How GCC locates included files
It is a constantly raised issue: you write a C-program using a library. You #include "library_header.h" or #include <library_header.h>.
BUT: Not all is well: either your .h-file could not be found or another equally-named .h-file is included, ...
SO, one of the most asked questions in the interwebs about the C-compiler is: "where the hell does the C-compiler look for included files" (with more or less expletives included".
THE ANSWER is (as usual): it depends ...
As I was not really satisfied with what I could read, mainly because the concrete information was spread out all over the place, I'm trying to compile (sic!) the definitive information on the topic, but for one selected scenario:
- the AVR-GCC compiler used in the Arduino 1.8.12 development environment, running on Microsoft Windows 10. This should eliminate the "it depends.." factor by nailing down all variations to a specific constellation.
Quote-Syntax versus Angle-Bracket-Syntax for #include
- Quote-Syntax looks like:
#include "library_header.h" - Angle-Bracket-Syntax looks like
#include <library_header.h>
The difference between these 2 variants is the location(s) the compiler looks for the files to be included.
Built-in default locations in the compiler
The default locations where the compiler looks for include-files are determined when building the compiler itself.
For the AVR-GCC used in Arduino 1.8.12, the documentation says:
...
The Command
echo | gcc -Wp,-v -x c++ - -fsyntax-only
shows the directories the GCC compiler looks up for includes.
Example:
ignoring nonexistent directory "c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0" ignoring nonexistent directory "c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0/avr" ignoring nonexistent directory "c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0/backward" ignoring nonexistent directory "c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/../../../../avr/sys-include" ignoring nonexistent directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0" ignoring nonexistent directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0/avr" ignoring nonexistent directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/../../../../avr/include/c++/7.3.0/backward" ignoring duplicate directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/include" ignoring duplicate directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/include-fixed" ignoring nonexistent directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/../../../../avr/sys-include" ignoring duplicate directory "c:/program files (x86)/arduino/hardware/tools/avr/lib/gcc/../../lib/gcc/avr/7.3.0/../../../../avr/include" #include "..." search starts here: #include <...> search starts here: c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/include c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/include-fixed c:\program files (x86)\arduino\hardware\tools\avr\bin\../lib/gcc/avr/7.3.0/../../../../avr/include End of search list.