Comparing two files

[pxleyes]

I am trying to compare two files through command line arguments. The result is that the program returns the line number at which the files differ based on the first byte that it encounters that differs. It should be working just fine, but it always returns a line value of 1. I cant figure out why...

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

int main(int argc, char *argv[])
{

FILE *fp, *fr;
int line = 1;
char c, d;

fp=fopen(argv[1], "r");
fr=fopen(argv[2], "r");

while(((c=fgetc(fp)) != EOF) || ((d=fgetc(fr)) != EOF)){
   if(c != d){
     printf("File differs at line %d", line);
     exit(0);
   }
   if(c == '\n'){
     line++;
   }
}
      system("PAUSE");
      return 0;
}

[Dave_Sinkula]

while(((c=fgetc(fp)) != EOF) || ((d=fgetc(fr)) != EOF)){

Since the first part of the logical or is true, the second part is "short circuited" and not performed. So you are most likely comparing a value of 'c' read from the first file with an indeterminate value of 'd'. This compares false and you exit immediately.

[pxleyes]

Ah, ok. I just moved the d=fgetc(fr); into the actual while loop. This works fine. Thanks.

[EvBladeRunnervE]

or you could of done:

while(((c=fgetc(fp)) != EOF) && ((d=fgetc(fr)) != EOF)){

[pxleyes]

Doesn't work like you would expect it to if you do that though.

[Hammer]

char c, d;

while(((c=fgetc(fp)) != EOF)

This is incorrect. You should've declared c and d as ints not chars. Read this to understand why
http://faq.cprogramming.com/cgi-bin/...&id=1043284351

[BianConiglio]

I've done a similiar function and I have problems.

int FileCompare (char nomebuffer[], char nomebackup[]){
    
    FILE *namebuffer, *namebackup;
    
    char buffer[MAX_KEY];
    char backup[MAX_KEY];
    int  lineabuffer = 0;
    
    namebuffer = fopen(nomebuffer, "rb");
    namebackup = fopen(nomebackup, "rb");
    
    while((fgets(buffer, MAX_KEY, namebuffer)) != NULL){
        
        ++lineabuffer;
        fgets(backup, MAX_KEY, namebackup);
        
        if (buffer != backup){
             printf("Mismatch at line %d\n", lineabuffer);
             printf("Buffer -> %sBackup -> %s\n",buffer, backup);
             return 1;
        }//END IF

    }// END WHILE

        fclose(namebuffer);
        fclose(namebackup);       
        return 0;
    
}//END FileCompare

I have to compare 2 text files with written this :

NvCplDaemon
RUNDLL32.EXE C:\WINDOWS\System32\NvCpl.dll,NvStartup
AdslTaskBar
rundll32.exe stmctrl.dll,TaskBar

I copy the buffer file into a backup file, then I call FileCompare but it says there is allways a mismatch in line 1

output:

Mismatch at line 1:
Buffer -> NvCplDaemon
Backup -> NvCplDaemon

I'm sure the lines are not different coz backup it is a copy of buffer previously done by:

CopyFile(nomebuffer, nomebackup, FALSE);

any suggestion why the

if (buffer != backup)

fails?

[Dave_Sinkula]

any suggestion why the

if (buffer != backup)

fails?

FAQ > How do I... (Level 1) > Comparing strings (C)

[iwabee]

BianConiglio, you've made 5 mistakes:
1. Like Dave noted (buffer != backup) will compare adressess not strings
2. You assume that namebuffer file is longer than (or as long as) namebackup, since you never check if it reached the end (look at EvBladeRunnervE post)
3. You never close files if they aren't identical
4. You never check if fopen() calls failed
5. If you reading text, why open files with "rb"?

[BianConiglio]

what an idiot!!!! thanx guys

[BianConiglio]

Working code

void SicurezzaCompromessa(void){

    MessageBox( NULL,
                         "Error opening file.\n",
                         "Error:", 
                          MB_OK);
    exit(1);
}//END SicurezzaCompromessa

int FileCompare (char nomebuffer[], char nomebackup[]){
    
    FILE *namebuffer, *namebackup;
    
    char buffer[MAX_KEY];
    char backup[MAX_KEY];
    int  lineabuffer = 0;
    
    namebuffer = fopen(nomebuffer, "r");
    namebackup = fopen(nomebackup, "r");
    
    if (namebuffer == NULL){
        
        SicurezzaCompromessa();
    }    
    
    if (namebackup == NULL){
        
        SicurezzaCompromessa();
    }  
    
    while((fgets(backup, MAX_KEY, namebackup )) != NULL){
        
        ++lineabuffer;
        fgets(buffer, MAX_KEY, namebuffer );
        
        if (strcmp(buffer,backup)!=0){
             
             printf("Mismatch at line %d\n", lineabuffer);
             printf("Buffer -> %sBackup -> %s\n",buffer, backup);
          
             fclose(namebuffer);
             fclose(namebackup);
             
             return 1;
        }//END IF
    
      
    }// END WHILE

   fclose(namebuffer);
   fclose(namebackup);
          
   return 0;
    
}//END FileCompare

2. You assume that namebuffer file is longer than (or as long as) namebackup

yes I know it before calling that function

[Salem]

> fgets(buffer, MAX_KEY, namebuffer );
Unfortunately, if the 2nd file is shorter than the first, you repeatedly compare the extra lines of file1 with the last line of file2.

If both files have identical tails like

one
two
three

one
two
three
three

Your code will compare them equal.

[BianConiglio]

I hadn't noticed it before thanx

anyway that situation is avoided in the main{} (I call that function only if buffer <= backup)

anyway I will add (and post) an extra control so I can use this function everywhere safely