How do you reverse a string in place in C or C

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How do you reverse a string in C or C   without requiring a separate buffer to hold the reversed string

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include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt stdbool h gt   unsigned char   utf8 reverse const unsigned char    int   void assert true bool    int main void        unsigned char str      ma  ana man ana       unsigned char  ret   utf8 reverse str   strlen  const char    str    1        printf   s n   ret       assert true 0    strncmp  const char    ret   ana nam ana  am   strlen  ana nam ana  am     1         free ret        return EXIT SUCCESS     unsigned char   utf8 reverse const unsigned char  str  int size        unsigned char  ret   calloc size  sizeof unsigned char         int ret size   0      int pos   size - 2      int char size   0       if  str     NULL            fprintf stderr   failed to allocate memory  n            exit EXIT FAILURE              while  pos  gt  -1             if  str pos   lt  0x80                char size   1            else if  pos  gt  0  amp  amp  str pos - 1   gt  0xC1  amp  amp  str pos - 1   lt  0xE0                char size   2            else if  pos  gt  1  amp  amp  str pos - 2   gt  0xDF  amp  amp  str pos - 2   lt  0xF0                char size   3            else if  pos  gt  2  amp  amp  str pos - 3   gt  0xEF  amp  amp  str pos - 3   lt  0xF5                char size   4            else               char size   1                     pos -  char size          memcpy ret   ret size  str   pos   1  char size           ret size    char size                 ret ret size      0        return ret     void assert true bool boolean        puts boolean    true    true     false

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include  lt algorithm gt  std  reverse str begin    str end       This is the simplest way in C

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Non-evil C  assuming the common case where the string is a null-terminated char array    include  lt stddef h gt   include  lt string h gt      PRE  str must be either NULL or a pointer to a      possibly empty  null-terminated string     void strrev char  str      char temp   end ptr        If str is NULL or empty  do nothing      if  str    NULL       str        return     end ptr   str   strlen str  - 1        Swap the chars      while  end ptr  gt  str         temp    str       str    end ptr       end ptr   temp      str        end ptr--

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C   multi-byte UTF-8 reverser My thought is that you can never just swap ends  you must always move from beginning-to-end  move through the string and look for  quot how many bytes will this character require  quot  I attach the character starting at the original end position  and remove the character from the front of the string  void StringReverser std  string  original      int eos   original- gt length   - 1    while  eos  gt  0        char c     original  0       int characterBytes      switch   c  amp  0xF0   gt  gt  4         case 0xC      case 0xD     U 000080-U 0007FF  two bytes           characterBytes   2        break      case 0xE     U 000800-U 00FFFF  three bytes           characterBytes   3        break      case 0xF     U 010000-U 10FFFF  four bytes           characterBytes   4        break      default        characterBytes   1        break             for  int i   0  i  lt  characterBytes  i            original- gt insert eos i  1    original  i              original- gt erase 0  characterBytes       eos -  characterBytes

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If you are using ATL MFC CString  simply call CString  MakeReverse

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include  lt algorithm gt  std  reverse str begin    str end       This is the simplest way in C

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Non-evil C  assuming the common case where the string is a null-terminated char array    include  lt stddef h gt   include  lt string h gt      PRE  str must be either NULL or a pointer to a      possibly empty  null-terminated string     void strrev char  str      char temp   end ptr        If str is NULL or empty  do nothing      if  str    NULL       str        return     end ptr   str   strlen str  - 1        Swap the chars      while  end ptr  gt  str         temp    str       str    end ptr       end ptr   temp      str        end ptr--

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Another C   way  though I would probably use std  reverse   myself    as being more expressive and faster   str   std  string str rbegin    str rend       The C way  more or less      and please  be careful about XOR trick for swapping   compilers sometimes cannot optimize that   In such case it is usually much slower   char  reverse char  s        char  beg   s   end   s  tmp      while   end  end        while  end--  gt  beg                 tmp     beg            beg      end             end    tmp            return s       fixed  check history for details  as those are interesting ones

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I like Evgeny s K amp R answer  However  it is nice to see a version using pointers  Otherwise  it s essentially the same    include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   char  reverse char  str        if  str    NULL       str    return NULL      int i  j   strlen str -1      char  sallocd      sallocd   malloc sizeof char     j 1        for i 0  j gt  0  i    j--              sallocd i      str j             return sallocd     int main void        char  s    a man a plan a canal panama       char  sret   reverse s       printf   s n   reverse sret        free sret       return 0

