Index: uspace/lib/c/generic/async.c
===================================================================
--- uspace/lib/c/generic/async.c	(revision 623bab8f043f0e60c5e6d04af58ed83d9338327b)
+++ uspace/lib/c/generic/async.c	(revision eceff5f61ce871257409875152a1d0af843c1455)
@@ -350,5 +350,5 @@
 static async_client_conn_t client_connection = default_client_connection;
 static async_interrupt_handler_t interrupt_received = default_interrupt_received;
-static size_t interrupt_handler_stksz = (size_t) -1;
+static size_t interrupt_handler_stksz = FIBRIL_DFLT_STK_SIZE;
 
 /** Setter for client_connection function pointer.
@@ -375,5 +375,5 @@
 /** Set the stack size for the interrupt handler notification fibrils.
  *
- * @param size Stack size. Use -1 to use the system default stack size.
+ * @param size Stack size in bytes.
  */
 void async_set_interrupt_handler_stack_size(size_t size)
Index: uspace/lib/c/generic/fibril.c
===================================================================
--- uspace/lib/c/generic/fibril.c	(revision 623bab8f043f0e60c5e6d04af58ed83d9338327b)
+++ uspace/lib/c/generic/fibril.c	(revision eceff5f61ce871257409875152a1d0af843c1455)
@@ -256,5 +256,5 @@
  * @param func Implementing function of the new fibril.
  * @param arg Argument to pass to func.
- * @param stksz Stack size, -1 for the system default stack size.
+ * @param stksz Stack size in bytes.
  *
  * @return 0 on failure or TLS of the new fibril.
@@ -269,5 +269,6 @@
 		return 0;
 	
-	size_t stack_size = (stksz == (size_t) -1) ? stack_size_get() : stksz;
+	size_t stack_size = (stksz == FIBRIL_DFLT_STK_SIZE) ?
+	    stack_size_get() : stksz;
 	fibril->stack = as_area_create((void *) -1, stack_size,
 	    AS_AREA_READ | AS_AREA_WRITE | AS_AREA_CACHEABLE | AS_AREA_GUARD |
Index: uspace/lib/c/include/fibril.h
===================================================================
--- uspace/lib/c/include/fibril.h	(revision 623bab8f043f0e60c5e6d04af58ed83d9338327b)
+++ uspace/lib/c/include/fibril.h	(revision eceff5f61ce871257409875152a1d0af843c1455)
@@ -86,6 +86,8 @@
 extern void context_restore(context_t *ctx) __attribute__((noreturn));
 
+#define FIBRIL_DFLT_STK_SIZE	0
+
 #define fibril_create(func, arg) \
-	fibril_create_generic((func), (arg), (size_t) -1)
+	fibril_create_generic((func), (arg), FIBRIL_DFLT_STK_SIZE)
 extern fid_t fibril_create_generic(int (*func)(void *), void *arg, size_t);
 extern void fibril_destroy(fid_t fid);