Compare Two Files

One of the K&R exercises is to write a small piece of software to compare two files and print the first line in which they differ.

Here is my attempt:

#include 
#include 
#include 
#include 

#define SET 1
#define UNSET 0

#define BUFFERSIZE 100

void error(char *fmt, ...);

int main(int argc, char *argv[])
{
    if (argc != 3)
        error("Wrong argument size. Usage: %s FILE1 FILE2", *argv);

    char *prog = *argv++;
    char *fname1 = *argv++;
    char *fname2 = *argv;
    FILE *fp1 = fopen(fname1, "r");
    FILE *fp2 = fopen(fname2, "r");

    if(fp1 == NULL)
        error("Failed to open \"%s\"");
    if(fp2 == NULL)
        error("Failed to open \"%s\"");

    char *b1 = malloc(BUFFERSIZE * sizeof(char));
    char *b2 = malloc(BUFFERSIZE * sizeof(char));

    char EOF_FLAG1 = UNSET;
    char EOF_FLAG2 = UNSET;

    do {
        EOF_FLAG1 = fgets(b1, BUFFERSIZE, fp1) == NULL ? SET : UNSET;
        EOF_FLAG2 = fgets(b2, BUFFERSIZE, fp2) == NULL ? SET : UNSET;
    } while(strcmp(b1, b2) == 0 && EOF_FLAG1 == UNSET && EOF_FLAG2 == UNSET);

    if(strcmp(b1, b2) == 0 && EOF_FLAG1 == EOF_FLAG2)
        printf("The contents of \"%s\" and \"%s\" are equal.\n", fname1, fname2);
    else {
        printf("Difference found: \n\n");
        printf("%10s:\t%s\n", fname1, b1);
        printf("%10s:\t%s\n", fname2, b2); 
    }

    free(b1);
    free(b2);

    exit(0);
}

void error(char *fmt, ...)
{
    va_list ap;
    va_start(ap, fmt);

    fprintf(stderr, "error: ");
    vfprintf(stderr, fmt, ap);
    putc('\n', stderr);

    va_end(ap);
    exit(1);
}

Things I like about your program include

• The use of pointer arithmetic to parse your file arguments. I have never seen that done, and I think that is a good use of it.

• I also like how your error function works, It gives you a good way to exit your program in a hurry, consider adding the ability to give warning levels to this.

-
Your output statements are very clear, and user friendly. However, it is not very unixy, in that you could pipe the output of this to another tool easily. Consider fixing this, just because it violates the rule of silence.


Rule of Silence
Developers should design programs so that they do not print unnecessary output. This rule aims to allow other programs and developers to pick out the information they need from a program's output without having to parse verbosity. ~ Wikipedia

Things I dislike:

• You don't close your file handles using fclose, this is good practice and should be done.

• if statements should always be curly bracketed, as it makes them easier to expand if you need to. if(){ .... } else { .... }.

-
You don't necessarily need to wrap conditionals in parenthesis, but I feel that code is made clearer when you wrap each conditional in a compound conditional in parenthesis.

if((COND1) && (COND2)){ ... }

Things you could improve

You could return a code to the operating system to indicate that the files do not match, this would make it more useful, as you could add it to a shell script workflow. See the snippet below.

YourProg f1 f2
if [ $? -eq 0 ] 
then
    echo File match
else
    echo Files do not match
fi

StackExchange Code Review Q#110920, answer score: 3