Comparing two files in c [closed]

Question

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center.

Closed 9 years ago.

As the title says I need to compare 2 files.If a line comes by ,that isnt the same between this 2 files,print that line (From both files).The code is giving a don't send error.I can't seem to be able to find the error in my code.

#include <stdio.h>
#include <string.h>
#define MAX 1000

int main(int argc, char *argv[]) {
    char c1, c2;
    char s1[MAX], s2[MAX];
    char *p1;
    char *p2;
    FILE *fp1;
    FILE *fp2;
    fp1 = fopen(argv[1], "r");
    fp2 = fopen(argv[2], "r");
    p1 = s1;
    p2 = s2;
    if (argc != 3) {
        printf("Wrong use of program \n ");
        return -1;
    }
    if (fp1 == NULL || fp2 == NULL) {
        printf("One or both of the files can't be used \n ");
        return -1;
    }
    while ((c1 = getc(fp1)) != EOF || (c2 = getc(fp2)) != EOF) {
        *p1 = c1;
        *p2 = c2;
        p1++;
        p2++;

        for (c1 = getc(fp1); c1 != '\n'; p1++) {
            *p1 = c1;
        }
        *p1 = '\0';

        for (c2 = getc(fp2); c2 != '\n'; p2++) {
            *p2 = c2;
        }
        *p2 = '\0';
        if (!(strcmp(s1, s2))) {
            printf("%s \n ", s1);
            printf("%s \n ", s2);
            return 0;
        }
    }
    return 0;
}

Answer 1

On the two for loops you have the getc on the initialisation part of the loop, it will put a character in c1 and c2 once and then do the loop until you go beyond the limit of s1 and s2 through your p1 and p2 pointers. Since the c1 != '\n' is never met unless your first character in the line is a '\n' it will probably throw a segmentation fault error.

On the while, at least with gcc and on my system, it does lazy evaluation (EDIT: Not lazy but short-circuit evaluation, see comments) and the c2 = getc(fp2) part is not executed if the other part was true.

You also don't reset p1 and p2 after each line.

strcmp returns 0 if both strings are equal, and in C, 0 is false and non-zero is true, so you are exiting on the first match.

Here is a barely functioning version of your code, you still need to work on it and take into account cases like one file being shorter than the other, one line in a file being bigger than 1000 characters (as it is right now it would overflow s1 or s2), etc.

#include <stdio.h>
#include <string.h>
#define MAX 1000

int main(int argc, char *argv[]) {
  char c1, c2;
  char s1[MAX], s2[MAX];
  char *p1;
  char *p2;
  FILE *fp1;
  FILE *fp2;
  p1 = s1;
  p2 = s2;
  if (argc != 3) {
    printf("Wrong use of program \n ");
    return -1;
  }
  fp1 = fopen(argv[1], "r");
  fp2 = fopen(argv[2], "r");
  if (fp1 == NULL || fp2 == NULL) {
    printf("One or both of the files can't be used \n ");
    return -1;
  }
  c1 = getc(fp1);
  c2 = getc(fp2);
  while ((c1 != EOF) && (c2 != EOF)) {
    for (; c1 != '\n'; p1++) {
        *p1 = c1;
        c1 = getc(fp1);
    }
    *p1 = '\0';

    for (; c2 != '\n'; p2++) {
        *p2 = c2;
        c2 = getc(fp2);
    }
    *p2 = '\0';
    if ((strcmp(s1, s2)) != 0) {
        printf("%s\n", s1);
        printf("%s\n", s2);
        return 0;
    }
    c1 = getc(fp1);
    c2 = getc(fp2);
    p1 = s1;
    p2 = s2;
  }
  if (c1 != EOF || c2 != EOF)
    printf("One of the files ended prematurely\n");
  return 0;
}

Answer 2

If the two files are text files then, honestly, I would start from scratch with a much simpler program that uses fgets() rather than getc() and compares line by line using strcmp() rather than character by character. There are just too many errors in the code as it is - you will complete the task more quickly if you discard what you have and start again with the simpler solution.

By the way, strcmp returns zero if the two strings are the same, non-zero if they are different.