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In the interest of completeness  it should be pointed out that there are representations of strings on various platforms in which the number of bytes per character varies depending on the character   Old-school programmers would refer to this as DBCS  Double Byte Character Set   Modern programmers more commonly encounter this in UTF-8  as well as UTF-16 and others    There are other such encodings as well   In any of these variable-width encoding schemes  the simple algorithms posted here  evil  non-evil or otherwise  would not work correctly at all   In fact  they could even cause the string to become illegible or even an illegal string in that encoding scheme   See Juan Pablo Califano s answer for some good examples   std  reverse   potentially would still work in this case  as long as your platform s implementation of the Standard C   Library  in particular  string iterators  properly took this into account

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include  lt cstdio gt   include  lt cstdlib gt   include  lt string gt   void strrev char  str            if  str    NULL                   return           char  end ptr    amp str strlen str  - 1           char temp          while  end ptr  gt  str                             temp    str                   str      end ptr                   end ptr--   temp               int main int argc  char  argv              char buffer 32            strcpy buffer   testing            strrev buffer           printf   s n   buffer            strcpy buffer   a            strrev buffer           printf   s n   buffer            strcpy buffer   abc            strrev buffer           printf   s n   buffer            strcpy buffer               strrev buffer           printf   s n   buffer            strrev NULL            return 0      This code produces this output   gnitset a cba

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The standard algorithm is to use pointers to the start   end  and walk them inward until they meet or cross in the middle   Swap as you go     Reverse ASCII string  i e  a 0-terminated array where every character fits in 1 char    Or other non-multibyte character sets    void strrev char  head      if   head  return    char  tail   head    while  tail    tail        find the 0 terminator  like head strlen   --tail                   tail points to the last real char                            head still points to the first   for    head  lt  tail    head  --tail             walk pointers inwards until they meet or cross in the middle       char h    head  t    tail         head   t               swapping as we go        tail   h               test program that reverses its args  include  lt stdio h gt   int main int argc  char   argv      do       printf   s     argv argc-1        strrev argv argc-1        printf   s n   argv argc-1        while --argc      return 0      The same algorithm works for integer arrays with known length  just use tail   start   length - 1 instead of the end-finding loop    Editor s note  this answer originally used XOR-swap for this simple version  too   Fixed for the benefit of future readers of this popular question   XOR-swap is highly not recommended  hard to read and making your code compile less efficiently   You can see on the Godbolt compiler explorer how much more complicated the asm loop body is when xor-swap is compiled for x86-64 with gcc -O3      Ok  fine  let s fix the UTF-8 chars      This is XOR-swap thing  Take care to note that you must avoid swapping with self  because if  p and  q are the same location you ll zero it with a a  0   XOR-swap depends on having two distinct locations  using them each as temporary storage    Editor s note  you can replace SWP with a safe inline function using a tmp variable    include  lt bits types h gt   include  lt stdio h gt    define SWP x y   x  y  y  x  x  y   void strrev char  p      char  q   p    while q  amp  amp   q    q     find eos      for --q  p  lt  q    p  --q  SWP  p   q      void strrev utf8 char  p      char  q   p    strrev p      call base case          Ok  now fix bass-ackwards UTF chars       while q  amp  amp   q    q     find eos      while p  lt  --q      switch    q  amp  0xF0   gt  gt  4         case 0xF     U 010000-U 10FFFF  four bytes           SWP   q-0     q-3          SWP   q-1     q-2          q -  3        break      case 0xE     U 000800-U 00FFFF  three bytes           SWP   q-0     q-2          q -  2        break      case 0xC     fall-through        case 0xD     U 000080-U 0007FF  two bytes           SWP   q-0     q-1          q--        break           int main int argc  char   argv      do       printf   s     argv argc-1        strrev utf8 argv argc-1        printf   s n   argv argc-1        while --argc      return 0       Why  yes  if the input is borked  this will cheerfully swap outside the place  Useful link when vandalising in the UNICODE  http   www macchiato com unicode chart  Also  UTF-8 over 0x10000 is untested  as I don t seem to have any font for it  nor the patience to use a hexeditor    Examples       strrev R  ksm  rg  s                                       R  ksm  rg  s s  gr  msk  R    strrev verrts

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C   multi-byte UTF-8 reverser My thought is that you can never just swap ends  you must always move from beginning-to-end  move through the string and look for  quot how many bytes will this character require  quot  I attach the character starting at the original end position  and remove the character from the front of the string  void StringReverser std  string  original      int eos   original- gt length   - 1    while  eos  gt  0        char c     original  0       int characterBytes      switch   c  amp  0xF0   gt  gt  4         case 0xC      case 0xD     U 000080-U 0007FF  two bytes           characterBytes   2        break      case 0xE     U 000800-U 00FFFF  three bytes           characterBytes   3        break      case 0xF     U 010000-U 10FFFF  four bytes           characterBytes   4        break      default        characterBytes   1        break             for  int i   0  i  lt  characterBytes  i            original- gt insert eos i  1    original  i              original- gt erase 0  characterBytes       eos -  characterBytes

