[vlc-devel] [PATCH 4/4] seek config.h include via angled bracket search

Steve Lhomme robux4 at ycbcr.xyz
Tue Oct 6 08:33:23 CEST 2020

On 2020-10-04 20:44, Lyndon Brown wrote:
 > I only just received this email of yours today for some reason, even
 > though it's marked as sent on Friday.
 > It would help clarify things for me if you could possibly please point
 > out the bits of the build system that would allow such a thing to
 > happen.

I just gave it a try. I put a dummy config.h in my 
contrib/x86_64-w64-mingw32/include directory with some #error in it.
"" or <> the result is the same, it's not used. So at least as far as 
clang is concerned it won't be used.

It may also be because we include a lot of folders. Here's an example 
for a module:

This is called from <build>/modules so -I.. is the path where the local 
config.h is. It comes before the contribs one so it's safe to say it 
will be picked before contrib ones. The rest in our source tree is our 

 > As I said before, my understanding of the difference between quoting
 > and angle brackets is that with quoting, the directory of the file
 > doing the include is checked first, before falling back to the angle
 > bracket path search.

It seems more subtle than that and is left to compilers to decide. For 
example in gcc you have this: 

"Directories specified with -iquote apply only to the quote form of the 
directive, #include "file". Directories specified with -I, -isystem, or 
-idirafter apply to lookup for both the #include "file" and #include 
<file> directives."

Which is what should be used for local includes to make sure you don't 
include similar files from other places. That's probably overkill.

These are the rules it applies:

- For the quote form of the include directive, the directory of the 
current file is searched first.
- For the quote form of the include directive, the directories specified 
by -iquote options are searched in left-to-right order, as they appear 
on the command line.
- Directories specified with -I options are scanned in left-to-right order.
- Directories specified with -isystem options are scanned in 
left-to-right order.
- Standard system directories are scanned.
- Directories specified with -idirafter options are scanned in 
left-to-right order.

 > On that basis, the existing common use of quotes always fails to find
 > the config.h file, since it exists within the build directory, not
 > within the <source>/{src|modules|bin|lib|compat} etc subdirectories
 > that those files doing the includes are located within, and thus always
 > falls back to the same search as if we were using angle brackets.
 > Switching to angle brackets thus only has the difference of skipping
 > the pointless initial same-directory check done when using quotes.
 > Thus the only way to unintentionally pick up a config.h from a contrib
 > rather than the vlc one in our build directory would be for the contrib
 > one to appear in a "-I" path, and for that path to be given and thus
 > checked before the build directory, and if this were the case, we'd
 > already be affected by it.
 > Am I wrong? I may well be, but if so could someone please explain.

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