How do you convert a byte array to a hexadecimal string in C

Question

I have   uint8 buf      0  1  10  11     I want to convert the byte array to a string such that I can print the string using printf   printf   s n   str     and get  the colons aren t necessary     00 01 0A 0B    Any help would be greatly appreciated

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This is one way of performing the conversion    include lt stdio h gt   include lt stdlib h gt    define l word 15  define u word 240  char  hex str     0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F     main int argc char  argv             char  str   malloc 50        char  tmp       char  tmp2        int i 0         while  i  lt   argc-1               tmp   hex str   argv i    amp  l word             tmp2   hex str   argv i    amp  u word              if i    0    memcpy str tmp2 1   strcat str tmp              else   strcat str tmp2   strcat str tmp              i               printf   n            s                   n   str

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Slightly modified Yannith version  It is just I like to have it as a return value   x000D   x000D  typedef struct   x000D     size t len  x000D     uint8 t  bytes  x000D    vdata  x000D   x000D  char  vdata get hex const vdata data  x000D    x000D     char hex str     0123456789abcdef   x000D   x000D     char  out  x000D     out    char   malloc data len   2   1   x000D      out  data len   2    0  x000D      x000D     if   data len  return NULL  x000D      x000D     for  size t i   0  i  lt  data len  i      x000D         out  i   2   0    hex str  data bytes i   gt  gt  4   amp  0x0F   x000D         out  i   2   1    hex str  data bytes i         amp  0x0F   x000D       x000D     return out  x000D    x000D   x000D   x000D

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There s no primitive for this in C  I d probably malloc  or perhaps alloca  a long enough buffer and loop over the input  I ve also seen it done with a dynamic string library with semantics  but not syntax   similar to C   s ostringstream  which is a plausibly more generic solution but it may not be worth the extra complexity just for a single case

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What complex solutions  Malloc and sprints and casts oh my   OZ quote  and not a single rem anywhere  Gosh  How about something like this   main            the value     int value   16          create a string array with a   0  ending ie  0 0 0     char hex     0 0   0         char  hex p hex         a working variable     int TEMP int 0          get me how many 16s are in this code     TEMP int value 16          load the first character up with         48 0 gives you ascii 0  55 10 gives you ascii A     if  TEMP int lt 10    hex p 48 TEMP int           else   hex p 55 TEMP int           move that pointer to the next  less significant byte  lt BR gt      hex p            get me the remainder after I have divied by 16     TEMP int value 16          48 0 gives you ascii 0  55 10 gives you ascii A     if  TEMP int lt 10    hex p 48 TEMP int           else   hex p 55 TEMP int           print the result     printf   i   0x s  value hex

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I know this question already has an answer but I think my solution could help someone   So  in my case I had a byte array representing the key and I needed to convert this byte array to char array of hexadecimal values in order to print it out in one line  I extracted my code to a function like this   char const   keyToStr uint8 t const  key        uint8 t offset   0      static char keyStr 2   KEY SIZE   1        for  size t i   0  i  lt  KEY SIZE  i                  offset    sprintf keyStr   offset    02X   key i              sprintf keyStr   offset    c     0         return keyStr      Now  I can use my function like this   Serial print  Public key      Serial println keyToStr m publicKey      Serial object is part of Arduino library and m publicKey is member of my class with the following declaration uint8 t m publicKey 32

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You can solve with snprintf and malloc   char c buff 50    u8 number val       0xbb  0xcc  0xdd  0x0f  0xef  0x0f  0x0e  0x0d  0x0c     char  s temp   malloc u8 size   2   1    for  uint8 t i   0  i  lt  u8 size  i          snprintf s temp    i   2  3    02x   u8 number val i       snprintf c buff  strlen s temp  1    s   s temp     printf   s n  c buff    free s     OUT  bbccdd0fef0f0e0d0c