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include  lt cstdio gt   include  lt cstdlib gt   include  lt string gt   void strrev char  str            if  str    NULL                   return           char  end ptr    amp str strlen str  - 1           char temp          while  end ptr  gt  str                             temp    str                   str      end ptr                   end ptr--   temp               int main int argc  char  argv              char buffer 32            strcpy buffer   testing            strrev buffer           printf   s n   buffer            strcpy buffer   a            strrev buffer           printf   s n   buffer            strcpy buffer   abc            strrev buffer           printf   s n   buffer            strcpy buffer               strrev buffer           printf   s n   buffer            strrev NULL            return 0      This code produces this output   gnitset a cba

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If you re looking for reversing NULL terminated buffers  most solutions posted here are OK  But  as Tim Farley already pointed out  these algorithms will work only if it s valid to assume that a string is semantically an array of bytes  i e  single-byte strings   which is a wrong assumption  I think    Take for example  the string  a  o   year in Spanish    The Unicode code points are 0x61  0xf1  0x6f   Consider some of the most used encodings   Latin1   iso-8859-1  single byte encoding  1 character is 1 byte and vice versa       Original       0x61  0xf1  0x6f  0x00      Reverse       0x6f  0xf1  0x61  0x00      The result is OK   UTF-8      Original       0x61  0xc3  0xb1  0x6f  0x00      Reverse       0x6f  0xb1  0xc3  0x61  0x00      The result is gibberish and an illegal UTF-8 sequence   UTF-16 Big Endian      Original       0x00  0x61  0x00  0xf1  0x00  0x6f  0x00  0x00      The first byte will be treated as a NUL-terminator  No reversing will take place    UTF-16 Little Endian      Original       0x61  0x00  0xf1  0x00  0x6f  0x00  0x00  0x00      The second byte will be treated as a NUL-terminator  The result will be 0x61  0x00  a string containing the  a  character

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If you re looking for reversing NULL terminated buffers  most solutions posted here are OK  But  as Tim Farley already pointed out  these algorithms will work only if it s valid to assume that a string is semantically an array of bytes  i e  single-byte strings   which is a wrong assumption  I think    Take for example  the string  a  o   year in Spanish    The Unicode code points are 0x61  0xf1  0x6f   Consider some of the most used encodings   Latin1   iso-8859-1  single byte encoding  1 character is 1 byte and vice versa       Original       0x61  0xf1  0x6f  0x00      Reverse       0x6f  0xf1  0x61  0x00      The result is OK   UTF-8      Original       0x61  0xc3  0xb1  0x6f  0x00      Reverse       0x6f  0xb1  0xc3  0x61  0x00      The result is gibberish and an illegal UTF-8 sequence   UTF-16 Big Endian      Original       0x00  0x61  0x00  0xf1  0x00  0x6f  0x00  0x00      The first byte will be treated as a NUL-terminator  No reversing will take place    UTF-16 Little Endian      Original       0x61  0x00  0xf1  0x00  0x6f  0x00  0x00  0x00      The second byte will be treated as a NUL-terminator  The result will be 0x61  0x00  a string containing the  a  character

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In the interest of completeness  it should be pointed out that there are representations of strings on various platforms in which the number of bytes per character varies depending on the character   Old-school programmers would refer to this as DBCS  Double Byte Character Set   Modern programmers more commonly encounter this in UTF-8  as well as UTF-16 and others    There are other such encodings as well   In any of these variable-width encoding schemes  the simple algorithms posted here  evil  non-evil or otherwise  would not work correctly at all   In fact  they could even cause the string to become illegible or even an illegal string in that encoding scheme   See Juan Pablo Califano s answer for some good examples   std  reverse   potentially would still work in this case  as long as your platform s implementation of the Standard C   Library  in particular  string iterators  properly took this into account

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Read Kernighan and Ritchie   include  lt string h gt   void reverse char s          int length   strlen s        int c  i  j       for  i   0  j   length - 1  i  lt  j  i    j--                c   s i           s i    s j           s j    c

