reading hex values from a file in c

Question

As a part of an assignment I'm working on, I'm trying to read hex values from a file using the following code steps:

char buf[2048];
FILE *fp = fopen("/home/httpd/AS1/binary.bin", "r");
fgets(buf, 1024, fp);

I created a binary file that has values like for example /xff/xff/xff .... etc

and every time I read the file using the provided code, the hex values (shellcode) get change into different numerical values

I also cannot modify to ready the values in a loop one at a time as I am supposed to use the code as is.

I tried creating the file containing the values in various ways:

$ vi shcode.bin
$ vi shcode

I input the values into the file in various ways:

/xff/xff/xff
"/xff/xff/xff"
ÿÿÿ

and every time I read the file, the values gets changed.

I spent countless hours trying to find a solution with no result. Could you please help with what I'm doing wrong

The code you give has nothing to do with numerical values, so how can they get changed? All you are doing is to read a character string from a file, which might contain anything. — Weather Vane, Sep 13 '15 at 17:05
At least you must open the file with `"rb"`, that is, in binary mode. And vi is a _text_ editor, not a binary file editor. — Paul Ogilvie, Sep 13 '15 at 17:52

user3121023 · Answer 1 · 2015-09-13T20:08:10.880

This opens a text file and using fgets, reads a line containing the hex values. The sscanf format, " /x%x%n" skips any whitespace, scans a /, then an x, a hex value and by way of %n saves the number of characters processed in offset. used and offset allow sscanf to work through the hex buffer.

#include <stdio.h>
#include <stdlib.h>

int main( int argc, char *argv[])
{
    FILE *fp = NULL;
    char hex[1024] = "";
    int each = 0;
    size_t used = 0;
    size_t offset = 0;

    if ( ( fp = fopen ( "shcodes", "r")) == NULL) {
        printf ( "could not open file\n");
        return 1;
    }
    while ( fgets ( hex, sizeof ( hex), fp)) {
        used = 0;
         //work through the string as long the format is matched
        while ( ( sscanf ( hex + used, " /x%x%n", &each, &offset)) == 1) {
            printf ( "sscanf this int %d as hex %x\n", each, each);
            used += offset;
        }
    }
    fclose ( fp);
    return 0;
}

The text file used is

/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff/xff
/x0f/x1f/x2f/x3f/x4f/x5f/x6f/x7f/x8f/x9f/xaf/xbf/xcf/xff/xff
/x01/x11/x21/x31/x41/x51/x61/x71/x81/x91/xa1/xb1/xc1/xf1/xff

This should work for a binary file. fread is a better choice for reading binary data. bytes will store the number of bytes read.

#include <stdio.h>
#include <stdlib.h>

int main( int argc, char *argv[])
{
    FILE *fp = NULL;
    unsigned char hex[1024] = "";
    int each = 0;
    size_t bytes = 0;

    if ( ( fp = fopen ( "shcodes.bin", "rb")) == NULL) {
        printf ( "could not open file\n");
        return 1;
    }
    //loop as long as there are bytes to read
    while ( ( bytes = fread ( &hex, 1, 1024, fp)) > 0) {
        for ( each = 0; each < bytes; each++) {
            printf ( "read this char as int %u and as hex %x\n", hex[each], hex[each]);
        }
    }
    fclose ( fp);
    return 0;
}

reading hex values from a file in c

1 Answers1