How to pack a hexadecimal value in an unsigned char variable in a C program?

Question

I have a hexadecimal value "F69CF355B6231FDBD91EB1E22B61EA1F" in a string and I am using this value in my program by hardcoding the value in an unsigned char variable like this: unsigned char a[] = { 0xF6 ,0x9C ,0xF3 ,0x55 ,0xB6 ,0x23 ,0x1F ,0xDB ,0xD9 ,0x1E ,0xB1 ,0xE2 ,0x2B ,0x61 ,0xEA ,0x1F}; Is there any function or any other method by which I can take the value from a string and put it into an unsigned variable in the hexadecimal format by packing it?

Presumably you meaan `unsigned char a[]`, not `unsigned char a`. — Oliver Charlesworth, May 18 '12 at 13:26
I do not think that there are some functions. Why don't you want to create your own? — besworland, May 18 '12 at 13:28
possible duplicate of [How to turn a hex string into an unsigned char array?](http://stackoverflow.com/questions/3221170/how-to-turn-a-hex-string-into-an-unsigned-char-array) — Mat, May 18 '12 at 13:33

BLUEPIXY · Answer 1 · 2012-05-18T17:01:17.683

#include <stdio.h>
#include <ctype.h>

int hctoi(const char h){
    if(isdigit(h))
        return h - '0';
    else
        return toupper(h) - 'A' + 10;
}

int main(void){
    const char cdata[]="F69CF355B6231FDBD91EB1E22B61EA1F";
    unsigned char udata[(sizeof(cdata)-1)/2];
    const char *p;
    unsigned char *up;

    for(p=cdata,up=udata;*p;p+=2,++up){
        *up = hctoi(p[0])*16 + hctoi(p[1]);
    }

    {   //check code
        int i;
        for(i=0;i<sizeof(udata);++i)
            printf("%02X", udata[i]);
    }
    return 0;
}

score 0 · Answer 2 · answered May 18 '12 at 13:33

You can translate a hexadecimal value in a string into a value using sscanf. If you want an array of values, then you could write a function to split up the input string into two character segments and use sscanf to convert each piece. (I haven't done C for an eternity so I don't know if that's a good way to do it.)

score 0 · Answer 3 · answered May 18 '12 at 15:57

If it's for 32 single hex-values (16 bytes, 128bit) only then you might take look at the methods provided by libuuid.

libuuid is part of the e2fsprogs package. Anyhow some linux distros, Debian for example, ship libuuid as a separate package. To use the Debian package for your developement you also need to look here.

score 0 · Answer 4 · edited May 23 '17 at 12:11

Check this answer for doing this stuff in c++ using sscanf().

For c, it would be something like this:

char *str           = "F69CF355B6231FDBD91EB1E22B61EA1F";
char substr[3]      = "__";    
unsigned char *a    = NULL;    
len                 = strlen(str);
a                   = malloc(sizeof(unsigned char)*(len/2)+1);
for (  i = 0; i < len/2; i++) {
    substr[0]       = str[i*2];
    substr[1]       = str[i*2 + 1];
    sscanf( substr, "%hx", &a[i] );
}
free(a);

score 0 · Answer 5 · answered Sep 11 '14 at 18:06

Introduce auxiliary functions data_length and data_get to easily iterate over your data. The following program dumps unpacked unsigned chars on stdout, one per line:

#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>

/* htoi(H)
    Return the value associated to the hexadecimal digit H. */
int
htoi(char h)
{
  int a = -1;
  if(isdigit(h))
  {
    a = h - '0';
  }
  else
  {
    a = toupper(h) - 'A' + 10;
  }
  return a;
}

/* data_length(D)
    The length of the data stored at D. */
int
data_length(const char* d)
{
  return strlen(d) / 2;
}

/* data_get(D, K)
    Return the K-th unsigned char located encoded in d. */
unsigned char
data_get(const char *d, int k)
{
  return htoi(d[2*k]) * 0x10 +
    htoi((d+1)[2*k]);
}

int
main()
{
    const char cdata[]="F69CF355B6231FDBD91EB1E22B61EA1F";
    for(int i = 0; i < data_length(cdata); ++i)
    {
      printf("0x%02hhx\n", data_get(cdata, i));
    }
    return EXIT_SUCCESS;
}

How to pack a hexadecimal value in an unsigned char variable in a C program?

5 Answers5

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