diff --git a/checkpassword.c b/checkpassword.c
index 6e5c576..07609b9 100644
--- a/checkpassword.c
+++ b/checkpassword.c
@@ -4,6 +4,11 @@ extern char *crypt();
 extern char *malloc();
 extern char **environ;
 
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+
 int error_txtbsy =
 #ifdef ETXTBSY
 ETXTBSY;
@@ -28,6 +33,8 @@ static struct spwd *spw;
 static struct userpw *upw;
 #endif
 
+#include <stdlib.h>	/* for getenv */
+
 void doit(login)
 char *login;
 {
@@ -70,7 +77,7 @@ char *str1e2(name,value) char *name; char *value;
 char up[513];
 int uplen;
 
-void main(argc,argv)
+int main(argc,argv)
 int argc;
 char **argv;
 {
@@ -81,13 +88,16 @@ char **argv;
   int i;
   char **newenv;
   int numenv;
- 
+  int unit = 3;
+  char* unit_str = getenv("CHECKPASSWORD_UNIT");
+  if (unit_str) unit = atoi(unit_str);
+  
   if (!argv[1]) _exit(2);
  
   uplen = 0;
   for (;;) {
     do
-      r = read(3,up + uplen,sizeof(up) - uplen);
+      r = read(unit, up + uplen,sizeof(up) - uplen);
     while ((r == -1) && (errno == EINTR));
     if (r == -1) _exit(111);
     if (r == 0) break;
@@ -95,7 +105,7 @@ char **argv;
     if (uplen >= sizeof(up)) _exit(1);
   }
 
-  close(3);
+  close(unit);
 
   i = 0;
   login = up + i;
@@ -108,10 +118,14 @@ char **argv;
  
   encrypted = crypt(password,stored);
  
-  for (i = 0;i < sizeof(up);++i) up[i] = 0;
+  for (i = 0;i < sizeof(up);++i) up[i] = 0;   // don't leave it lying around
  
   if (!*stored || strcmp(encrypted,stored)) _exit(1);
  
+// OK, the password checks out.
+
+  if (argv[1][0] == '-' && argv[1][1] == 0) _exit(0);
+
   if (prot_gid((int) pw->pw_gid) == -1) _exit(1);
   if (prot_uid((int) pw->pw_uid) == -1) _exit(1);
   if (chdir(pw->pw_dir) == -1) _exit(111);