(Only C) conversion special characters from string char to hexadecimal

Question

I'm trying to use this code: https://www.includehelp.com/c/convert-ascii-string-to-hexadecimal-string-in-c.aspx

This code works perfect on my program. It converts from utf-8 to hexa characters like A,m,n,d,0,9 perfectly.

Please, can anybody tell me or modify this program, when inside the string we have "special characters", like vocals with accents (ñ,ç,à,á,...). because, when I run this program don't works as I expected.

I'm working in a RHEL 7 with native C (sorry but I don't know the version) The special characters that I'm trying to conver to hex are in UTF-8.

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

//function to convert ascii char[] to hex-string (char[])
void string2hexString(char* input, char* output)
{
    int loop;
    int i; 

    i=0;
    loop=0;

    while(input[loop] != '\0')
    {
        sprintf((char*)(output+i),"%02X", input[loop]);
        loop+=1;
        i+=2;
    }
    //insert NULL at the end of the output string
    output[i++] = '\0';
}

int main(){
    char ascii_str[] = "Hello world!";
    //declare output string with double size of input string
    //because each character of input string will be converted
    //in 2 bytes
    int len = strlen(ascii_str);
    char hex_str[(len*2)+1];

    //converting ascii string to hex string
    string2hexString(ascii_str, hex_str);

    printf("ascii_str: %s\n", ascii_str);
    printf("hex_str: %s\n", hex_str);

    return 0;
}

Output

ascii_str: Hello world!

hex_str: 48656C6C6F20776F726C6421

I would like entry ascii_str like "ñáéíóúàèìòùç" and be able to obtain this hex codes on a string:

letra-> á // cod.hex--> e1
letra-> é // cod.hex--> e9
letra-> í // cod.hex--> ed
letra-> ó // cod.hex--> f3
letra-> ú // cod.hex--> fa
letra-> à // cod.hex--> e0
letra-> è // cod.hex--> e8
letra-> ì // cod.hex--> ec
letra-> ò // cod.hex--> f2
letra-> ù // cod.hex--> f9
letra-> ç // cod.hex--> e7

Answer 1

Change this:

sprintf((char*)(output+i), "%02X", input[loop]);

To this (which solves your problem):

sprintf((char*)(output+i), "%02X", (unsigned char)input[loop]);

Or better yet, to this (which gets rid of a redundant cast):

sprintf(output+i, "%02X", (unsigned char)input[loop]);