A C program compares 2 files and find its first difference if they are different

This C program will compare two files and find the first line of difference if there is a difference. This is just a basic example of how to read files using C programming, in unix like systems, there is already a command line tool (diff) you can use to compare two files.

To compile the program: gcc diff.c
To run the program: ./a.out file1.txt file2.txt

[code language=”c”]
#include<stdio.h>
#include<stdlib.h>

int main(int argc, char *argv[]){
FILE *fp1, *fp2;
int nl;
char r, r2;
int identical;
//fp1 = fopen(file1, "r");/* open file1 and file2 for reading */
//fp2 = fopen(file2, "r");
/* if either of these file does not exist, print error and return */
if((fp1 = fopen(argv[1], "r")) == NULL){
printf("Error — Cannot open file: %s\n\n", argv[1]);
return;
}
if((fp2 = fopen(argv[2], "r")) == NULL){
printf("Error — Cannot open file: %s\n\n", argv[2]);
return;
}
nl=1;/* Set the number line equals to 1 first.*/
while(!feof(fp1)) {
r = fgetc(fp1);/*use r to read each character from file1, use r2 to read each character from file2. */
r2 = fgetc(fp2);
if(r ==’\n’)/* if the character read if a new line, increment nl by 1*/
++nl;
if(r != r2){/* if r is not equal to r2, we have find the difference, print the following message */
printf("The files are different; the first difference is in line %d.\n\n", nl);
identical = 0;
return;
}
}
if(identical)
printf("The files are identical.\n\n");
if((fclose( fp1 ) == EOF)||(fclose( fp2 ) == EOF)) {/* close the file */
printf("Error closing first file.\n\n");
return;
}
return;
}
[/code]