Convert string array into hex numbers in C

Question

I have a file that contains a matrix of many rows and columns. It looks like something below:

fa ff 00 10 00
ee ee 00 00 30
dd d1 00 aa 00

Each entry in the matrix is a hex number of an eight bit value. I would like to read this file into a two dimensional array.

I have two problems:

Using the read method in my code, *it contains an array that has each entry (two characters) of the matrix. How can I pass each entry into a single variable instead of two characters?
When I pass into the single variable, how to convert it from character to hex? I mean "ff" should be converted to 0xff.

Part of my code is below. I can avoid the tokenize function if better methods can be uesd.

char** tokens;
char** it;

while (fgets(line, sizeof(line), file) != NULL){ /* read a line */
    tokens = tokenize(line);    // split line

    for(it=tokens; it && *it; ++it){
        printf("%s\n", *it);
        free(*it);
    } // end for
} // end while

char** tokenize(const char* str){
    int count = 0;
    int capacity = 10;
    char** result = malloc(capacity*sizeof(*result));

    const char* e=str;

    if (e) do {
        const char* s=e;
        e=strpbrk(s," ");

        if (count >= capacity)
            result = realloc(result, (capacity*=2)*sizeof(*result));

        result[count++] = e? strndup(s, e-s) : strdup(s);
    } while (e && *(++e));

    if (count >= capacity)
        result = realloc(result, (capacity+=1)*sizeof(*result));
    result[count++] = 0;

    return result;
}

Do you mean take fa ff 00 10 00 as a number, then we will get 1078020018176? — prehistoricpenguin
– prehistoricpenguin, Commented Oct 1, 2013 at 1:35
No, fa is a single entry. So i will get 0xfa instead of "fa". — drdot
– drdot, Commented Oct 1, 2013 at 1:36

Floris · Accepted Answer · 2014-01-21 13:39:40Z

7

Here is half an answer: to read a hex string and turn it into a value, you can do the following:

#include <stdio.h>

int main(void) {
  char *myString="8e";
  int myVal;
  sscanf(myString, "%x", &myVal);
  printf("The value was read in: it is %0x in hex, or %d in decimal\n", myVal, myVal);
}

This gets you the answer to the "how do I read two characters into one variable" part of your question, I hope.

As for the second half - if you did the following:

while (fgets(line, sizeof(line), file) != NULL){ /* read a line */
    tokens = tokenize(line);    // split line

    for(int ii=0; tokens[ii]; i++) {
        printf("ii=%d; token = %s\n", ii, tokens[ii]);
    } // end for
} // end while

you would see that your tokens array already contains what you are asking for: the strings. If you convert each of them using sscanf - for example like so:

int myValues[10];
for(int ii=0; ii<10; ii++) {
  sscanf(tokens+ii, "%x", myValues+ii);
  printf("The %dth converted value is %d - or %x in hex\n", ii, myValues[ii], myValues[ii]);
}

You can see that this does everything you want. I used a fixed size array in the above - you clearly know how to use malloc to make a dynamically allocated array of the right size (instead of a fixed value of 10).

One more note - the address of an array element myArray[0] can be written either as &myArray[0], or simply as myArray. For other elements, &myArray[5] is the same as myArray+5. One of the marvels of pointer math in C. I hope this helps.

edited Jan 21, 2014 at 13:39

answered Oct 1, 2013 at 1:01

Floris

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6 Comments

drdot Over a year ago

Thank you for the answer! What about reading each entry into a single variable?

Floris Over a year ago

Do you know ahead of time how many tokens per line, and how many lines?

drdot Over a year ago

Yes. I know those numbers, they are passed in from stdin.

drdot Over a year ago

But i think tokens[i] is the answer as @Theodoros suggested.

drdot Over a year ago

Thank you for the answer. One last question. The line: for(it=tokens; it && *it; ++it){ // Although I assign it = tokens, but it[0] is not equal to tokens[0]. What is the reason for this? Thank you!

|

Theodoros Chatzigiannakis · Accepted Answer · 2013-10-01 01:02:32Z

2

Converting to a hexadecimal number is kind of meaningless, since hexadecimal/decimal/binary/etc are just representations of a value that's stored in the same way in memory anyway.

That being said, a simple way to convert the hexadecimal string to the number it represents is this:

Create a temporary buffer where you prepend "0x" to your hex string (so if you have the string "ff" make the buffer contain "0xff").
Give that buffer to a scanning function (sscanf will work for your case) and in the format string give the correct format specifier (in this case %x for hex).
Retrieve the value from the variable (whose address you have provided to sscanf) and use it however you like.

answered Oct 1, 2013 at 1:02

Theodoros Chatzigiannakis

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5 Comments

drdot Over a year ago

Thank you. This problem is solved. What about reading each matrix entry into a single variable?

Theodoros Chatzigiannakis Over a year ago

How about just tokens[0], tokens[1]? (Unless you mean something else.)

drdot Over a year ago

That solves the problem. Could you add a few lines in your answer and I will take it. But I still don't understand why it[0] != tokens[0].

Theodoros Chatzigiannakis Over a year ago

Could you be more specific about what lines you want?

drdot Over a year ago

for(it=tokens; it && *it; ++it){ // Although I assign it = tokens, but it[0] is not equal to tokens[0]. What is the reason for this? Thank you!

prehistoricpenguin · Accepted Answer · 2013-10-01 01:46:24Z

0

You may just use cin of scanf to read in the data as digit, they automatically take space or newline as split character, take this example:

int main() {
  int val;
  while (scanf("%x", &val) != EOF)
    printf("val we get : %x\n", val);

  return 0;
}

Here is the test.

answered Oct 1, 2013 at 1:46

prehistoricpenguin

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Comments

liruqi · Accepted Answer · 2015-07-12 11:20:57Z

Checkout Apple's implementation from hexString.c

static char byteMap[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
static int byteMapLen = sizeof(byteMap);

/* utility function to convert hex character representation to their nibble (4 bit) values */
static uint8_t
nibbleFromChar(char c)
{
    if(c >= '0' && c <= '9') return c - '0';
    if(c >= 'a' && c <= 'f') return c - 'a' + 10;
    if(c >= 'A' && c <= 'F') return c - 'A' + 10;
    return 255;
}


/* Utility function to convert nibbles (4 bit values) into a hex character representation */
static char
nibbleToChar(uint8_t nibble)
{
    if(nibble < byteMapLen) return byteMap[nibble];
    return '*';
}

/* Convert a buffer of binary values into a hex string representation */
char * bytesToHexString(uint8_t *bytes, size_t buflen)
{
    char *retval;
    int i;

    retval = malloc(buflen*2 + 1);
    for(i=0; i<buflen; i++) {
        retval[i*2] = nibbleToChar(bytes[i] >> 4);
        retval[i*2+1] = nibbleToChar(bytes[i] & 0x0f);
    }
    retval[i] = '\0';
    return retval;
}

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