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include  lt cstdio gt   include  lt cstdlib gt   include  lt string gt   void strrev char  str            if  str    NULL                   return           char  end ptr    amp str strlen str  - 1           char temp          while  end ptr  gt  str                             temp    str                   str      end ptr                   end ptr--   temp               int main int argc  char  argv              char buffer 32            strcpy buffer   testing            strrev buffer           printf   s n   buffer            strcpy buffer   a            strrev buffer           printf   s n   buffer            strcpy buffer   abc            strrev buffer           printf   s n   buffer            strcpy buffer               strrev buffer           printf   s n   buffer            strrev NULL            return 0      This code produces this output   gnitset a cba

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The standard algorithm is to use pointers to the start   end  and walk them inward until they meet or cross in the middle   Swap as you go     Reverse ASCII string  i e  a 0-terminated array where every character fits in 1 char    Or other non-multibyte character sets    void strrev char  head      if   head  return    char  tail   head    while  tail    tail        find the 0 terminator  like head strlen   --tail                   tail points to the last real char                            head still points to the first   for    head  lt  tail    head  --tail             walk pointers inwards until they meet or cross in the middle       char h    head  t    tail         head   t               swapping as we go        tail   h               test program that reverses its args  include  lt stdio h gt   int main int argc  char   argv      do       printf   s     argv argc-1        strrev argv argc-1        printf   s n   argv argc-1        while --argc      return 0      The same algorithm works for integer arrays with known length  just use tail   start   length - 1 instead of the end-finding loop    Editor s note  this answer originally used XOR-swap for this simple version  too   Fixed for the benefit of future readers of this popular question   XOR-swap is highly not recommended  hard to read and making your code compile less efficiently   You can see on the Godbolt compiler explorer how much more complicated the asm loop body is when xor-swap is compiled for x86-64 with gcc -O3      Ok  fine  let s fix the UTF-8 chars      This is XOR-swap thing  Take care to note that you must avoid swapping with self  because if  p and  q are the same location you ll zero it with a a  0   XOR-swap depends on having two distinct locations  using them each as temporary storage    Editor s note  you can replace SWP with a safe inline function using a tmp variable    include  lt bits types h gt   include  lt stdio h gt    define SWP x y   x  y  y  x  x  y   void strrev char  p      char  q   p    while q  amp  amp   q    q     find eos      for --q  p  lt  q    p  --q  SWP  p   q      void strrev utf8 char  p      char  q   p    strrev p      call base case          Ok  now fix bass-ackwards UTF chars       while q  amp  amp   q    q     find eos      while p  lt  --q      switch    q  amp  0xF0   gt  gt  4         case 0xF     U 010000-U 10FFFF  four bytes           SWP   q-0     q-3          SWP   q-1     q-2          q -  3        break      case 0xE     U 000800-U 00FFFF  three bytes           SWP   q-0     q-2          q -  2        break      case 0xC     fall-through        case 0xD     U 000080-U 0007FF  two bytes           SWP   q-0     q-1          q--        break           int main int argc  char   argv      do       printf   s     argv argc-1        strrev utf8 argv argc-1        printf   s n   argv argc-1        while --argc      return 0       Why  yes  if the input is borked  this will cheerfully swap outside the place  Useful link when vandalising in the UNICODE  http   www macchiato com unicode chart  Also  UTF-8 over 0x10000 is untested  as I don t seem to have any font for it  nor the patience to use a hexeditor    Examples       strrev R  ksm  rg  s                                       R  ksm  rg  s s  gr  msk  R    strrev verrts

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If you don t need to store it  you can reduce the time spent like this   void showReverse char s    int length        printf  Reversed String without storing is           could use another variable to test for length  keeping length whole        assumes contiguous memory     for    length  gt  0  length--                printf   c     s  length-1               printf   n

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I like Evgeny s K amp R answer  However  it is nice to see a version using pointers  Otherwise  it s essentially the same    include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   char  reverse char  str        if  str    NULL       str    return NULL      int i  j   strlen str -1      char  sallocd      sallocd   malloc sizeof char     j 1        for i 0  j gt  0  i    j--              sallocd i      str j             return sallocd     int main void        char  s    a man a plan a canal panama       char  sret   reverse s       printf   s n   reverse sret        free sret       return 0

