diff --git a/backend/agfafocus.c b/backend/agfafocus.c
index 1255b35bf..0b59d2da4 100644
--- a/backend/agfafocus.c
+++ b/backend/agfafocus.c
@@ -954,7 +954,7 @@ do_cancel (AgfaFocus_Scanner * s)
       /* ensure child knows it's time to stop: */
       sanei_thread_kill (s->reader_pid);
       sanei_thread_waitpid (s->reader_pid, &exit_status);
-      s->reader_pid = -1;
+      sanei_thread_invalidate(s->reader_pid);
     }
 
   if (s->fd >= 0)
diff --git a/backend/artec_eplus48u.c b/backend/artec_eplus48u.c
index f31bf5cc3..0e81b068b 100644
--- a/backend/artec_eplus48u.c
+++ b/backend/artec_eplus48u.c
@@ -3505,7 +3505,7 @@ do_cancel (Artec48U_Scanner * s, SANE_Bool closepipe)
 	{
 	  XDBG ((1, "sanei_thread_waitpid() failed !\n"));
 	}
-      s->reader_pid = -1;
+      sanei_thread_invalidate (s->reader_pid);
       XDBG ((1, "reader_process killed\n"));
     }
   if (SANE_TRUE == closepipe)
@@ -4337,7 +4337,7 @@ sane_read (SANE_Handle handle, SANE_Byte * data,
 	  if (s->eof == SANE_TRUE)
 	    {
 	      sanei_thread_waitpid (s->reader_pid, 0);
-	      s->reader_pid = -1;
+	      sanei_thread_invalidate (s->reader_pid);
 	      artec48u_scanner_stop_scan (s);
 	      artec48u_carriage_home (s->dev);
 	      return close_pipe (s);
diff --git a/backend/avision.c b/backend/avision.c
index 3e9871546..faf71babe 100644
--- a/backend/avision.c
+++ b/backend/avision.c
@@ -6207,7 +6207,7 @@ do_eof (Avision_Scanner *s)
   /* join our processes - without a wait() you will produce zombies
      (defunct children) */
   sanei_thread_waitpid (s->reader_pid, &exit_status);
-  s->reader_pid = -1;
+  sanei_thread_invalidate (s->reader_pid);
 
   DBG (3, "do_eof: returning %d\n", exit_status);
   return (SANE_Status)exit_status;
@@ -6229,7 +6229,7 @@ do_cancel (Avision_Scanner* s)
     /* ensure child knows it's time to stop: */
     sanei_thread_kill (s->reader_pid);
     sanei_thread_waitpid (s->reader_pid, &exit_status);
-    s->reader_pid = -1;
+    sanei_thread_invalidate (s->reader_pid);
   }
 
   return SANE_STATUS_CANCELLED;
@@ -7844,7 +7844,7 @@ sane_open (SANE_String_Const devicename, SANE_Handle *handle)
   s->av_con.scsi_fd = -1;
   s->av_con.usb_dn = -1;
 
-  s->reader_pid = -1;
+  sanei_thread_initialize (s->reader_pid);
   s->read_fds = -1;
 
   s->hw = dev;
diff --git a/backend/coolscan.c b/backend/coolscan.c
index dd17d67de..4904afcd9 100644
--- a/backend/coolscan.c
+++ b/backend/coolscan.c
@@ -2037,7 +2037,7 @@ do_cancel (Coolscan_t * scanner)
       sanei_thread_kill (scanner->reader_pid);
       while (sanei_thread_waitpid(scanner->reader_pid, &exit_status) !=
                                                         scanner->reader_pid );
-      scanner->reader_pid = -1;
+      sanei_thread_invalidate (scanner->reader_pid);
     }
 
   if (scanner->sfd >= 0)
@@ -4155,7 +4155,7 @@ sane_cancel (SANE_Handle handle)
     {
       sanei_thread_kill   ( s->reader_pid );
       sanei_thread_waitpid( s->reader_pid, NULL );
-      s->reader_pid = -1;
+      sanei_thread_invalidate (s->reader_pid);
     }
   swap_res (s);
   s->scanning = SANE_FALSE;
diff --git a/backend/hp3500.c b/backend/hp3500.c
index 7733796a2..97b96f18a 100644
--- a/backend/hp3500.c
+++ b/backend/hp3500.c
@@ -1134,7 +1134,7 @@ do_cancel (struct hp3500_data *scanner)
 
