Converting a binary string to hex

I have a string in binary and I need to convert it to hex. (Obvious right?)

What I'm trying to do is this:

Code:

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for (z = 0; z < 20; z++)
        sprintf(string, "%02x", out[z]);

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Both out and string are defined as unsigned char[20]

Now, I know that hex characters take two bytes, so I also tried

Code:

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for (z = 0; z < 20; z++)
        y += sprintf(string+y, "%02x", out[z]);

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Where y is initialized to 0. I keep getting damage after normal block errors. I thought it was becuase I was overwriting memory so I increased the size of string to 200, which should be MORE than enough room, but I get the same error. Any suggestions?

You need to look at 4 binary digits to make 1 hex digit. Try:

Code:

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itsme@itsme:~/C$ cat bintohex.c
#include <stdio.h>
#include <string.h>

int main(void)
{
  unsigned char binstr[] = "1001110001100111";
  unsigned char hexstr[5];
  unsigned int hexnum;
  unsigned char four[4];
  int i, j;

  for(i = 0;i < 4;++i)
  {
    hexnum = 0;

    strncpy(four, binstr + (i * 4), 4);
    for(j = 0;j < 4;++j)
      hexnum += (four[j] - '0') << (3 - j);

    sprintf(hexstr + i, "%X", hexnum);
  }

  puts(hexstr);

  return 0;
}

---

Code:

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itsme@itsme:~/C$ ./bintohex
9C67
itsme@itsme:~/C$

---

There is a "secret" standard library function called strtol() [String To Long]. It can convert a string representation in (almost) any base to a type long.

Once you have a "regular" integer value, you can display it in decimal, octal, or hex with cout, or you can use sprintf() if you want a string representation.

code fragment...

Code:

---


  int Base, x;
  char InputString[40];
  char *pEnd = NULL;          // Required for strtol()

  cout << "Base? (2-36, 0 to exit) " ;
  cin >> Base;

  if(!Base)
            return 0;

    cout << "Number? ";
    cin >> InputString;
    x = (int)strtol(InputString, &pEnd, Base);    // String to long

    cout << "\t" << dec << x << "\t\t Decimal" << endl;
    cout << "\t" << oct << x <<  "\t\t Octal" << endl;
    cout << "\t" << hex << x << "\t\t Hex" << endl;

---

If I wanted to print x to a string using sprintf, what flag would I use within the sprintf? %x?

It occurs to me that I was unclear, what I'm trying to convert is not a string of ASCII characters representing binary, it's pure, raw binary data (Specifically from a SHA-1 message digest).

So you basically have an array of 20 unsigned 1-byte numbers? Are you just wanting to convert those 20 numbers into a string that represents them in hex?

Try changing the code you had:

Code:

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for (z = 0; z < 20; z++)
        y += sprintf(string+y, "%02x", out[z]);

---

to...

Code:

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for (z = 0; z < 20; z++)
  sprintf(string+(z*2), "%02x", out[z]);

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You were close, but y is not needed. Your string only needs to be exactly 41 chars big to fit the hex string (2 chars for each of the 20 numbers plus 1 for the terminating zero).

Thanks much itsme86, that worked.