Assigning hex characters dynamically

**IronyOfLife** · 07-14-2009

Hi,

I would appreciate if anyone can help with this newbie problem. This is required for me to run a sample C program from the open source GnuTLS package.

I have a string containing some hexadecimal characters.

I would like to assign it to another string.

Code:

char * hexdata = "4042a55e"
unsigned char * data;

The sample program runs if I assign the characters like this:

Code:

data[0] = 0x40;
data[1] = 0x42;
data[2] = 0xa5;
data[3] = 0x5e;

However if I assign it like below, it does not work.

Code:

data = hexdata;

The hexdata can be dynamic for me, so I'm trying to assign it dynamically like this:

Code:

for (i = 0; i < 4; i++)
{
    sprintf(&data[i], "0x%c%c", *hexdata,*(hexdata + 1));
}

There must be something wrong with this because the sample program is not working.

Can you please point out any other way of achieving this:

Code:

data[0] = 0x40;
....

Thanks in advance

Pete

**tabstop** · 07-14-2009

Are you actually trying to turn eight characters into four? If so, you're going to have to parse the string yourself -- read two characters, figure out what it's "worth" as a hex value, and then assign that value to data[wherever]. (I suppose it's worth mentioning that '4' is not represented with a 4, but with the (almost surely) ASCII value for '4' instead.....)

**MK27** · 07-14-2009

This is wrong for a few reasons:

Code:

for (i = 0; i < 4; i++)
{
    sprintf(&data[i], "0x%c%c", *hexdata,*(hexdata + 1));
}

you copy 4 bytes "0" "x" "%c" and "%c" starting at data[i].
then you advance one byte and copy four more bytes, three of which are exactly the same
"data" itself does not appear to have any allocated memory, so there is an overflow on the very first sprintf write

The last point is where we should start.

Code:

unsigned char * data;

"data" is thus an unsigned char pointer, with no memory allocated to it. It is fine to do this

Code:

data = hexdata;

data will now point to hexdata, which is a string literal. However, that means that you cannot make any changes to either one. It would be possible if hexdata was declared this way instead:

Code:

char hexdata[] = "4042a55e";

But in any case, assigning data to hexdata IS NOT the same thing as copying the contents of hexdata to data. That would look something like this:

Code:

data = malloc(strlen(hexdata)+1);
strcpy(data, hexdata);

Notice memory is allocated to data first.

As for what you are trying to do in the for() loop, you probably mean something like this:

Code:

int len = strlen(hexdata);
for (i=0;i<len;i++) {
    data[i] = hexdata[i];
}

Finally, you should be aware that doing this:

Code:

char * hexdata = "4042a55e"

Does not create a 4-byte string with values 0x40, 0x42, 0xa5, 0x5e! It creates an eight byte string, one for each character. The values for each byte are the ASCII values for the character -- eg. "404" is 0x34, 0x30, 0x34.

**itCbitC** · 07-14-2009

Can you explain what you're trying to do and why isn't this working for you?

Code:

data = hexdata; /* works just fine for me */

**IronyOfLife** · 07-14-2009

Thank you tabstop, MK27 and itCbitC for your replies.

To explain what I'm trying to do and why isn't this working for me - I'm running a sample program which is part of the open source GnuTLS package. This program has a client which sends a password key as :

Code:

typedef struct
  {
    unsigned char *data;
    unsigned int size;
  } gnutls_datum_t;
  
const gnutls_datum_t key = { (char*) "DEADBEEF", 8 };

The server sample does the key assignment as follows:

Code:

  key->data = gnutls_malloc (4);
  key->data[0] = 0xDE;
  key->data[1] = 0xAD;
  key->data[2] = 0xBE;
  key->data[3] = 0xEF;
  key->size = 4;

I'm trying to find to out how internally it works but right now I do not have the answer.

What I was trying to do was follow the same way as the server sample but assign key->data dynamically. What I'm trying to figure out is how can I use sscanf or sprintf or any other way to make an assignment similar to

Code:

key->data[0] = 0xDE;

Thanks again

Pete

**slingerland3g** · 07-14-2009

What do you think this line is doing:

Code:

   const gnutls_datum_t key = { (char*) "DEADBEEF", 8 };

IT appears this server code is hard coding those values,clearing them out with gnutls_malloc first, then overwritting the datums as previously assigned.