 	  sanei_thread_waitpid (scanner->reader_pid, &exit_status);
 	}
-      scanner->reader_pid = -1;
+      sanei_thread_invalidate (scanner->reader_pid);
     }
   if (scanner->pipe_r >= 0)
     {
diff --git a/backend/microtek2.c b/backend/microtek2.c
index e650f4762..f7c63d62c 100644
--- a/backend/microtek2.c
+++ b/backend/microtek2.c
@@ -554,7 +554,7 @@ sane_open(SANE_String_Const name, SANE_Handle *handle)
     ms->cancelled = SANE_FALSE;
     ms->current_pass = 0;
     ms->sfd = -1;
-    ms->pid = -1;
+    sanei_thread_initialize(ms->pid);
     ms->fp = NULL;
     ms->gamma_table = NULL;
     ms->buf.src_buf = ms->buf.src_buffer[0] = ms->buf.src_buffer[1] = NULL;
@@ -1353,7 +1353,7 @@ cleanup_scanner(Microtek2_Scanner *ms)
     if ( ms->sfd != -1 )
       sanei_scsi_close(ms->sfd);
     ms->sfd = -1;
-    ms->pid = -1;
+    sanei_thread_invalidate(ms->pid);
     ms->fp = NULL;
     ms->current_pass = 0;
     ms->scanning = SANE_FALSE;
diff --git a/backend/mustek.c b/backend/mustek.c
index 230ad0c58..5435d5fd2 100644
--- a/backend/mustek.c
+++ b/backend/mustek.c
@@ -3008,7 +3008,7 @@ do_stop (Mustek_Scanner * s)
 	    status = exit_status;
 	}
 
-      s->reader_pid = -1;
+      sanei_thread_invalidate (s->reader_pid);
     }
 
   if (s->fd >= 0)
diff --git a/backend/pie.c b/backend/pie.c
index 76cbb47b7..0e56a5af7 100644
--- a/backend/pie.c
+++ b/backend/pie.c
@@ -2901,7 +2901,7 @@ do_cancel (Pie_Scanner * scanner)
       DBG (DBG_sane_info, "killing reader_process\n");
       sanei_thread_kill (scanner->reader_pid);
       sanei_thread_waitpid (scanner->reader_pid, 0);
-      scanner->reader_pid = -1;
+      sanei_thread_invalidate (scanner->reader_pid);
       DBG (DBG_sane_info, "reader_process killed\n");
     }
 
diff --git a/backend/pixma.c b/backend/pixma.c
index 7527da461..9f6927604 100644
--- a/backend/pixma.c
+++ b/backend/pixma.c
@@ -1105,7 +1105,7 @@ terminate_reader_task (pixma_sane_t * ss, int *exit_code)
 /*      pixma_cancel (ss->s);   What is this for ? Makes end-of-scan buggy => removing */
     }
   result = sanei_thread_waitpid (pid, &status);
-  ss->reader_taskid = -1;
+  sanei_thread_invalidate (ss->reader_taskid);
 
   if (ss->sp.source != PIXMA_SOURCE_ADF && ss->sp.source != PIXMA_SOURCE_ADFDUP)
     ss->idle = SANE_TRUE;
@@ -1376,7 +1376,7 @@ sane_open (SANE_String_Const name, SANE_Handle * h)
     return SANE_STATUS_NO_MEM;
   ss->next = first_scanner;
   first_scanner = ss;
-  ss->reader_taskid = -1;
+  sanei_thread_initialize (ss->reader_taskid);
   ss->wpipe = -1;
   ss->rpipe = -1;
   ss->idle = SANE_TRUE;
diff --git a/backend/plustek.c b/backend/plustek.c
index 9b830e759..21e984564 100644
--- a/backend/plustek.c
+++ b/backend/plustek.c
@@ -604,7 +604,7 @@ do_cancel( Plustek_Scanner *scanner, SANE_Bool closepipe )
 #endif
 		}
 
-		scanner->reader_pid = -1;
+		sanei_thread_invalidate( scanner->reader_pid );
 		DBG( _DBG_PROC,"reader_process killed\n");
 #ifndef HAVE_SETITIMER
 		usb_StartLampTimer( scanner->hw );
@@ -2706,7 +2706,7 @@ sane_read( SANE_Handle handle, SANE_Byte *data,
 			if( s->bytes_read ==
 				(unsigned long)(s->params.lines * s->params.bytes_per_line)) {
 				sanei_thread_waitpid( s->reader_pid, 0 );
-				s->reader_pid = -1;
+				sanei_thread_invalidate( s->reader_pid );
 				s->scanning = SANE_FALSE;
 				drvclose( s->hw );
 				return close_pipe(s);
@@ -2735,7 +2735,7 @@ sane_read( SANE_Handle handle, SANE_Byte *data,
 			close_pipe(s);
 			return s->exit_code;
 		}
-		s->reader_pid = -1;
+		sanei_thread_invalidate( s->reader_pid );
 		s->scanning = SANE_FALSE;
 		return close_pipe(s);
 	}
diff --git a/backend/plustek_pp.c b/backend/plustek_pp.c
index adad34753..e1d4c02f7 100644
--- a/backend/plustek_pp.c
+++ b/backend/plustek_pp.c
@@ -506,7 +506,7 @@ static SANE_Status do_cancel( Plustek_Scanner *scanner, SANE_Bool closepipe  )
 #endif
 		}
 
