GCC compiler does not search sub-directories when looking for header files

Question

I can't make gcc compiler recognize complex paths in "includes".

Here's my toy "main.cpp" file (note the sub-directory in the include statement):

#include "sub/testlib.h"

int main()
{
    testlib(6);
    return 0;
}

Here's the path to the file "testlib.h" from the folder "main.cpp" lives in: ../lib/sub/testlib.h.

I'm specifying the include directory while compiling:

gcc -c -iquote../lib main.cpp

And the compiler yells at me:

main.cpp:1:10: fatal error: sub/testlib.h: No such file or directory
    1 | #include "sub/testlib.h"
      |          ^~~~~~~~~~~~~~~
compilation terminated.

Of course I can make it compile by removing sub-directory from the path. But this is just an experiment I make after having failed to compile a real-world project. I can't freely change the files there.

How do I force gcc to treat sub-directories well in the includes? Is there a flag or some option that I'm missing here?

Answer 1

How do I force gcc to treat sub-directories well in the includes? Is there a flag or some option that I'm missing here?

Read the documentation of GCC , in particular the Invoking GCC chapter, the section on preprocessor options (eg -I include-dir or -H or -M , etc...), the documentation of the preprocessor . Try also g++ --help ; the -I include-directory flag can be repeated a lot of times and is probably what you need. Of course, order of arguments to the g++ program matters a lot. You probably want to use g++ not gcc when compiling or linking C++ programs.

Read also some documentation of C++ (maybe even the n3337 "draft" standard). Be aware of translation units , and of the role of the linker .

In practice, you want to drive GCC compilation using some build automation tool, such as GNU make or ninja or many others.

If you use GNU make , read its documentation then try make -p which shows the many built-in rules known to that software. Be aware of the many functions of make .

If you use ninja , read its documentation , you probably want to generate the build.ninja script it is using. You could generate it using a Python script or a Guile one (or your own C++ program, etc...).

Be aware that of course g++ will invoke some GNU binutils utilities (eg the assembler as or the linker ld ).

Practically speaking, invoke g++ as g++ -Wall -Wextra -g to get warnings and debug information (of course with additional -I include-directory flags). Then use the gdb debugger . Once your program has few bugs, add optimization flags such as -O2

See also Clang , its static analyzer , Frama-C , and CompCert and, at end of 2020, Bismon .

Consider in some cases to generate some #include -d C++ code (eg with SWIG or ANTLR or Qt or your own script) or to extend GCC with your plugins .

Be of course aware of the Joel Test .