Hexadecimal string to byte array in C

Question

Is there any standard C function that converts from hexadecimal string to byte array  I do not want to write my own function

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I found this question by Googling for the same thing  I don t like the idea of calling sscanf   or strtol   since it feels like overkill  I wrote a quick function which does not validate that the text is indeed the hexadecimal presentation of a byte stream  but will handle odd number of hex digits   uint8 t tallymarker hextobin const char   str  uint8 t   bytes  size t blen       uint8 t  pos     uint8 t  idx0     uint8 t  idx1         mapping of ASCII characters to hex values    const uint8 t hashmap               0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00            amp        0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00          -        0x00  0x01  0x02  0x03  0x04  0x05  0x06  0x07     01234567      0x08  0x09  0x00  0x00  0x00  0x00  0x00  0x00     89   lt   gt        0x00  0x0a  0x0b  0x0c  0x0d  0x0e  0x0f  0x00      ABCDEFG      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     HIJKLMNO      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     PQRSTUVW      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     XYZ           0x00  0x0a  0x0b  0x0c  0x0d  0x0e  0x0f  0x00      abcdefg      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     hijklmno      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     pqrstuvw      0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00     xyz           0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                   0x00  0x00  0x00  0x00  0x00  0x00  0x00  0x00                        bzero bytes  blen      for  pos   0    pos  lt   blen 2    amp  amp   pos  lt  strlen str     pos    2             idx0    uint8 t str pos 0         idx1    uint8 t str pos 1         bytes pos 2     uint8 t  hashmap idx0   lt  lt  4    hashmap idx1             return 0

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Here is a solution to deal with files  which may be used more frequently    int convert char  infile  char  outfile     char  source   NULL  FILE  fp   fopen infile   quot r quot    long bufsize  if  fp    NULL           Go to the end of the file         if  fseek fp  0L  SEEK END     0               Get the size of the file             bufsize   ftell fp           if  bufsize    -1       Error                  Allocate our buffer to that size             source   malloc sizeof char     bufsize   1                Go back to the start of the file             if  fseek fp  0L  SEEK SET     0       Error                  Read the entire file into memory             size t newLen   fread source  sizeof char   bufsize  fp           if   ferror  fp      0                 fputs  quot Error reading file quot   stderr             else               source newLen        0      Just to be safe                         fclose fp           int sourceLen   bufsize - 1  int destLen   sourceLen 2  unsigned char  dest   malloc destLen   short i    unsigned char highByte  lowByte           for  i   0  i  lt  sourceLen  i    2                highByte   toupper source i              lowByte    toupper source i   1                 if  highByte  gt  0x39                highByte -  0x37            else               highByte -  0x30               if  lowByte  gt  0x39                lowByte -  0x37            else               lowByte -  0x30               dest i   2     highByte  lt  lt  4    lowByte                  FILE  fop   fopen outfile   quot w quot    if  fop    NULL  return 1  fwrite dest  1  destLen  fop   fclose fop   free source   free dest   return 0

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This is a modified function from a similar question  modified as per the suggestion of https   stackoverflow com a 18267932 700597   This function will convert a hexadecimal string - NOT prepended with  0x  - with an even number of characters to the number of bytes specified  It will return -1 if it encounters an invalid character  or if the hex string has an odd length  and 0 on success     convert hexstring to len bytes of data   returns 0 on success  -1 on error   data is a buffer of at least len bytes   hexstring is upper or lower case hexadecimal  NOT prepended with  0x  int hex2data unsigned char  data  const unsigned char  hexstring  unsigned int len        unsigned const char  pos   hexstring      char  endptr      size t count   0       if   hexstring 0       0       strlen hexstring    2               hexstring contains no data           or hexstring has an odd length         return -1             for count   0  count  lt  len  count              char buf 5      0    x   pos 0   pos 1   0           data count    strtol buf   amp endptr  0           pos    2   sizeof char            if  endptr 0       0                   non-hexadecimal character encountered             return -1                       return 0

