리눅스 cat 옵션 구현 질문

#include <stdio.h>
#include <unistd.h> 
void output_file(FILE *fp);
int flag = 0;
main(int argc, char *argv[])
{
 FILE *fp;
 int opt;
 if (argc == 1) { 
 printf("Usage : a.out [-n] filename ...\n");
 exit(1);
 }
 
 while ((opt = getopt(argc, argv, "nbv")) != -1) {
 switch (opt) {
 
 case 'n':
 flag = 1;
 break;
 default:
 printf("unknown option : %c\n", optopt);
 }
 }
 
 if (argc != 2)
 argv++;
 
 while (*++argv) {
 printf("\n[filename : %s]\n", *argv);
 if ((fp = fopen(*argv, "r")) == NULL) {
 perror("fopen failed");
 exit(2);
 }
 output_file(fp);
 fclose(fp);
 }
 exit(0);
}
void output_file(FILE *fp)
{
 int ch, line = 1;
 if (flag)
 printf("1 ");
 while ((ch = getc(fp)) != EOF) {
 if (flag && ch == '\n') {
 
 if ((ch = getc(fp)) == EOF)
 break;
 
 ungetc(ch, fp);
 printf("\n%-4d", ++line);
 }
 else
 putc(ch, stdout);
 }
 
}
 printf("\n");