How to compare two files and show all the differences in C?

Question

I'm currently trying to make a program that compare 2 files and that show all the differences.

The problems I'm having are:

• First line of the result doesn't show the first character.

• The differences don't have right results.

I've two input files.

file.txt

AAA
BBB
CCC
DDD
EEE

file2.txt

AAA
111
BBB
222
333
CCC
DDD
EEE
444

The output (1st line is bugged) I'm getting is:

11
BBB
222
333
CCC

And the output (without the 1st line bug) I desire to get must be:

111
222
333
444

This is currently my code:

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

int compare(char *fname1, char *fname2)
{
    FILE *fp1 = fopen(fname1, "r"); 
    FILE *fp2 = fopen(fname2, "r");
    int ch1, ch2;

    if (fp1 == NULL) 
    {
        printf("Can't open %s", fname1);
        exit(1);
    } 
    else if (fp2 == NULL)
    {
        printf("Can't open %s", fname2);
        exit(1);
    } 
    else
    {
        ch1 = getc(fp1);    
        ch2 = getc(fp2);

        while ((ch1 != EOF) && (ch2 != EOF) && (ch1 == ch2)) 
        {
            ch1 = getc(fp1);
            ch2 = getc(fp2);
        }

        if (ch1 == ch2)
        {
            printf("Same. \n");
        }
        else if (ch1 != ch2)
        {
            printf("Different strings:\n");

            while(!feof(fp1) && !feof(fp2))
            {
                fgets(fname1, ch1, fp1);
                fgets(fname2, ch2, fp2);

                if(strcmp(fname1, fname2) != 0)
                {
                    printf("%s", fname2);
                }
            }
        }
    }

    fclose(fp1);
    fclose(fp2);
    return 0;
}

And the main function:

int main(int argc, char *argv[])
{
    if (argc == 3){
        compare(argv[1], argv[2]);
    }else{
        printf("Usage: ./what file.txt file2.txt \n");
    }
    return 0;
}

Comparing file.txt and file2.txt or file2.txt and file.txt should give the same result.

Answer 1

The first character you're missing is being digested by getc(), in the first loop. To fix that, try using only the second while() or seek back one char before starting the next while() loop

Answer 2

Your two calls to fgets() are wrong. According to the fgets() documentation:

char * fgets ( char * str, int num, FILE * stream );

str: Pointer to an array of chars where the string read is copied.

num: Maximum number of characters to be copied into str (including the terminating null-character).

stream: Pointer to a FILE object that identifies an input stream. stdin can be used as argument to read from the standard input.

Trying to write into your programs's argv[] probably won't end well. At a minimum you'd want to do something like:

char string1[100];
char string2[100];

fgets(string1, 99, fp1);
fgets(string2, 99, fp2);

Also be aware that using feof() like this is probably the wrong thing to do.

As for your comparison problem...trying to do a file comparison and checking for added/deleted strings is not an easy problem. You can start at this question for some background.