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A fleshed out version of Michael Foukarakis post  since I don t have the  reputation  to add a comment to that post yet     include  lt stdio h gt   include  lt string h gt   void print unsigned char  byte array  int byte array size        int i   0      printf  0x        for   i  lt  byte array size  i                  printf   02x   byte array i              printf   n       int convert const char  hex str  unsigned char  byte array  int byte array max        int hex str len   strlen hex str       int i   0  j   0          The output array size is half the hex str length  rounded up      int byte array size    hex str len 1  2       if  byte array size  gt  byte array max                   Too big for the output array         return -1             if  hex str len   2    1                   hex str is an odd length  so assume an implicit  0  prefix         if  sscanf  amp  hex str 0      1hhx    amp  byte array 0       1                        return -1                     i   j   1             for    i  lt  hex str len  i  2  j                  if  sscanf  amp  hex str i      2hhx    amp  byte array j       1                        return -1                       return byte array size     void main         char  examples            5    D    5D    5Df    deadbeef10203040b00b1e50    02invalid55         unsigned char byte array 128       int i   0       for    i  lt  sizeof examples  sizeof char     i                  int size   convert examples i   byte array  128           if  size  lt  0                        printf  Failed to convert   s  n   examples i                      else if  size    0                        printf  Nothing to convert for   s  n   examples i                      else                       print byte array  size

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Here s my version      Convert a hex char digit to its integer value     int hexDigitToInt char digit        digit   tolower digit       if   0   lt   digit  amp  amp  digit  lt    9     if it s decimal         return  int  digit -  0        else if   a   lt   digit  amp  amp  digit  lt    f     if it s abcdef         return  int  digit -   a  - 10        else         return -1    value not in  0-9  a-f  range       Decode a hex string     char  decodeHexString const char  hexStr        char  decoded   malloc strlen hexStr  2 1       char  hexStrPtr    char   hexStr      char  decodedPtr   decoded       while   hexStrPtr      0        Step through hexStr  two chars at a time              decodedPtr   16   hexDigitToInt  hexStrPtr    hexDigitToInt   hexStrPtr 1            hexStrPtr    2          decodedPtr                decodedPtr     0      final null char        return decoded

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hextools h   ifndef HEX TOOLS H  define HEX TOOLS H  char  bin2hex unsigned char   int    unsigned char  hex2bin const char      endif    HEX TOOLS H   hextools c   include  lt stdlib h gt   char  bin2hex unsigned char  p  int len        char  hex   malloc   2 len    1        char  r   hex       while len  amp  amp  p                  r       p   amp  0xF0   gt  gt  4            r       r   lt   9    0      r     A  - 10     r            r              r       p   amp  0x0F             r       r   lt   9    0      r     A  - 10     r            r            p            len--             r     0        return hex     unsigned char  hex2bin const char  str        int len  h      unsigned char  result   err   p  c       err   malloc 1        err   0       if   str          return err       if    str          return err       len   0      p    unsigned char   str      while   p            len         result   malloc  len 2  1       h     len 2    4      p   result       p   0       c    str      while c                if   0   lt   c   amp  amp   c  lt    9                 p     c -  0    lt  lt  h          else if   A   lt   c   amp  amp   c  lt    F                 p     c -  A    10   lt  lt  h          else if   a   lt   c   amp  amp   c  lt    f                 p     c -  a    10   lt  lt  h          else             return err           str            c    str           if  h              h   0          else                       h   4              p                 p   0                       return result      main c   include  lt stdio h gt   include  hextools h   int main void        unsigned char s       0xa0  0xf9  0xc3  0xde  0x44         char  hex   bin2hex s  sizeof s       puts hex        unsigned char  bin      bin   hex2bin hex        puts bin2hex bin  5         size t k      for k 0  k lt 5  k            printf   02X   bin k         putchar   n         return 0

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Following is the solution I wrote up for performance reasons   void hex2bin const char  in  size t len  unsigned char  out       static const unsigned char TBL            0    1    2    3    4    5    6    7    8    9   58   59   60   61      62   63   64   10   11   12   13   14   15   71   72   73   74   75      76   77   78   79   80   81   82   83   84   85   86   87   88   89      90   91   92   93   94   95   96   10   11   12   13   14   15         static const unsigned char  LOOKUP   TBL - 48     const char  end   in   len     while in  lt  end    out      LOOKUP   in      lt  lt  4   LOOKUP   in           Example   unsigned char seckey 32    hex2bin  351aaaec0070d13d350afae2bc43b68c7e590268889869dde489f2f7988f3fee   64  seckey         seckey          53   26  170  236    0  112  209   61   53   10  250  226  188   67  182  140       126   89    2  104  136  152  105  221  228  137  242  247  152  143   63  238           If you don t need to support lowercase   static const unsigned char TBL            0    1    2    3    4    5    6    7    8    9   58   59      60   61   62   63   64   10   11   12   13   14   15

