliant systems, you have the standard byteswap(3) functions:

#include <arpa/inet.h>

uint32_t htonl(uint32_t hostlong);
uint16_t htons(uint16_t hostshort);
uint32_t ntohl(uint32_t netlong);
uint16_t ntohs(uint16_t netshort);

These are intended specifically for network use, as suggested by the name (host-to-network-long, host-to-network-short, etc.)

GNU and BSD systems also provide the endian(3) functions:

#define _BSD_SOURCE
#include <endian.h>

uint16_t htobe16(uint16_t host_16bits);
uint16_t htole16(uint16_t host_16bits);
uint16_t be16toh(uint16_t big_endian_16bits);
uint16_t le16toh(uint16_t little_endian_16bits);

uint32_t htobe32(uint32_t host_32bits);
uint32_t htole32(uint32_t host_32bits);
uint32_t be32toh(uint32_t big_endian_32bits);
uint32_t le32toh(uint32_t little_endian_32bits);

uint64_t htobe64(uint64_t host_64bits);
uint64_t htole64(uint64_t host_64bits);
uint64_t be64toh(uint64_t big_endian_64bits);
uint64_t le64toh(uint64_t little_endian_64bits);

(On OpenBSD, the number for bit width is always at the end of the function: be64toh() vs. betoh64(), for example.)

Otherwise, most people define their own macros or functions:

#define bswap16(x) ((x)>>8 | ((x)&255)<<8)
#deinfe bswap32(x) ((bswap16((x)>>16)&65535)|(bswap16((x)&65535)<<16))
/* etc. */

You can also use assembly intrinsics, like with the bswap instruction on x86. __builtin_bswap64 in GCC and ICC. Something similar is in VS since VS7.0.