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include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt stdbool h gt   unsigned char   utf8 reverse const unsigned char    int   void assert true bool    int main void        unsigned char str      ma  ana man ana       unsigned char  ret   utf8 reverse str   strlen  const char    str    1        printf   s n   ret       assert true 0    strncmp  const char    ret   ana nam ana  am   strlen  ana nam ana  am     1         free ret        return EXIT SUCCESS     unsigned char   utf8 reverse const unsigned char  str  int size        unsigned char  ret   calloc size  sizeof unsigned char         int ret size   0      int pos   size - 2      int char size   0       if  str     NULL            fprintf stderr   failed to allocate memory  n            exit EXIT FAILURE              while  pos  gt  -1             if  str pos   lt  0x80                char size   1            else if  pos  gt  0  amp  amp  str pos - 1   gt  0xC1  amp  amp  str pos - 1   lt  0xE0                char size   2            else if  pos  gt  1  amp  amp  str pos - 2   gt  0xDF  amp  amp  str pos - 2   lt  0xF0                char size   3            else if  pos  gt  2  amp  amp  str pos - 3   gt  0xEF  amp  amp  str pos - 3   lt  0xF5                char size   4            else               char size   1                     pos -  char size          memcpy ret   ret size  str   pos   1  char size           ret size    char size                 ret ret size      0        return ret     void assert true bool boolean        puts boolean    true    true     false

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Read Kernighan and Ritchie   include  lt string h gt   void reverse char s          int length   strlen s        int c  i  j       for  i   0  j   length - 1  i  lt  j  i    j--                c   s i           s i    s j           s j    c

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Yet another    include  lt stdio h gt   include  lt strings h gt   int main int argc  char   argv       char  reverse   argv argc-1     char  left   reverse    int length   strlen reverse     char  right   reverse length-1    char temp     while right-left gt  1        temp  left       left  right       right temp        left      --right          printf   s n   reverse

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Note that the beauty of std  reverse is that it works with char   strings and std  wstrings just as well as std  strings  void strrev char  str        if  str    NULL          return      std  reverse str  str   strlen str

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In the interest of completeness  it should be pointed out that there are representations of strings on various platforms in which the number of bytes per character varies depending on the character   Old-school programmers would refer to this as DBCS  Double Byte Character Set   Modern programmers more commonly encounter this in UTF-8  as well as UTF-16 and others    There are other such encodings as well   In any of these variable-width encoding schemes  the simple algorithms posted here  evil  non-evil or otherwise  would not work correctly at all   In fact  they could even cause the string to become illegible or even an illegal string in that encoding scheme   See Juan Pablo Califano s answer for some good examples   std  reverse   potentially would still work in this case  as long as your platform s implementation of the Standard C   Library  in particular  string iterators  properly took this into account

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include  lt algorithm gt  std  reverse str begin    str end       This is the simplest way in C

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Note that the beauty of std  reverse is that it works with char   strings and std  wstrings just as well as std  strings  void strrev char  str        if  str    NULL          return      std  reverse str  str   strlen str

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In case you are using GLib  it has two functions for that  g strreverse   and g utf8 strreverse

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Read Kernighan and Ritchie   include  lt string h gt   void reverse char s          int length   strlen s        int c  i  j       for  i   0  j   length - 1  i  lt  j  i    j--                c   s i           s i    s j           s j    c

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It s been a while and I don t remember which book taught me this algorithm  but I thought it was quite ingenious and simple to understand  char input      quot moc wolfrevokcats quot    int length   strlen input   int last pos   length-1  for int i   0  i  lt  length 2  i          char tmp   input i       input i    input last pos - i       input last pos - i    tmp     printf  quot  s n quot   input    A visualization of this algorithm  courtesy of slashdottir

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In case you are using GLib  it has two functions for that  g strreverse   and g utf8 strreverse

