diff(1) first diff line

diff.c

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

#define MAXLINE 100

char *fgets(char *line, int maxline, FILE *fp);

void diff_line(char *a, char *b){
	if(strcmp(a, b) < 0 || strcmp(a, b) > 0){
		fprintf(stderr, "%s%s\n", a, b);
	}
}

int main(int argc, char *argv[])
{
	FILE *fa, *fb;
	char fa_line[MAXLINE], fb_line[MAXLINE];

	if(argc < 3){
		fprintf(stderr, "invalid arguments %s \n", *argv);
		exit(1);
	}

	if((fa = fopen(argv[1], "r")) == NULL){
		fprintf(stderr, "cannot open %s", argv[1]);
		exit(1);
	}

	if((fb = fopen(argv[2], "r")) == NULL){
		fprintf(stderr, "cannot open %s", argv[2]);
		exit(1);
	}

	while((fgets(fa_line, MAXLINE, fa) != NULL)
			&& (fgets(fb_line, MAXLINE, fb) != NULL)){
		diff_line(fa_line, fb_line);
	}
}

char *fgets(char *line, int maxline, FILE *fp)
{
	register int c;
	register char *cs;

	cs = line;

	while(--maxline > 0 && (c = getc(fp)) != EOF){
		if((*cs++ = c) == '\n'){
			break;
		}
	}

	*cs = '\0';

	return (c == EOF && cs == line) ? NULL : line;
}