diff --git a/cmp.c b/cmp.c
index 56c34c6..92424d8 100755
--- a/cmp.c
+++ b/cmp.c
@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 int main(int argc, char ** argv) {
   int silent_mode = 0;
@@ -11,29 +12,25 @@ int main(int argc, char ** argv) {
   int files_differ = 0;
   int i, f1num = 1;
   FILE *file1, *file2;
-  char c1, c2;
+  char c1, c2, c;
   int line = 1, byte = 1;
 
-  if(argv[1][0] == '-' && argv[1][1] == 's' && argv[1][2] == '\0') {
-    silent_mode = 1;
-    f1num++;
-    if(argv[2][0] == '-' && argv[2][1] == 'l' &&
-       argv[2][2] == '\0') {
-      f1num++;
+  while((c = getopt(argc, argv, "sl")) != -1) {
+    switch(c) {
+      case 's': silent_mode = 1; print_all_diff = 0; break;
+      case 'l': print_all_diff = 1; silent_mode = 0; break;
     }
   }
-  else if(argv[1][0] == '-' && argv[1][1] == 'l' &&
-	  argv[1][2] == '\0') {
-    print_all_diff = 1;
-    f1num++;
-    if(argv[2][0] == '-' && argv[2][1] == 's' &&
-       argv[2][2] == '\0') {
-      f1num++;
-    }
+
+  f1num = optind;
+
+  if(argc - f1num < 2) {
+    fprintf(stderr, "%s: missing file operand\n", argv[0]);
+    return 3;
   }
 
   if(strcmp(argv[f1num], "-") == 0 && strcmp(argv[f1num+1], "-") == 0) {
-    fprintf(stderr, "%s: Cannot accept stdin for both files\n", argv[0]);
+    fprintf(stderr, "%s: cannot accept stdin for both files\n", argv[0]);
     return 2;
   }
   else if(strcmp(argv[f1num], "-") == 0) {
@@ -49,6 +46,16 @@ int main(int argc, char ** argv) {
     file2 = fopen(argv[f1num+1], "r");
   }
 
+  if(file1 == NULL) {
+    fprintf(stderr, "%s: could not open file %s\n", argv[0], argv[f1num]);
+    return 4;
+  }
+
+  if(file2 == NULL) {
+    fprintf(stderr, "%s: could not open file %s\n", argv[0], argv[f1num + 1]);
+    return 4;
+  }
+
   if(silent_mode) {
     c1 = fgetc(file1);
     c2 = fgetc(file2);