Please see my note book http://mbed.org/users/kenjiArai/notebook/freertos-on-mbed-board-with-mbed-cloud-ide--never-/

This is too old.
Below is another FreeRTOS on mbed.
http://developer.mbed.org/users/rgrover1/code/FreeRTOS/
I don't know it works well or not.
I have not evaluated it.

Revision:
0:d4960fcea8ff
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/syscalls.c	Sat Jan 01 11:17:45 2011 +0000
@@ -0,0 +1,91 @@
+/* Don't need anything here. */
+
+#include <stdlib.h>
+#if 0
+#include <sys/stat.h>
+#else
+
+#endif
+
+//---------------------------------
+// from types.h
+typedef    char *    caddr_t;
+//----------------------------------
+
+int _read_r (struct _reent *r, int file, char * ptr, int len)
+{
+    ( void ) r;
+    ( void ) file;
+    ( void ) ptr;
+    ( void ) len;
+    return -1;
+}
+
+/***************************************************************************/
+
+int _lseek_r (struct _reent *r, int file, int ptr, int dir)
+{
+    ( void ) r;
+    ( void ) file;
+    ( void ) ptr;
+    ( void ) dir;
+    
+    return 0;
+}
+
+/***************************************************************************/
+
+int _write_r (struct _reent *r, int file, char * ptr, int len)
+{  
+    ( void ) r;
+    ( void ) file;
+    ( void ) ptr;
+    ( void ) len;
+    
+    return 0;
+}
+
+/***************************************************************************/
+
+int _close_r (struct _reent *r, int file)
+{
+    ( void ) r;
+    ( void ) file;
+
+    return 0;
+}
+
+/***************************************************************************/
+
+caddr_t _sbrk_r (struct _reent *r, int incr)
+{
+    ( void ) r;
+    ( void ) incr;
+    
+    return 0;
+}
+
+/***************************************************************************/
+
+int _fstat_r (struct _reent *r, int file, struct stat * st)
+{
+    ( void ) r;
+    ( void ) file;
+    ( void ) st;
+    
+    return 0;
+}
+
+/***************************************************************************/
+
+int _isatty_r(struct _reent *r, int fd)
+{
+    ( void ) r;
+    ( void ) fd;
+    
+    return 0;
+}
+
+
+
+