g++ generate dependency file miss user defined headers

Question

The structure of the folder of my source code is defined as follows:

src
 |--c
    |--c.h
    |--c.cpp

'ch' declares a class called 'B' and 'c.cpp' defined the class 'B'.

Suppose we are in folder 'src' now. I run

   g++ -I./ -MM -MT c/c.o -MF c/c.d c/c.cpp

to generate the dependency file 'c/cd' for 'c/c.cpp'. The content of the file 'c/c.d', however, does not contain 'c/ch' even if I have included 'c/ch' in 'c/c.cpp' by

   #include "c/c.h".

However, the result is different if we are in folder 'c' and run the above command. By replacing 'c/ch' with 'ch' in the above process, I can get a correct dependency file (meaning that 'ch' is in the dependency file).

Anyone knows the reason of why the first process misses the header dependency?

Answer 1

According to this GCC webpage , the

"preprocessor looks for header files included by the quote form of the directive #include "file" first relative to the directory of the current file , and then in a preconfigured list of standard system directories."

That means when it sees #include "c/ch" , it checks for files in a hypothetical subdirectory named "c" from the current file's location.

When you replaced that with #include "ch" , the preprocessor then checks the directory of the current file.

Another option is to add -I../ to the command line parameters for g++.

This GCC webpage provides the complete order in which the preprocessor searches directories for include files. The lookup order is as follows:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. 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.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

Note that the directory from which you ran g++ does not appear on the above list. This means, the preprocessor does not check the directory from which you ran g++ on the command line. The reason is so that you can run g++ from any directory and still get the same build results.

Answer 2

This weird output is caused by the variable CPLUS_INCLUDE_PATH. I have set it to some value in the following:

CPLUS_INCLUDE_PATH=some_path:

This tail ':' of the variable CPLUS_INCLUDE_PATH is the cause of my problem. With ':', the compiler considers './' as the system folder, so it automatically removes the headers related to './', such as 'c/c.h', from the dependency list. Accordingly, if I set CPLUS_INCLUDE_PATH to

CPLUS_INCLUDE_PATH=some_path

then the problem is solved.