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The best way I know   int hex2bin by zibri char  source str  char  dest buffer      char  line   source str    char  data   line    int offset    int read byte    int data len   0     while  sscanf data     02x n    amp read byte   amp offset     1        dest buffer data len      read byte      data    offset        return data len      The function returns the number of converted bytes saved in dest buffer  The input string can contain spaces and mixed case letters    01  02  03  04 ab Cd eF garbage AB   translates to dest buffer containing 01 02 03 04 ab cd ef  and also  01020304abCdeFgarbageAB   translates as before   Parsing stops at the first  error

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For short strings  strtol  strtoll  and strtoimax will work just fine  note that the third argument is the base to use in processing the string   set it to 16   If your input is longer than number-of-bits-in-the-longest-integer-type 4 then you ll need one of the more flexible methods suggested by other answers

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Could it be simpler    uint8 t hex char ch        uint8 t r    ch  gt  57     ch - 55     ch - 48       return r  amp  0x0F     int to byte array const char  in  size t in size  uint8 t  out        int count   0      if  in size   2            while   in  amp  amp  out                 out   hex  in                 if    in                  return count               out     out  lt  lt  4    hex  in                  out                count                      return count        else           while   in  amp  amp  out                 out      hex  in     lt  lt  4    hex  in                 count                      return count           int main         char hex in      deadbeef10203040b00b1e50       uint8 t out 32       int res   to byte array hex in  sizeof hex in  - 1  out        for  size t i   0  i  lt  res  i            printf   02x    out i         printf   n        system  pause        return 0

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Try the following code   static unsigned char ascii2byte char  val        unsigned char temp    val       if temp  gt  0x60  temp -  39      convert chars a-f     temp -  48      convert chars 0-9     temp    16       temp      val 1       if   val 1   gt  0x60  temp -  39      convert chars a-f     temp -  48      convert chars 0-9         return temp

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As far as I know  there s no standard function to do so  but it s simple to achieve in the following manner    include  lt stdio h gt   int main int argc  char   argv        const char hexstring      DEadbeef10203040b00b1e50    pos   hexstring      unsigned char val 12            WARNING  no sanitization or error-checking whatsoever        for  size t count   0  count  lt  sizeof val sizeof  val  count              sscanf pos    2hhx    amp val count            pos    2             printf  0x        for size t count   0  count  lt  sizeof val sizeof  val  count            printf   02x   val count        printf   n         return 0        Edit  As Al pointed out  in case of an odd number of hex digits in the string  you have to make sure you prefix it with a starting 0  For example  the string  f00f5  will be evaluated as  0xf0  0x0f  0x05   erroneously by the above example  instead of the proper  0x0f  0x00  0xf5    Amended the example a little bit to address the comment from  MassimoCallegari

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In main         printf  enter string   n        fgets buf  200  stdin   unsigned char str len   strlen buf   k 0  unsigned char bytearray 100        for j 0 j lt str len-1 j j 2            bytearray k    converttohex  amp buffer j                       printf    02X  bytearray k-1                          Use this       int converttohex char   val                    unsigned char temp   toupper  val           unsigned char fin 0          if temp gt 64          temp 10  temp-65            else         temp temp-48           fin  temp lt  lt 4  amp 0xf0           temp   toupper   val 1                 if temp gt 64              temp 10  temp-65                else             temp temp-48           fin fin  temp  amp  0x0f                return fin

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By some modification form user411313 s code  following works for me    include  lt stdio h gt   include  lt stdint h gt    include  lt string h gt   int main          char  hexstring    deadbeef10203040b00b1e50       int i      unsigned int bytearray 12       uint8 t str len   strlen hexstring        for  i   0  i  lt   str len   2   i              sscanf hexstring   2 i    02x    amp bytearray i            printf  bytearray  d   02x n   i  bytearray i               return 0

