How can i modify this program so it calculates the hash without modifying the file?

Question

i have after alot of troubles finally come up with this code. It calculates the hash of a text file, and adds the hash to the file. Obviously, that changes the hash, so when i run it again i will get another hash.

However, if i just want to get the hash in its current state without changing it - what should i be changing in my code? Is it the "f = fopen (apszArgV[1], "rb+");" who causes the hash to change?

#include <stdio.h>
#include "md5.h"

#define BUFFER_SIZE 1024

void print_hash(char hash[]);

int main (int iArgC, char *apszArgV[])
{
   FILE *f;
   MD5_CTX ctx;
   BYTE byHash[16];
   BYTE byBuffer[BUFFER_SIZE]; 
   int iReadBytes;

   if (iArgC < 2) {
      printf ("Usage: md5_add <file name>\n");
      return 1;
   }
   f = fopen (apszArgV[1], "rb+");
   if (f != NULL) {

      md5_init(&ctx); 

      while (!feof(f)) {
         iReadBytes = fread(byBuffer, sizeof(BYTE), BUFFER_SIZE, f);
         md5_update(&ctx, byBuffer, iReadBytes);  
         if (iReadBytes < BUFFER_SIZE) break;
      }

      md5_final(&ctx, byHash);

      f = fopen("fil1.txt", "a");

      for (int i = 0; i < 15; i++) {
          fprintf (f, "%02X", byHash[i]);
      }
      fprintf(f, "\n");
      fclose (f);
   }

   print_hash(byHash);

}

void print_hash(char hash[]) 
{
   int idx; 
   for (idx=0; idx < 16; idx++) 
      printf("%02x",(int)((unsigned char)hash[idx])); 
   printf("\n"); 
}

thanks

new to c btw

You haven't fixed anything from [your previous question](https://stackoverflow.com/questions/46670913/how-do-i-write-this-hash-array-to-a-text-file). — Eugene Sh., Oct 10 '17 at 18:07
Not the code, no. But the way i ask the question. I learnt that the reason i was getting different hashes was because the process itself, of printing the hash to the file was changing the hash itself (duh). But now my question is, how can i simply get the hash without modifying the file? — , Oct 10 '17 at 18:12
you seem to miss a `fclose (f);` just after `md5_final(&ctx, byHash);` — Support Ukraine, Oct 10 '17 at 18:18
if you add, remove, or change even one bit in the file, the hash for it will change (collisions, and perhaps some padding excluded,, but that's the spirit of a hash). There's no getting around that. If you want to save the hash somewhere, you'll have to save it in another file, or you'll have to save it in the original file and parse it out and only hash the original data of the file to compare the two. — yano, Oct 10 '17 at 18:19
Read the manual page on `fopen`. If you just want to read a file, you don't want to open it with mode `rb+`. You also need to `fclose(f)` before using `f` again to open a different file. Finally, you were warned in your prior question post about the perils of using `while (!feof(f))`, so you may want to avoid that. — lurker, Oct 10 '17 at 18:24

score 0 · Answer 1 · answered Oct 10 '17 at 18:25

Sorry, cannot comment by now. In difference to the question before you actually already open 2 different files. To be honest i dont see in your code why it should still print to the same file that you first read for hashing but i see you need to clean up.

1) name pointers in a way you can see what they actually do:

Instead of

   FILE *f;

name it

File * file_to_be_hashed;

Then better declare a new name for your output file.

FILE * listOfHashes = fopen("fil1.txt", "a");

Also do not forget to close the files when you are done with them:

md5_final(&ctx, byHash)
fclose(f);

But as said, i dont really see why your code would still print into your input file, except maybe fopen to "fil2.txt" dont work. My guess is the file fil2.txt does not yet exist so it cannot be opened for appending as you specify in fopen("fil1.txt", "a");

Also there is no reason for the first fopen to be rb+ as r provides enough rights for reading ;-)

Maybe you learn about what the open flags of fopen actually mean... http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html

How can i modify this program so it calculates the hash without modifying the file?

1 Answers1