-		scanner->reader_pid = -1;
+		sanei_thread_invalidate( scanner->reader_pid );
 		DBG( _DBG_PROC,"reader_process killed\n");
 	}
 
@@ -2084,7 +2084,7 @@ SANE_Status sane_read( SANE_Handle handle, SANE_Byte *data,
 			if( s->bytes_read ==
 				(unsigned long)(s->params.lines * s->params.bytes_per_line)) {
 				sanei_thread_waitpid( s->reader_pid, 0 );
-				s->reader_pid = -1;
+				sanei_thread_invalidate( s->reader_pid );
 				drvclose( s->hw );
 				return close_pipe(s);
 			}
@@ -2112,7 +2112,7 @@ SANE_Status sane_read( SANE_Handle handle, SANE_Byte *data,
 			close_pipe(s);
 			return s->exit_code;
 		}
-		s->reader_pid = -1;
+		sanei_thread_invalidate( s->reader_pid );
 		return close_pipe(s);
 	}
 
diff --git a/backend/snapscan.c b/backend/snapscan.c
index c88537be6..d9054ebbb 100644
--- a/backend/snapscan.c
+++ b/backend/snapscan.c
@@ -1286,7 +1286,7 @@ static SANE_Status start_reader (SnapScan_Scanner *pss)
 
     pss->nonblocking = SANE_FALSE;
     pss->rpipe[0] = pss->rpipe[1] = -1;
-    pss->child = -1;
+    sanei_thread_initialize (pss->child);
 
     if (pipe (pss->rpipe) != -1)
     {
@@ -1812,7 +1812,7 @@ SANE_Status sane_read (SANE_Handle h,
       if (sanei_thread_is_valid (pss->child))
         {
             sanei_thread_waitpid (pss->child, 0);        /* ensure no zombies */
-            pss->child = -1;
+            sanei_thread_invalidate (pss->child);
         }
         release_unit (pss);
         close_scanner (pss);
@@ -1904,7 +1904,7 @@ void sane_cancel (SANE_Handle h)
                 sanei_thread_sendsig( pss->child, SIGKILL );
 #endif
             }
-            pss->child = -1;
+            sanei_thread_invalidate( pss->child );
             DBG( DL_INFO,"reader_process killed\n");
         }
         release_unit (pss);
diff --git a/backend/sp15c.c b/backend/sp15c.c
index 203bfd9dc..20b2c8077 100644
--- a/backend/sp15c.c
+++ b/backend/sp15c.c
@@ -1769,7 +1769,7 @@ do_cancel (struct sp15c *scanner)
       sanei_thread_kill (scanner->reader_pid);
       DBG (50, "wait for scanner to stop\n");
       sanei_thread_waitpid (scanner->reader_pid, &exit_status);
-      scanner->reader_pid = -1;
+      sanei_thread_invalidate (scanner->reader_pid);
     }
 
   if (scanner->sfd >= 0)
diff --git a/backend/tamarack.c b/backend/tamarack.c
index 3fda5a7c3..feb9ee1c6 100644
--- a/backend/tamarack.c
+++ b/backend/tamarack.c
@@ -479,7 +479,7 @@ do_cancel (Tamarack_Scanner *s)
       /* ensure child knows it's time to stop: */
       sanei_thread_kill (s->reader_pid);
       sanei_thread_waitpid (s->reader_pid, &exit_status);
-      s->reader_pid = -1;
+      sanei_thread_invalidate (s->reader_pid);
     }
 