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Solution  Function btox converts arbitrary data  bb to an unterminated string  xp of n hexadecimal digits   void btox char  xp  const char  bb  int n         const char xx     0123456789ABCDEF       while  --n  gt   0  xp n    xx  bb n gt  gt 1   gt  gt    1 -  n amp 1    lt  lt  2    amp  0xF       Example   include  lt stdio h gt   typedef unsigned char uint8   void main void         uint8 buf      0  1  10  11       int n   sizeof buf  lt  lt  1      char hexstr n   1        btox hexstr  buf  n       hexstr n    0     Terminate         printf   s n   hexstr       Result  00010A0B   Live  Tio run

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I just wanted to add the following  even if it is slightly off-topic  not standard C   but I find myself looking for it often  and stumbling upon this question among the first search hits  The Linux kernel print function  printk  also has format specifiers for outputting array memory contents  directly  through a singular format specifier   https   www kernel org doc Documentation printk-formats txt  Raw buffer as a hex string        ph    00 01 02       3f       phC   00 01 02       3f       phD   00-01-02-     -3f       phN   000102     3f      For printing a small buffers  up to 64 bytes long  as a hex string with     certain separator  For the larger buffers consider to use     print hex dump           however  these format specifiers do not seem to exist for the standard  user-space  s printf

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Here is a method that is way way faster      include  lt stdlib h gt   include  lt stdio h gt   unsigned char       bin to strhex const unsigned char  bin  unsigned int binsz                                    unsigned char   result      unsigned char     hex str     0123456789abcdef     unsigned int      i     if     result    unsigned char   malloc binsz   2   1        return  NULL        result  binsz   2    0     if   binsz      return  NULL      for  i   0  i  lt  binsz  i                  result  i   2   0    hex str  bin i   gt  gt  4   amp  0x0F           result  i   2   1    hex str  bin i         amp  0x0F           return   result      int                 main         the calling   unsigned char     buf      0 1 10 11     unsigned char     result     printf  result    s n   bin to strhex  unsigned char   buf  sizeof buf    amp result      free result      return 0

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If you want to store the hex values in a char   string  you can use snprintf  You need to allocate space for all the printed characters  including the leading zeros and colon   Expanding on Mark s answer   char str buf    malloc 3 X   1        X is the number of bytes to be converted  int i  for  i   0  i  lt  x  i          if  i  gt  0  snprintf str buf  1            snprintf str buf  2    02X   num buf i        need 2 characters for a single hex value   snprintf str buf  2    n 0       dont forget the NULL byte   So now str buf will contain the hex string

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ZincX s solution adapted to include colon delimiters   char buf      0 1 10 11   int i  size   sizeof buf    sizeof char   char  buf str    char   malloc 3   size    buf ptr   buf str  if  buf str      for  i   0  i  lt  size  i        buf ptr    sprintf buf ptr  i  lt  size - 1     02X       02X 0   buf i      printf   s n   buf str     free buf str

