Convert to ASCII String from hex String in C

Question

I am trying to convert a hex string to its equivalent ASCII. I am given a "hex" value of a string as a string, i.e instead of "ABCD" I get "41424344". I just need to extract 41 which will be my hexvalue and recode as "ABCD". Here is what I have.

   int main(int argc, char *argv[]){

    char *str = "ABCD";
    unsigned int val = 0;
    int i = 0;
    int MAX = 4;
    for (i = 0; i<MAX; i++){
        val = (str[i] & 0xFF);
        //printf("dec val= %d\n", val);
        //printf("hex val= %02x\n", val);
    }
    val = 0;
    char *hexstr = "41424344";
    char *substr = (char*)malloc(3);
    char *ptr = hexstr;

    for (i = 0; i<8; i++){
        strncpy(substr, ptr, 2);
        printf("substr = %s\n", substr);
        int s = atoi(substr);
        printf("s= %d\n", s);
        ptr= ptr+2;
        i = i+2;
    }
    return 0;

}

The thing is from here on, I have to make this "s" value to be a hex value and not an int. How can this be done?

UPDATE:

Here is what I have after your answers:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[]){

    char *str = "ABCD";
    unsigned int val;
    val = 0;
    int i = 0;
    int MAX = 4;
    for (i = 0; i<MAX; i++){
        val = (str[i] & 0xFF);
        //printf("dec val= %d\n", val);
        //printf("hex val= %02x\n", val);
    }
    val = 0;
    char *hexstr = "41424344";
    char *substr = (char*)malloc(3);
    char *ptr = hexstr;
    char *retstr = (char *)malloc(5);
    char *retptr = retstr;

    for (i = 0; i<8; i+1){
        strncpy(substr, ptr, 2);
        printf("substr = %s\n", substr);
        int s = strtol(substr, NULL, 16);
        printf("s= %d\n", s);
        ptr= ptr+2;
        i = i+2;
        sprintf(retptr, "%c", s);
        retptr = retptr +1;
    }
    printf("retstr= %s\n", retstr);
    return 0;

}

you have strtol to convert a string in an other base and %x or %X in printf to display an hexa value — Alexis
– Alexis, Commented Jul 16, 2013 at 9:25

slaterade · Accepted Answer · 2013-07-16 09:39:14Z

2

Change your "s" variable line from

int s = atoi(substr);

to

int s = strtol(substr, NULL, 16);

edited Jul 16, 2013 at 9:39

answered Jul 16, 2013 at 9:26

slaterade

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4 Comments

simonc Over a year ago

substr doesn't have a nul terminator so the return from atoi will still be undefined

slaterade Over a year ago

yeah, also there's an interesting typo in the for loop increment step. i+i. anyway updated the answer hope it helps

user900785 Over a year ago

hi sudopunk, why do I get a warning in the for loop statement: statement has no effect?

slaterade Over a year ago

your loop needs to assign a value to i. (i = 0; i<8; i+1) should be changed to (i = 0; i<8; i=i+2) and you should remove the following statement inside the for loop i = i+2;

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