c - Converting HEX data in a file to ascii

Question

i want to decode an ASN.1 standard binary file. i have converted the binary file to hex and stored it in a file. now i want to convert this hex to ascii. The problem im having now is how to read the hex file.

the file looks this way, 81 01 32 82 0D 35 31 34 32 34 31 38 38 38 where 81 is a header, 01 is the size and 32 is the data. again 82 is the header and this goes on. how do i read from this file and differentiate between the various fields present.

i searched all over the internet for this, but couldnt get a satisfactory answer. so can someone help me with the way forward . i dont want any code, just want the procedure how i can do it.

Answer 1

I would first read the header and then in a loop the data. You can read hexadecimal numbers with the "x"-specifier (say your file is named hexfile.txt):

#include <stdio.h>

int main ()
{

    FILE *stream;
    unsigned int h, l, d;

    if( (stream  = fopen( "hexfile.txt", "r" )) == NULL ) return 1;

    while (EOF != fscanf (stream, " %x %x", &h, &l))
    {
        printf ("%02X %02X
",h,l);

        for (unsigned i=0; i<l; ++i)
        {
            if (EOF == fscanf (stream, " %x", &d)) break;
            printf ("%02X ",d);
        }
        puts ("");
    }

    fclose (stream);

    return 0;
}