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For completude  you can also easily do it without calling any heavy library function  no snprintf  no strcat  not even memcpy   It can be useful  say if you are programming some microcontroller or OS kernel where libc is not available    Nothing really fancy you can find similar code around if you google for it  Really it s not much more complicated than calling snprintf and much faster    include  lt stdio h gt   int main        unsigned char buf      0  1  10  11          target buffer should be large enough        char str 12        unsigned char   pin   buf      const char   hex    0123456789ABCDEF       char   pout   str      int i   0      for   i  lt  sizeof buf -1    i            pout     hex   pin gt  gt 4  amp 0xF            pout     hex   pin    amp 0xF            pout                     pout     hex   pin gt  gt 4  amp 0xF        pout     hex   pin  amp 0xF        pout   0       printf   s n   str       Here is another slightly shorter version  It merely avoid intermediate index variable i and duplicating laste case code  but the terminating character is written two times     include  lt stdio h gt  int main        unsigned char buf      0  1  10  11          target buffer should be large enough        char str 12        unsigned char   pin   buf      const char   hex    0123456789ABCDEF       char   pout   str      for   pin  lt  buf sizeof buf   pout  3  pin             pout 0    hex   pin gt  gt 4   amp  0xF           pout 1    hex   pin      amp  0xF           pout 2                   pout -1    0       printf   s n   str       Below is yet another version to answer to a comment saying I used a  trick  to know the size of the input buffer  Actually it s not a trick but a necessary input knowledge  you need to know the size of the data that you are converting   I made this clearer by extracting the conversion code to a separate function  I also added boundary check code for target buffer  which is not really necessary if we know what we are doing    include  lt stdio h gt   void tohex unsigned char   in  size t insz  char   out  size t outsz        unsigned char   pin   in      const char   hex    0123456789ABCDEF       char   pout   out      for   pin  lt  in insz  pout   3  pin             pout 0    hex   pin gt  gt 4   amp  0xF           pout 1    hex   pin      amp  0xF           pout 2                 if  pout   3 - out  gt  outsz                  Better to truncate output string than overflow buffer                   it would be still better to either return a status                   or ensure the target buffer is large enough and it never happen                break                      pout -1    0     int main        enum  insz   4  outsz   3 insz       unsigned char buf      0  1  10  11       char str outsz       tohex buf  insz  str  outsz       printf   s n   str

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This function is suitable where user caller wants hex string to be put in a charactee array buffer  With hex string in a character buffer  user caller can use its own macro function to display or log it to any place it wants  e g  to a file   This function also allows caller to control number of  hex  bytes to put in each line           fn     get hex        brief     Converts a char into bunary string         param in            buf Value to be converted to hex string     param in            buf len Length of the buffer     param in            hex  Pointer to space to put Hex string into     param in            hex len Length of the hex string space     param in            num col Number of columns in display hex string     param out            hex  Contains the hex string     return  void     static inline void get hex char  buf  int buf len  char  hex   int hex len  int num col        int i   define ONE BYTE HEX STRING SIZE   3   unsigned int byte no   0     if  buf len  lt   0          if  hex len  gt  0            hex  0      0                 return         if hex len  lt  ONE BYTE HEX STRING SIZE   1            return         do            for  i   0    i  lt  num col   amp  amp   buf len  gt  0   amp  amp   hex len  gt  0      i                          snprintf hex   hex len    02X    buf byte no     amp  0xff               hex     ONE BYTE HEX STRING SIZE              hex len - ONE BYTE HEX STRING SIZE              buf len--                      if  buf len  gt  1                          snprintf hex   hex len    n                 hex     1                 while   buf len   gt  0  amp  amp   hex len  gt  0         Example  Code   define DATA HEX STR LEN 5000     char      data hex str DATA HEX STR LEN        get hex pkt  pkt len  data hex str  DATA HEX STR LEN  16                                                                            Input byte array                              Number of  hex  byte             to be converted to hex string                 columns in hex string      printf  pkt  n s  data hex str     OUTPUT  pkt  BB 31 32 00 00 00 00 00 FF FF FF FF FF FF DE E5  A8 E2 8E C1 08 06 00 01 08 00 06 04 00 01 DE E5  A8 E2 8E C1 67 1E 5A 02 00 00 00 00 00 00 67 1E  5A 01

