htobe64, htole64, be64toh, le64toh - convert values  between  host  and
       big-/little-endian byte order

       #define _BSD_SOURCE             /* See feature_test_macros(7) */
       #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);

       These  functions  convert  the byte encoding of integer values from the
       byte order that the current CPU (the "host") uses, to and from  little-
       endian and big-endian byte order.

       The  number,  nn,  in  the  name of each function indicates the size of
       integer handled by the function, either 16, 32, or 64 bits.

       The functions with names of the form "htobenn" convert from  host  byte
       order to big-endian order.

       The  functions  with names of the form "htolenn" convert from host byte
       order to little-endian order.

       The functions with names of the form "benntoh" convert from  big-endian
       order to host byte order.

       The  functions  with  names  of the form "lenntoh" convert from little-
       endian order to host byte order.

       These function were added to glibc in version 2.9.

       These functions are nonstandard.  Similar functions are present on  the
       BSDs,  where  the  required  header  file  is <sys/endian.h> instead of
       <endian.h>.  Unfortunately, NetBSD, FreeBSD, and glibc haven't followed
       the original OpenBSD naming convention for these functions, whereby the
       nn component always appears at the end of the function name (thus,  for
       example,  in  NetBSD,  FreeBSD,  and  glibc, the equivalent of OpenBSDs
       "betoh32" is "be32toh").

       host byte order is either little-endian  or  big-endian,  only  one  of
       these  conversions  will have an effect.  When we run this program on a
       little-endian system such as x86-32, we see the following:

           $ ./a.out
           x.u32 = 0x44332211
           htole32(x.u32) = 0x44332211
           htobe32(x.u32) = 0x11223344

   Program source

       #include <endian.h>
       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>

       main(int argc, char *argv[])
           union {
            uint32_t u32;
            uint8_t arr[4];
           } x;

           x.arr[0] = 0x11;     /* Lowest-address byte */
           x.arr[1] = 0x22;
           x.arr[2] = 0x33;
           x.arr[3] = 0x44;     /* Highest-address byte */

           printf("x.u32 = 0x%x\n", x.u32);
           printf("htole32(x.u32) = 0x%x\n", htole32(x.u32));
           printf("htobe32(x.u32) = 0x%x\n", htobe32(x.u32));



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