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Two short routines to parse a byte or a word  using strchr      x000D   x000D     HexConverter h x000D   ifndef HEXCONVERTER H x000D   define HEXCONVERTER H x000D  unsigned int hexToByte  const char  hexString   x000D  unsigned int hexToWord  const char  hexString   x000D   endif x000D   x000D   x000D     HexConverter c x000D   include  lt string h gt     for strchr   x000D   include  lt ctype h gt      for toupper   x000D   x000D  unsigned int hexToByte  const char  hexString  x000D    x000D      unsigned int value  x000D      const char  hexDigits    0123456789ABCDEF   x000D   x000D      value   0  x000D      if  hexString    NULL  x000D        x000D          char  ptr  x000D   x000D          ptr   strchr  hexDigits  toupper hexString 0     x000D          if  ptr    NULL  x000D            x000D              value    ptr - hexDigits   lt  lt  4  x000D   x000D              ptr   strchr  hexDigits  toupper hexString 1     x000D              if  ptr    NULL  x000D                x000D                  value   value    ptr - hexDigits   x000D                x000D            x000D        x000D   x000D      return value  x000D    x000D   x000D  unsigned int hexToWord  const char  hexString  x000D    x000D      unsigned int value  x000D   x000D      value   0  x000D      if  hexString    NULL  x000D        x000D          value    hexToByte   amp hexString 0    lt  lt  8    x000D                   hexToByte   amp hexString 2     x000D        x000D   x000D      return value  x000D    x000D   x000D   x000D     HexConverterTest c x000D   include  lt stdio h gt  x000D   x000D   include  HexConverter h  x000D   x000D  int main  int argc  char   argv  x000D    x000D       void argc     not used x000D       void argv     not used x000D   x000D      unsigned int value  x000D      char  hexString  x000D   x000D      hexString    2a   x000D      value   hexToByte  hexString   x000D      printf    s     x   u  n   hexString  value  value   x000D   x000D      hexString    1234   x000D      value   hexToWord  hexString   x000D      printf    s     x   u  n   hexString  value  value   x000D   x000D      hexString    0102030405060708090a10ff   x000D      printf   Hex String   s n   hexString   x000D      for  unsigned int idx   0  idx  lt  strlen hexString   idx    2  x000D        x000D          value   hexToByte   amp hexString idx    x000D          printf    c c     x   u  n   hexString idx   hexString idx 1   x000D                  value  value   x000D        x000D   x000D      return EXIT SUCCESS  x000D    x000D   x000D   x000D

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No   But it s relatively trivial to achieve using sscanf in a loop

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Here is HexToBin and BinToHex relatively clean and readable   Note originally there were returned enum error codes through an error logging system not a simple -1 or -2    typedef unsigned char ByteData  ByteData HexChar  char c        if   0   lt   c  amp  amp  c  lt    9   return  ByteData  c -  0        if   A   lt   c  amp  amp  c  lt    F   return  ByteData  c -  A    10       if   a   lt   c  amp  amp  c  lt    f   return  ByteData  c -  a    10       return  ByteData  -1      ssize t HexToBin  const char  s  ByteData   buff  ssize t length        ssize t result   0      if   s     buff    length  lt   0  return -2       while   s                ByteData nib1   HexChar  s             if   signed nib1  lt  0  return -3          ByteData nib2   HexChar  s             if   signed nib2  lt  0  return -4           ByteData bin    nib1  lt  lt  4    nib2           if  length--  lt   0  return -5           buff     bin            result            return result     void BinToHex  const ByteData   buff  ssize t length  char   output  ssize t outLength        char binHex      0123456789ABCDEF        if   output    outLength  lt  4  return  void  -6        output     0        if   buff    length  lt   0    outLength  lt   2   length                memcpy output   ERR   4           return  void  -7              for    length  gt  0  --length  outLength -  2                ByteData byte    buff              output     binHex  byte  gt  gt  4   amp  0x0F            output     binHex byte  amp  0x0F             if  outLength--  lt   0  return  void  -8        output       0

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Apart from the excellent answers above I though I would write a C function that does not use any libraries and has some guards against bad strings   uint8 t  datahex char  string         if string    NULL          return NULL       size t slength   strlen string       if  slength   2     0     must be even        return NULL       size t dlength   slength   2       uint8 t  data   malloc dlength       memset data  0  dlength        size t index   0      while  index  lt  slength            char c   string index           int value   0          if c  gt    0   amp  amp  c  lt    9             value    c -  0            else if  c  gt    A   amp  amp  c  lt    F              value    10    c -  A             else if  c  gt    a   amp  amp  c  lt    f             value    10    c -  a             else             free data             return NULL                     data  index 2      value  lt  lt     index   1    2    4            index               return data      Explanation    a  index   2   Division between integers will round down the value  so 0 2   0  1 2   0  2 2   1  3 2   1  4 2   2  5 2   2  etc  So  for every 2 string characters we add the value to 1 data byte   b   index   1    2   We want odd numbers to result to 1 and even to 0 since the first digit of a hex string is the most significant and needs to be multiplied by 16  so for index 0    0   1   2   1  index 1    1   1   2   0 etc   c   lt  lt  4   Shift by 4 is multiplying by 16  example  b00000001  lt  lt  4   b00010000

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char  hexstring    deadbeef10203040b00b1e50    pos   hexstring  unsigned char val 12   while   pos       if     pos-hexstring  amp 1        sscanf pos   02x   amp val  pos-hexstring  gt  gt 1        pos      sizeof val  sizeof val 0   is redundant

[c] Hexadecimal string to byte array in C

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