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Similar answers already exist above  I added this one to explain how the following line of code works exactly   ptr    sprintf ptr    02X   buf i     It s quiet tricky and not easy to understand  I put the explanation in the comments below   uint8 buf      0  1  10  11       Allocate twice the number of bytes in the  buf  array because each byte would    be converted to two hex characters  also add an extra space for the terminating    null byte      size  is the size of the buf array    char output  size   2    1       pointer to the first item  0 index  of the output array    char  ptr    amp output 0    int i   for  i   0  i  lt  size  i              sprintf  converts each byte in the  buf  array into a 2 hex string        characters appended with a null byte  for example 10   gt   0A 0                 This string would then be added to the output array starting from the        position pointed at by  ptr   For example if  ptr  is pointing at the 0        index then  0A 0  would be written as output 0     0   output 1     A  and        output 2      0                  sprintf  returns the number of chars in its output excluding the null        byte  in our case this would be 2  So we move the  ptr  location two        steps ahead so that the next hex string would be written at the new        location  overriding the null byte from the previous hex string                We don t need to add a terminating null byte because it s been already         added for us from the last hex string           ptr    sprintf ptr    02X   buf i       printf   s n   output

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printf   02X  02X  02X  02X   buf 0   buf 1   buf 2   buf 3      for a more generic way   int i  for  i   0  i  lt  x  i          if  i  gt  0  printf           printf   02X   buf i      printf   n      to concatenate to a string  there are a few ways you can do this    i d probably keep a pointer to the end of the string and use sprintf   you should also keep track of the size of the array to make sure it doesnt get larger than the space allocated   int i  char  buf2   stringbuf  char  endofbuf   stringbuf   sizeof stringbuf   for  i   0  i  lt  x  i             i use 5 here since we are going to add at most         3 chars  need a space for the end   n  and need        a null terminator        if  buf2   5  lt  endofbuf                if  i  gt  0                        buf2    sprintf buf2                          buf2    sprintf buf2    02X   buf i            buf2    sprintf buf2    n

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Based on Yannuth s answer but simplified   Here  length of dest   is implied to be twice of len  and its allocation is managed by the caller   void create hex string implied const unsigned char  src  size t len  unsigned char  dest        static const unsigned char table      0123456789abcdef        for    len  gt  0  --len                unsigned char c    src             dest     table c  gt  gt  4            dest     table c  amp  0x0f

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For simple usage I made a function that encodes the input string  binary data       Encodes string to hexadecimal string reprsentation     Allocates a new memory for supplied lpszOut that needs to be deleted after use     Fills the supplied lpszOut with hexadecimal representation of the input        void StringToHex unsigned char  szInput  size t size szInput  char   lpszOut        unsigned char  pin   szInput      const char  hex    0123456789ABCDEF       size t outSize   size szInput   2   2       lpszOut   new char outSize       char  pout    lpszOut      for    pin  lt  szInput   size szInput  pout    2  pin                  pout 0    hex   pin  gt  gt  4   amp  0xF           pout 1    hex  pin  amp  0xF             pout 0    0      Usage   unsigned char input      This is a very long string that I want to encode   char  szHexEncoded   NULL  StringToHex input  strlen  const char   input    amp szHexEncoded    printf szHexEncoded       The allocated memory needs to be deleted after usage delete   szHexEncoded

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I ll add the C   version here for anyone who is interested    include  lt iostream gt   include  lt iomanip gt  inline void print bytes char const   buffer  std  size t count  std  size t bytes per line  std  ostream  amp  out        std  ios  fmtflags flags out flags        Save flags before manipulation      out  lt  lt  std  hex  lt  lt  std  setfill  0        out setf std  ios  uppercase       for  std  size t i   0  i    count    i            auto current byte number   static cast lt unsigned int gt  static cast lt unsigned char gt  buffer i             out  lt  lt  std  setw 2   lt  lt  current byte number          bool is end of line    bytes per line    0   amp  amp    i   1    count       i   1    bytes per line    0            out  lt  lt   is end of line     n                    out flush        out flags flags      Restore original flags      It will print the hexdump of the buffer of length count to std  ostream out  you can make it default to std  cout   Every line will contain bytes per line bytes  each byte is represented using uppercase two digit hex  There will be a space between bytes  And at end of line or end of buffer it will print a newline  If bytes per line is set to 0  then it will not print new line  Try for yourself

[c] How do you convert a byte array to a hexadecimal string in C?

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