clang tools cannot open file

Hi Everyone,

I am using clang by libtooling, and meet something I can't figure out.As for the given example clang-check, when running on all files in a source tree, clang-check frequently produces a fatal error - cannot open file... But when I run those files one by one, clang-check perfectly works. By the way, there is a .json file in the top level of source tree.
I also find that changing files order can influence the behavior of clang-check:
*************************************************************************$ clang-check /home/jiazhouyang/httpd-2.4.10/server/mpm/event/event.c /home/jiazhouyang/httpd-2.4.10/os/unix/unixd.c
warning: argument unused during compilation: '-fsyntax-only'

*************************************************************************$ clang-check /home/jiazhouyang/httpd-2.4.10/os/unix/unixd.c /home/jiazhouyang/httpd-2.4.10/server/mpm/event/event.cIn file included from event.c:79:
/home/jiazhouyang/httpd-2.4.10/include/ap_config.h:136:10: fatal error: cannot open file './os.h': No such file or directory
#include "os.h"
         ^
1 error generated.
Error while processing /home/jiazhouyang/httpd-2.4.10/server/mpm/event/event.c.
warning: argument unused during compilation: '-fsyntax-only'
In file included from event.c:79:
/home/jiazhouyang/httpd-2.4.10/include/ap_config.h:136:10: fatal error: cannot open file './os.h': No such file or directory
#include "os.h"
         ^
1 error generated.
Error while processing /home/jiazhouyang/httpd-2.4.10/server/mpm/event/event.c.*************************************************************************Any ideas what I am doing wrong?-regards,
Zhouyang