   if (s->fd >= 0)
diff --git a/backend/test.c b/backend/test.c
index 239dcfa12..3ead456dc 100644
--- a/backend/test.c
+++ b/backend/test.c
@@ -1375,7 +1375,7 @@ finish_pass (Test_Device * test_device)
 	  DBG (2, "finish_pass: reader process terminated with status: %s\n",
 	       sane_strstatus (status));
 	}
-      test_device->reader_pid = -1;
+      sanei_thread_invalidate (test_device->reader_pid);
     }
   /* this happens when running in thread context... */
   if (test_device->reader_fds >= 0)
@@ -1641,7 +1641,7 @@ sane_init (SANE_Int * __sane_unused__ version_code, SANE_Auth_Callback __sane_un
       test_device->eof = SANE_FALSE;
       test_device->scanning = SANE_FALSE;
       test_device->cancelled = SANE_FALSE;
-      test_device->reader_pid = -1;
+      sanei_thread_initialize (test_device->reader_pid);
       test_device->pipe = -1;
       DBG (4, "sane_init: new device: `%s' is a %s %s %s\n",
 	   test_device->sane.name, test_device->sane.vendor,
diff --git a/backend/u12.c b/backend/u12.c
index 4725f5b2d..78e917573 100644
--- a/backend/u12.c
+++ b/backend/u12.c
@@ -421,7 +421,7 @@ static SANE_Status do_cancel( U12_Scanner *scanner, SANE_Bool closepipe )
 			sanei_thread_sendsig( scanner->reader_pid, SIGKILL );
 #endif
 		}
-		scanner->reader_pid = -1;
+		sanei_thread_invalidate( scanner->reader_pid );
 		DBG( _DBG_PROC, "reader_process killed\n");
 
 		if( scanner->hw->fd >= 0 ) {
@@ -1773,7 +1773,7 @@ SANE_Status sane_read( SANE_Handle handle, SANE_Byte *data,
 			if( s->bytes_read ==
 				(unsigned long)(s->params.lines * s->params.bytes_per_line)) {
 				sanei_thread_waitpid( s->reader_pid, 0 );
-				s->reader_pid = -1;
+				sanei_thread_invalidate( s->reader_pid );
 				drvClose( s->hw );
 				return drvClosePipes(s);
 			}
@@ -1801,7 +1801,7 @@ SANE_Status sane_read( SANE_Handle handle, SANE_Byte *data,
 			drvClosePipes(s);
 			return s->exit_code;
 		}
-		s->reader_pid = -1;
+		sanei_thread_invalidate( s->reader_pid );
 		return drvClosePipes(s);
 	}
 
diff --git a/backend/umax.c b/backend/umax.c
index e9fd449d4..94f8aa4e6 100644
--- a/backend/umax.c
+++ b/backend/umax.c
@@ -4686,7 +4686,7 @@ static SANE_Status do_cancel(Umax_Scanner *scanner)
       DBG(DBG_sane_info, "do_cancel: reader_process terminated with status: %s\n", sane_strstatus(status));
     }
 
-    scanner->reader_pid = -1;
+    sanei_thread_invalidate (scanner->reader_pid);
 
     if (scanner->device->pixelbuffer != NULL)					      /* pixelbuffer exists? */
     {
@@ -7285,9 +7285,9 @@ SANE_Status sane_start(SANE_Handle handle)
 
   DBG(DBG_sane_init,"sane_start\n");
 
-  /* Initialize reader_pid to invalid so a subsequent error and following call
-   * to do_cancel() won't trip over it. */
-  scanner->reader_pid = -1;
+  /* Invalidate reader_pid so a subsequent error and following call to
+   * do_cancel() won't trip over it. */
+  sanei_thread_invalidate(scanner->reader_pid);
 
   mode = scanner->val[OPT_MODE].s;
 
diff --git a/include/sane/sanei_thread.h b/include/sane/sanei_thread.h
index f2323cedd..6e689642a 100644
--- a/include/sane/sanei_thread.h
+++ b/include/sane/sanei_thread.h
@@ -94,6 +94,38 @@ extern SANE_Bool sanei_thread_is_forked (void);
  */
 extern SANE_Bool sanei_thread_is_valid (SANE_Pid pid);
 
+/** Invalidate a SANE_Pid
+ *
+ *  This "function" should be used to invalidate a SANE_Pid in a
+ *  portable manner.
+ *
+ *  @note
+ *  When using pthreads, this only works for those implementations
+ *  that opted to make pthread_t an arithmatic type.  This is *not*
+ *  required by the POSIX threads specification.  The choice to do
+ *  SANE_Pid invalidation by means of a macro rather than a proper
+ *  function circumvents to need to pass a pointer.
+ *  If we decide to implement SANE_Pid with a void* in the future,
+ *  this can be changed into a proper function without the need to
+ *  change existing code.
+ *
+ *  For details on the pthread_t type, see in particular Issue 6 of
+ *  http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_types.h.html
+ */
+#define sanei_thread_invalidate(pid) ((pid) = (SANE_Pid)(-1))
+
+/** Initialize a SANE_Pid
+ *
+ *  This "function" should be used to initialize a SANE_Pid in a
+ *  portable manner.
+ *
+ *  @note
+ *  This is at present just an alias of sanei_thread_invalidate.
+ *  It seemed misleading to use the latter when intent clearly has
+ *  initialization written all over it, hence the alias.
+ */
+#define sanei_thread_initialize sanei_thread_invalidate
+
 /** Spawn a new task.
  *
  * This function should be used to start a new task.