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The standard algorithm is to use pointers to the start   end  and walk them inward until they meet or cross in the middle   Swap as you go     Reverse ASCII string  i e  a 0-terminated array where every character fits in 1 char    Or other non-multibyte character sets    void strrev char  head      if   head  return    char  tail   head    while  tail    tail        find the 0 terminator  like head strlen   --tail                   tail points to the last real char                            head still points to the first   for    head  lt  tail    head  --tail             walk pointers inwards until they meet or cross in the middle       char h    head  t    tail         head   t               swapping as we go        tail   h               test program that reverses its args  include  lt stdio h gt   int main int argc  char   argv      do       printf   s     argv argc-1        strrev argv argc-1        printf   s n   argv argc-1        while --argc      return 0      The same algorithm works for integer arrays with known length  just use tail   start   length - 1 instead of the end-finding loop    Editor s note  this answer originally used XOR-swap for this simple version  too   Fixed for the benefit of future readers of this popular question   XOR-swap is highly not recommended  hard to read and making your code compile less efficiently   You can see on the Godbolt compiler explorer how much more complicated the asm loop body is when xor-swap is compiled for x86-64 with gcc -O3      Ok  fine  let s fix the UTF-8 chars      This is XOR-swap thing  Take care to note that you must avoid swapping with self  because if  p and  q are the same location you ll zero it with a a  0   XOR-swap depends on having two distinct locations  using them each as temporary storage    Editor s note  you can replace SWP with a safe inline function using a tmp variable    include  lt bits types h gt   include  lt stdio h gt    define SWP x y   x  y  y  x  x  y   void strrev char  p      char  q   p    while q  amp  amp   q    q     find eos      for --q  p  lt  q    p  --q  SWP  p   q      void strrev utf8 char  p      char  q   p    strrev p      call base case          Ok  now fix bass-ackwards UTF chars       while q  amp  amp   q    q     find eos      while p  lt  --q      switch    q  amp  0xF0   gt  gt  4         case 0xF     U 010000-U 10FFFF  four bytes           SWP   q-0     q-3          SWP   q-1     q-2          q -  3        break      case 0xE     U 000800-U 00FFFF  three bytes           SWP   q-0     q-2          q -  2        break      case 0xC     fall-through        case 0xD     U 000080-U 0007FF  two bytes           SWP   q-0     q-1          q--        break           int main int argc  char   argv      do       printf   s     argv argc-1        strrev utf8 argv argc-1        printf   s n   argv argc-1        while --argc      return 0       Why  yes  if the input is borked  this will cheerfully swap outside the place  Useful link when vandalising in the UNICODE  http   www macchiato com unicode chart  Also  UTF-8 over 0x10000 is untested  as I don t seem to have any font for it  nor the patience to use a hexeditor    Examples       strrev R  ksm  rg  s                                       R  ksm  rg  s s  gr  msk  R    strrev verrts

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If you don t need to store it  you can reduce the time spent like this   void showReverse char s    int length        printf  Reversed String without storing is           could use another variable to test for length  keeping length whole        assumes contiguous memory     for    length  gt  0  length--                printf   c     s  length-1               printf   n

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The standard algorithm is to use pointers to the start   end  and walk them inward until they meet or cross in the middle   Swap as you go     Reverse ASCII string  i e  a 0-terminated array where every character fits in 1 char    Or other non-multibyte character sets    void strrev char  head      if   head  return    char  tail   head    while  tail    tail        find the 0 terminator  like head strlen   --tail                   tail points to the last real char                            head still points to the first   for    head  lt  tail    head  --tail             walk pointers inwards until they meet or cross in the middle       char h    head  t    tail         head   t               swapping as we go        tail   h               test program that reverses its args  include  lt stdio h gt   int main int argc  char   argv      do       printf   s     argv argc-1        strrev argv argc-1        printf   s n   argv argc-1        while --argc      return 0      The same algorithm works for integer arrays with known length  just use tail   start   length - 1 instead of the end-finding loop    Editor s note  this answer originally used XOR-swap for this simple version  too   Fixed for the benefit of future readers of this popular question   XOR-swap is highly not recommended  hard to read and making your code compile less efficiently   You can see on the Godbolt compiler explorer how much more complicated the asm loop body is when xor-swap is compiled for x86-64 with gcc -O3      Ok  fine  let s fix the UTF-8 chars      This is XOR-swap thing  Take care to note that you must avoid swapping with self  because if  p and  q are the same location you ll zero it with a a  0   XOR-swap depends on having two distinct locations  using them each as temporary storage    Editor s note  you can replace SWP with a safe inline function using a tmp variable    include  lt bits types h gt   include  lt stdio h gt    define SWP x y   x  y  y  x  x  y   void strrev char  p      char  q   p    while q  amp  amp   q    q     find eos      for --q  p  lt  q    p  --q  SWP  p   q      void strrev utf8 char  p      char  q   p    strrev p      call base case          Ok  now fix bass-ackwards UTF chars       while q  amp  amp   q    q     find eos      while p  lt  --q      switch    q  amp  0xF0   gt  gt  4         case 0xF     U 010000-U 10FFFF  four bytes           SWP   q-0     q-3          SWP   q-1     q-2          q -  3        break      case 0xE     U 000800-U 00FFFF  three bytes           SWP   q-0     q-2          q -  2        break      case 0xC     fall-through        case 0xD     U 000080-U 0007FF  two bytes           SWP   q-0     q-1          q--        break           int main int argc  char   argv      do       printf   s     argv argc-1        strrev utf8 argv argc-1        printf   s n   argv argc-1        while --argc      return 0       Why  yes  if the input is borked  this will cheerfully swap outside the place  Useful link when vandalising in the UNICODE  http   www macchiato com unicode chart  Also  UTF-8 over 0x10000 is untested  as I don t seem to have any font for it  nor the patience to use a hexeditor    Examples       strrev R  ksm  rg  s                                       R  ksm  rg  s s  gr  msk  R    strrev verrts

