--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
      CC = gcc -g -ggdb $(PROF)
     CPP = g++ -g -ggdb $(PROF)
  CFLAGS = -Wall -O2 -Iinclude $(EXTRA)
-  EXTRA = -D_XOPEN_SOURCE
+  EXTRA =
 LDFLAGS = -L/usr/lib
      RM = rm -f
    FIND = find

--- a/connection.c
+++ b/connection.c
@@ -67,7 +67,10 @@
         return EXIT_FAILURE;
     }
 
-    write (sockfd, "GET /\r\n", 7);
+    if (write (sockfd, "GET /\r\n", 7) == -1) {
+        perror ("write");
+        return EXIT_FAILURE;
+    }
 
     while ((num = read (sockfd, buffer, 1023)) != 0) {
         buffer[num] = 0;

--- a/counter.c
+++ b/counter.c
@@ -25,7 +25,9 @@
 
     start = time_str->tm_hour * 60 * 60 + time_str->tm_min * 60 + time_str->tm_sec;
 
-    system (argv[1]);
+    if (system (argv[1]) == -1) {
+        return EXIT_FAILURE;
+    }
 
     time (&t);
     time_str = gmtime (&t);

--- a/crypt.c
+++ b/crypt.c
@@ -3,6 +3,8 @@
  * Copyright (C) 2008 Markus Broeker
  */
 
+#define _XOPEN_SOURCE 500
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>

--- a/daemon.c
+++ b/daemon.c
@@ -32,7 +32,8 @@
     strftime (time_stamp, sizeof (time_stamp), "%a %d %T", localtime (&t));
     len = snprintf (response, sizeof (response), "%s %s", time_stamp, buffer);
 
-    write (fd, response, len);
+    if (write (fd, response, len) == -1)
+        perror ("write");
 }
 
 void sigproc ()
@@ -80,7 +81,10 @@
 
     sanity_check (argc, argv);
 
-    daemon (0, 0);
+    if (daemon (0, 0) == -1) {
+        perror ("daemon");
+        return EXIT_FAILURE;
+    }
     signal (SIGTERM, sigproc);
 
     for (;;) {

--- a/file_demo.c
+++ b/file_demo.c
@@ -22,8 +22,10 @@
         return -1;
     mode = strtoul (argv[1], NULL, 8);
 
-    system ("/bin/rm -f /home/mbroeker/Desktop/test1.txt");
-    system ("/bin/rm -f /home/mbroeker/Desktop/test2.txt");
+    if (system ("/bin/rm -f /home/mbroeker/Desktop/test1.txt") != 0)
+        return EXIT_FAILURE;
+    if (system ("/bin/rm -f /home/mbroeker/Desktop/test2.txt") != 0)
+        return EXIT_FAILURE;
 
     /*
      * Opens ReadOnly

--- a/files.c
+++ b/files.c
@@ -58,7 +58,8 @@
             }
             close (fd);
         }
-        chdir ("..");
+        if (chdir ("..") == -1)
+            break;
     }
     return EXIT_SUCCESS;
 }

--- a/recursive_compiler.c
+++ b/recursive_compiler.c
@@ -98,7 +98,8 @@
 int main (int argc, char **argv)
 {
     printf ("Bitte geben Sie einen Ausdruck in Infix-Notation ein:\n\n\t");
-    fgets (expression, MAX, stdin);
+    if (fgets (expression, MAX, stdin) == NULL)
+        return EXIT_FAILURE;
 
     printf ("\nCompilierter Ausdruck in Postfix-Notation:\n\n\t");
     compile_mode = MODE_POSTFIX;