convert big endian to little endian in C without using provided func

Question

I need to write a function to convert big endian to little endian in C  I can not use any library function

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This code snippet can convert 32bit little Endian number to Big Endian number        include  lt stdio h gt  main            unsigned int i   0xfafbfcfd      unsigned int j          j    i amp 0xff000000  gt  gt 24     i amp 0xff0000  gt  gt 8      i amp 0xff00  lt  lt 8      i amp 0xff  lt  lt 24           printf  unsigned int j    x n    j

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EDIT  This function only swaps the endianness of aligned 16 bit words  A function often necessary for UTF-16 UCS-2 encodings  EDIT END   If you want to change the endianess of a memory block you can use my blazingly fast approach  Your memory array should have a size that is a multiple of 8    include  lt stddef h gt   include  lt limits h gt   include  lt stdint h gt   void ChangeMemEndianness uint64 t  mem  size t size     uint64 t m1   0xFF00FF00FF00FF00ULL  m2   m1  gt  gt  CHAR BIT   size    size    sizeof  uint64 t  - 1     sizeof  uint64 t   for   size  size--  mem       mem      mem  amp  m1   gt  gt  CHAR BIT       mem  amp  m2   lt  lt  CHAR BIT       This kind of function is useful for changing the endianess of Unicode UCS-2 UTF-16 files

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If you are running on a x86 or x86 64 processor  the big endian is native  so  for 16 bit values  unsigned short wBigE   value  unsigned short wLittleE     wBigE  amp  0xFF   lt  lt  8     wBigE  gt  gt  8     for 32 bit values  unsigned int   iBigE   value  unsigned int   iLittleE     iBigE  amp  0xFF   lt  lt  24                              iBigE  amp  0xFF00   lt  lt  8                              iBigE  gt  gt  8   amp  0xFF00                             iBigE  gt  gt  24     This isn t the most efficient solution unless the compiler recognises that this is byte level manipulation and generates byte swapping code   But it doesn t depend on any memory layout tricks and can be turned into a macro pretty easily

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As a joke     include  lt stdio h gt   int main  int argc  char  argv          size t sizeofInt   sizeof  int       int i       union               int x          char c sizeof  int          original  swapped       original x   0x12345678       for  i   0  i  lt  sizeofInt  i            swapped c sizeofInt - i - 1    original c i        fprintf  stderr    x n   swapped x        return 0

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By including    include  lt byteswap h gt    you can get an optimized version of machine-dependent byte-swapping functions  Then  you can easily use the following functions     bswap 32  uint32 t input    or    bswap 16  uint16 t input

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Here s a fairly generic version   I haven t compiled it  so there are probably typos  but you should get the idea   void SwapBytes void  pv  size t n        assert n  gt  0        char  p   pv      size t lo  hi      for lo 0  hi n-1  hi gt lo  lo    hi--                char tmp p lo           p lo    p hi           p hi    tmp           define SWAP x  SwapBytes  amp x  sizeof x      NB  This is not optimised for speed or space   It is intended to be clear  easy to debug  and portable   Update 2018-04-04  Added the assert   to trap the invalid case of n    0  as spotted by commenter  chux

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here s a way using the SSSE3 instruction pshufb using its Intel intrinsic  assuming you have a multiple of 4 ints   unsigned int  bswap unsigned int  destination  unsigned int  source  int length        int i        m128i mask    mm set epi8 12  13  14  15  8  9  10  11  4  5  6  7  0  1  2  3       for  i   0  i  lt  length  i    4             mm storeu si128    m128i    amp destination i            mm shuffle epi8  mm loadu si128    m128i    amp source i    mask              return destination

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Assuming what you need is a simple byte swap  try something like  Unsigned 16 bit conversion   swapped    num  8     num lt  lt 8    Unsigned 32-bit conversion   swapped     num gt  gt 24  0xff       move byte 3 to byte 0                       num lt  lt 8  0xff0000       move byte 1 to byte 2                       num gt  gt 8  0xff00       move byte 2 to byte 1                       num lt  lt 24  0xff000000      byte 0 to byte 3  This swaps the byte orders from positions 1234 to 4321  If your input was 0xdeadbeef  a 32-bit endian swap might have output of 0xefbeadde   The code above should be cleaned up with macros or at least constants instead of magic numbers  but hopefully it helps as is  EDIT  as another answer pointed out  there are platform  OS  and instruction set specific alternatives which can be MUCH faster than the above  In the Linux kernel there are macros  cpu to be32 for example  which handle endianness pretty nicely  But these alternatives are specific to their environments  In practice endianness is best dealt with using a blend of available approaches