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Read Kernighan and Ritchie   include  lt string h gt   void reverse char s          int length   strlen s        int c  i  j       for  i   0  j   length - 1  i  lt  j  i    j--                c   s i           s i    s j           s j    c

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void reverseString vector lt char gt  amp  s            int l   s size            char ch           int i   0           int j   l-1          while i  lt  j                   s i    s i  s j                   s j    s i  s j                   s i    s i  s j                   i                    j--                    for char c   s                  cout  lt  lt c           cout lt  lt  endl

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In case you are using GLib  it has two functions for that  g strreverse   and g utf8 strreverse

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Recursive function to reverse a string in place  no extra buffer  malloc    Short  sexy code  Bad  bad stack usage     include  lt stdio h gt      Store the each value and move to next char going down    the stack  Assign value to start ptr and increment     when coming back up the stack  return      Neat code  horrible stack usage        val - value of current pointer     s - start pointer    n - next char pointer in string      char  reverse r char val  char  s  char  n        if   n          s   reverse r  n  s  n 1       s   val     return s 1           expect the string to be passed as argv 1      int main int argc  char  argv          char  aString       if  argc  lt  2                printf  Usage  RSIP  lt string gt  n            return 0             aString   argv 1       printf  String to reverse   s n   aString         reverse r  aString  aString  aString 1        printf  Reversed String     s n   aString         return 0

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Non-evil C  assuming the common case where the string is a null-terminated char array    include  lt stddef h gt   include  lt string h gt      PRE  str must be either NULL or a pointer to a      possibly empty  null-terminated string     void strrev char  str      char temp   end ptr        If str is NULL or empty  do nothing      if  str    NULL       str        return     end ptr   str   strlen str  - 1        Swap the chars      while  end ptr  gt  str         temp    str       str    end ptr       end ptr   temp      str        end ptr--

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void reverseString vector lt char gt  amp  s            int l   s size            char ch           int i   0           int j   l-1          while i  lt  j                   s i    s i  s j                   s j    s i  s j                   s i    s i  s j                   i                    j--                    for char c   s                  cout  lt  lt c           cout lt  lt  endl

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If you are using ATL MFC CString  simply call CString  MakeReverse

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Yet another    include  lt stdio h gt   include  lt strings h gt   int main int argc  char   argv       char  reverse   argv argc-1     char  left   reverse    int length   strlen reverse     char  right   reverse length-1    char temp     while right-left gt  1        temp  left       left  right       right temp        left      --right          printf   s n   reverse

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Note that the beauty of std  reverse is that it works with char   strings and std  wstrings just as well as std  strings  void strrev char  str        if  str    NULL          return      std  reverse str  str   strlen str

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include  lt algorithm gt  std  reverse str begin    str end       This is the simplest way in C

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include  lt cstdio gt   include  lt cstdlib gt   include  lt string gt   void strrev char  str            if  str    NULL                   return           char  end ptr    amp str strlen str  - 1           char temp          while  end ptr  gt  str                             temp    str                   str      end ptr                   end ptr--   temp               int main int argc  char  argv              char buffer 32            strcpy buffer   testing            strrev buffer           printf   s n   buffer            strcpy buffer   a            strrev buffer           printf   s n   buffer            strcpy buffer   abc            strrev buffer           printf   s n   buffer            strcpy buffer               strrev buffer           printf   s n   buffer            strrev NULL            return 0      This code produces this output   gnitset a cba

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If you re looking for reversing NULL terminated buffers  most solutions posted here are OK  But  as Tim Farley already pointed out  these algorithms will work only if it s valid to assume that a string is semantically an array of bytes  i e  single-byte strings   which is a wrong assumption  I think    Take for example  the string  a  o   year in Spanish    The Unicode code points are 0x61  0xf1  0x6f   Consider some of the most used encodings   Latin1   iso-8859-1  single byte encoding  1 character is 1 byte and vice versa       Original       0x61  0xf1  0x6f  0x00      Reverse       0x6f  0xf1  0x61  0x00      The result is OK   UTF-8      Original       0x61  0xc3  0xb1  0x6f  0x00      Reverse       0x6f  0xb1  0xc3  0x61  0x00      The result is gibberish and an illegal UTF-8 sequence   UTF-16 Big Endian      Original       0x00  0x61  0x00  0xf1  0x00  0x6f  0x00  0x00      The first byte will be treated as a NUL-terminator  No reversing will take place    UTF-16 Little Endian      Original       0x61  0x00  0xf1  0x00  0x6f  0x00  0x00  0x00      The second byte will be treated as a NUL-terminator  The result will be 0x61  0x00  a string containing the  a  character

