There's no general rule in which order header files should be included but I think a good hint is to include local, project specific headers before system headers. This is mainly for two reasons:

1. It prevents that dependencies from local header files to system headers are hidden because a local header is included after a system header by chance. Instead, every local header should explicitly specify the system headers it depends on.

2. It enables the definition definition of local macros which control the behaviour of the system headers which are included later on.

Point two may be considered bad style, but we actually use it for example in src/share/vm/utilities/globalDefinitions.hpp where we define "__STDC_FORMAT_MACROS" before we include "<inttypes.h>" in the compiler specific global definitions file.

"__STDC_FORMAT_MACROS" controls the definition of the printf format macros in "<inttypes.h>" but this can only work if "<inttypes.h>" is really included AFTER the definition of "__STDC_FORMAT_MACROS". And this can only wok if every file includes the local HotSpot headers before any system headers, because otherwise "<inttypes.h>" may be indirectly included by a system header before we had a chance to define "__STDC_FORMAT_MACROS".

This is exactly what happened after the integration of 8050807 which added the system include "<fcntl.h>" to vmError.cpp as follows:

#include <fcntl.h>
#include "precompiled.hpp"
#include "code/codeCache.hpp"

This change broke the build on AIX because "<fcntl.h>" indirectly included "<inttypes.h>" BEFORE we defined "__STDC_FORMAT_MACROS".

To prevent such errors in the future I propose to change all HotSpot source files to always include the system headers AFTER the inclusion of the project specific HotSpot headers.

I’ve attached a small Pythin script to this bug which can be used as follows to detect the files which are currently violating this rule:

find src/ \( -name "*.cpp" -o -name "*.hpp" \) -type f -exec python include.py {} \;

src/os/solaris/dtrace/generateJvmOffsets.cpp: system header #include <proc_service.h> included before local header #include "code/codeBlob.hpp"
src/os/windows/vm/decoder_windows.hpp: system header #include <imagehlp.h> included before local header #include "utilities/decoder.hpp"
src/os_cpu/bsd_zero/vm/os_bsd_zero.cpp: system header # include <pthread_np.h> included before local header #include "assembler_zero.inline.hpp"
src/share/vm/adlc/adlc.hpp: system header #include <iostream> included before local header #include "string.h"
src/share/vm/libadt/port.hpp: system header #include <string.h> included before local header #include "port_tandem.hpp"
src/share/vm/runtime/os.hpp: system header # include <setjmp.h> included before local header # include "jvm_solaris.h"
src/share/vm/trace/traceDataTypes.hpp: system header #include <stddef.h> included before local header #include "utilities/globalDefinitions.hpp"
src/share/vm/utilities/dtrace.hpp: system header #include <sys/types.h> included before local header #include "dtracefiles/hotspot.h"
src/share/vm/utilities/elfFile.cpp: system header #include <new> included before local header #include "memory/allocation.inline.hpp"
src/share/vm/utilities/elfFile.hpp: system header #include <stdio.h> included before local header #include "globalDefinitions.hpp"
src/share/vm/utilities/vmError.cpp: system header #include <fcntl.h> included before local header #include "precompiled.hpp"

The script is written in Python intentionally such that it can be easily added as an automated hook to jcheck to prevent violations of this inclusion rule in the future.

Of course we can also try to not rely on the inclusion order at all. IMHO it actually seems that this is the cleanest solution, but it may require moving some macro definitions from header files right into the command line (e.g. -D__STDC_FORMAT_MACROS for the example above). By the way, that's actually the way how I've fixed the above mentioned compile error on AIX without the need to touch any shared files.

What do you think?