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If you need macros  e g  embedded system     define SWAP UINT16 x     x   gt  gt  8      x   lt  lt  8    define SWAP UINT32 x     x   gt  gt  24       x   amp  0x00FF0000   gt  gt  8       x   amp  0x0000FF00   lt  lt  8      x   lt  lt  24

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include  lt stdint h gt        Byte swap unsigned short uint16 t swap uint16  uint16 t val          return  val  lt  lt  8     val  gt  gt  8           Byte swap short int16 t swap int16  int16 t val          return  val  lt  lt  8      val  gt  gt  8   amp  0xFF          Byte swap unsigned int uint32 t swap uint32  uint32 t val         val     val  lt  lt  8   amp  0xFF00FF00       val  gt  gt  8   amp  0xFF00FF         return  val  lt  lt  16     val  gt  gt  16          Byte swap int int32 t swap int32  int32 t val         val     val  lt  lt  8   amp  0xFF00FF00      val  gt  gt  8   amp  0xFF00FF         return  val  lt  lt  16      val  gt  gt  16   amp  0xFFFF       Update   Added 64bit byte swapping  int64 t swap int64  int64 t val         val     val  lt  lt  8   amp  0xFF00FF00FF00FF00ULL       val  gt  gt  8   amp  0x00FF00FF00FF00FFULL        val     val  lt  lt  16   amp  0xFFFF0000FFFF0000ULL       val  gt  gt  16   amp  0x0000FFFF0000FFFFULL        return  val  lt  lt  32      val  gt  gt  32   amp  0xFFFFFFFFULL      uint64 t swap uint64  uint64 t val         val     val  lt  lt  8   amp  0xFF00FF00FF00FF00ULL       val  gt  gt  8   amp  0x00FF00FF00FF00FFULL        val     val  lt  lt  16   amp  0xFFFF0000FFFF0000ULL       val  gt  gt  16   amp  0x0000FFFF0000FFFFULL        return  val  lt  lt  32     val  gt  gt  32

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Here s a function I have been using - tested and works on any basic data type       SwapBytes h        Function to perform in-place endian conversion of basic types        Usage           double d        SwapBytes  amp d  sizeof d        inline void SwapBytes void  source  int size        typedef unsigned char TwoBytes 2       typedef unsigned char FourBytes 4       typedef unsigned char EightBytes 8        unsigned char temp       if size    2                TwoBytes  src    TwoBytes   source          temp     src  0             src  0      src  1             src  1    temp           return             if size    4                FourBytes  src    FourBytes   source          temp     src  0             src  0      src  3             src  3    temp           temp     src  1             src  1      src  2             src  2    temp           return             if size    8                EightBytes  src    EightBytes   source          temp     src  0             src  0      src  7             src  7    temp           temp     src  1             src  1      src  6             src  6    temp           temp     src  2             src  2      src  5             src  5    temp           temp     src  3             src  3      src  4             src  4    temp           return

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Will this work   be faster    uint32 t swapped  result     byte   amp swapped  0      byte   amp result  3     byte   amp swapped  1      byte   amp result  2     byte   amp swapped  2      byte   amp result  1     byte   amp swapped  3      byte   amp result  0

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Edit  These are library functions  Following them is the manual way to do it   I am absolutely stunned by the number of people unaware of   byteswap ushort    byteswap ulong  and   byteswap uint64  Sure they are Visual C   specific  but they compile down to some delicious code on x86 IA-64 architectures      Here s an explicit usage of the bswap instruction  pulled from this page  Note that the intrinsic form above will always be faster than this  I only added it to give an answer without a library routine   uint32 cq ntohl uint32 a          asm          mov eax  a          bswap eax

[c] convert big endian to little endian in C [without using provided func]

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