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Recursive function to reverse a string in place  no extra buffer  malloc    Short  sexy code  Bad  bad stack usage     include  lt stdio h gt      Store the each value and move to next char going down    the stack  Assign value to start ptr and increment     when coming back up the stack  return      Neat code  horrible stack usage        val - value of current pointer     s - start pointer    n - next char pointer in string      char  reverse r char val  char  s  char  n        if   n          s   reverse r  n  s  n 1       s   val     return s 1           expect the string to be passed as argv 1      int main int argc  char  argv          char  aString       if  argc  lt  2                printf  Usage  RSIP  lt string gt  n            return 0             aString   argv 1       printf  String to reverse   s n   aString         reverse r  aString  aString  aString 1        printf  Reversed String     s n   aString         return 0

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If you re looking for reversing NULL terminated buffers  most solutions posted here are OK  But  as Tim Farley already pointed out  these algorithms will work only if it s valid to assume that a string is semantically an array of bytes  i e  single-byte strings   which is a wrong assumption  I think    Take for example  the string  a  o   year in Spanish    The Unicode code points are 0x61  0xf1  0x6f   Consider some of the most used encodings   Latin1   iso-8859-1  single byte encoding  1 character is 1 byte and vice versa       Original       0x61  0xf1  0x6f  0x00      Reverse       0x6f  0xf1  0x61  0x00      The result is OK   UTF-8      Original       0x61  0xc3  0xb1  0x6f  0x00      Reverse       0x6f  0xb1  0xc3  0x61  0x00      The result is gibberish and an illegal UTF-8 sequence   UTF-16 Big Endian      Original       0x00  0x61  0x00  0xf1  0x00  0x6f  0x00  0x00      The first byte will be treated as a NUL-terminator  No reversing will take place    UTF-16 Little Endian      Original       0x61  0x00  0xf1  0x00  0x6f  0x00  0x00  0x00      The second byte will be treated as a NUL-terminator  The result will be 0x61  0x00  a string containing the  a  character

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It s been a while and I don t remember which book taught me this algorithm  but I thought it was quite ingenious and simple to understand  char input      quot moc wolfrevokcats quot    int length   strlen input   int last pos   length-1  for int i   0  i  lt  length 2  i          char tmp   input i       input i    input last pos - i       input last pos - i    tmp     printf  quot  s n quot   input    A visualization of this algorithm  courtesy of slashdottir

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In case you are using GLib  it has two functions for that  g strreverse   and g utf8 strreverse

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In the interest of completeness  it should be pointed out that there are representations of strings on various platforms in which the number of bytes per character varies depending on the character   Old-school programmers would refer to this as DBCS  Double Byte Character Set   Modern programmers more commonly encounter this in UTF-8  as well as UTF-16 and others    There are other such encodings as well   In any of these variable-width encoding schemes  the simple algorithms posted here  evil  non-evil or otherwise  would not work correctly at all   In fact  they could even cause the string to become illegible or even an illegal string in that encoding scheme   See Juan Pablo Califano s answer for some good examples   std  reverse   potentially would still work in this case  as long as your platform s implementation of the Standard C   Library  in particular  string iterators  properly took this into account

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Non-evil C  assuming the common case where the string is a null-terminated char array    include  lt stddef h gt   include  lt string h gt      PRE  str must be either NULL or a pointer to a      possibly empty  null-terminated string     void strrev char  str      char temp   end ptr        If str is NULL or empty  do nothing      if  str    NULL       str        return     end ptr   str   strlen str  - 1        Swap the chars      while  end ptr  gt  str         temp    str       str    end ptr       end ptr   temp      str        end ptr--

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Another C   way  though I would probably use std  reverse   myself    as being more expressive and faster   str   std  string str rbegin    str rend       The C way  more or less      and please  be careful about XOR trick for swapping   compilers sometimes cannot optimize that   In such case it is usually much slower   char  reverse char  s        char  beg   s   end   s  tmp      while   end  end        while  end--  gt  beg                 tmp     beg            beg      end             end    tmp            return s       fixed  check history for details  as those are interesting ones

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Note that the beauty of std  reverse is that it works with char   strings and std  wstrings just as well as std  strings  void strrev char  str        if  str    NULL          return      std  reverse str  str   strlen str

[c++] How do you reverse a string in place in C or C++?

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