A good read on this is found here:

http://www.gnu.org/software/gnutls/manual/gnutls.pdf

**MK27** · 07-14-2009

Originally Posted by IronyOfLife

What I'm trying to figure out is how can I use sscanf or sprintf or any other way to make an assignment similar to

Code:

key->data[0] = 0xDE;

You don't have to. That is the low-level "no function needed" byte op level that is the essence and fundament of C. If you want to assign an integer value to a byte, any kind of byte (signed, unsigned, char, void, whatever) just do it:

Code:

key->data[0] = 0xDE;

means exactly the same thing as "data[0] = 222". Look:

Code:

	unsigned char byte = 0xDE;
	printf("%d %x",byte,byte);

d is for decimal, x is for hexadecimal. This will print:
222 de

There are no characters with a value greater than 127, tho. If "byte" were signed it would print:
-34 de
because signed bytes use "two's complement" to produce positive and negative values (-128 to 127 instead of 0 to 255). Get it? If you did this:

Code:

char byte = '4';

The character '4', NOT the integer value 4, it will print
52 34

Assigning to a single char is more simple and straightforward that putting data into and array of chars (ie, a string), which is when you need the string functions.

**Kennedy** · 07-14-2009

Originally Posted by MK27

Code:

char byte = '4';

The character '4', NOT the integer value 4, it will print
20 14

Assigning to a single char is more simple and straightforward that putting data into and array of chars (ie, a string), which is when you need the string functions.

In which universe? I'm pretty sure the ASCII value of '4' is 0x34.

**MK27** · 07-14-2009

Originally Posted by Kennedy

In which universe? I'm pretty sure the ASCII value of '4' is 0x34.

Hey oops! You're right. I was looking at this:

Table of ASCII Characters

(so should IronyOfLife, nb!) and got the columns wrong. Sorry. I'll go back and correct those.

**nonoob** · 07-14-2009

Dynamically assign...

Code:

	char *hexdata = "4042a55e";
	char unsigned data[4];   /* caution!!! */
	int i, temp;

	for (i = 0; hexdata[i]; i += 2) {
		sscanf(&hexdata[i], "%02x", &temp);
		data[i / 2] = temp;
		}

**IronyOfLife** · 07-15-2009

Thank you for your suggestions. I tried out the code segment given by nonoob. But there is something lacking in my understanding of the GnuTLS sample program because of which it is not working. I'm going through MK27's posts and will update you with my progress.

By the way, this forum is the confluence of some very bright minds !

Thanks

Pete

**IronyOfLife** · 07-16-2009

Finally I could dynamically assign the hexadecimal keys as follows:

Code:

 
char * dynamickeys; //Could be any string with hex characters like DEADBEEF
atohx(key->data,dynamickeys);

Here is the atohx function I got from the following link:

No atoh() function in C ( Ascii To Hex )? - Well, Let's Create One

Code:

  
char * atohx(char * dst, const char * src)
{  
 int lsb,msb;
 char * ret;
 ret = dst;
 for(lsb = 0, msb = 0; *src; src += 2)
 { 
  msb = tolower(*src);
  lsb = tolower(*(src + 1));
  msb -= isdigit(msb) ? 0x30 : 0x57;
  lsb -= isdigit(lsb) ? 0x30 : 0x57;
  if((msb < 0x0 || msb > 0xf) || (lsb < 0x0 || lsb > 0xf))
  {
   *ret = 0;
   return NULL;
  }
  *dst++ = (char)(lsb | (msb << 4));  
 }
 *dst = 0;
 return ret;
}

Thanks to all for all your suggestions.

**KCfromNC** · 07-17-2009

Unfortunately, this function only works in limited cases. Try passing in a string with an odd length.

**newlearner** · 07-18-2009

Use the atoi function? And then just shift bits?

Code:

unsigned int hex = atoll(hexdata);
for (i = 0; i < 4; i++)
    sprintf(data[i], "0x%x", (hex >> ((3-i) * 8)) & 0xff);

EDIT: changed atoi to atoll...atoi maxes out at 0x7fffffff

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