Question

I use gcc (running as g++) and GNU make.
I use gcc to precompile a header file precompiled.h, creating precompiled.h.gch; the following line in a Makefile does it:

# MYCCFLAGS is a list of command-line parameters, e.g. -g -O2 -DNDEBUG
precompiled.h.gch: precompiled.h
    g++ $(MYCCFLAGS) -c $< -o $@

All was well until i had to run g++ with different command-line parameters.
In this case, even though precompiled.h.gch exists, it cannot be used, and the compilation will be much slower.
In the gcc documentation i have read that to handle this situation,
i have to make a directory called precompiled.h.gch and put
the precompiled header files there,
one file for each set of g++ command-line parameters.

So now i wonder how i should change my Makefile to tell g++ to create
the gch-files this way.
Maybe i can run g++ just to test whether it can use any existing file
in the precompiled.h.gch directory,
and if not, generate a new precompiled header with a unique file name.

Does gcc have support for doing such a test?

Maybe i can implement what i want in another way?

Answer 1

It seems weird to answer my own question; anyway, here goes.

To detect whether a suitable precompiled header file exists, i add a deliberate error to my header file:

// precompiled.h
#include <iostream>
#include <vector>
...
#error Precompiled header file not found

This works because if gcc finds a precompiled header, it will not read the .h file, and will not encounter the error.

To “compile” such a file, i remove the error first, placing the result in a temporary file:

grep -v '#error' precompiled.h > precompiled.h.h
g++ -c -x c++ $(MYCCFLAGS) precompiled.h.h -o MORE_HACKERY

Here MORE_HACKERY is not just a plain file name, but contains some code to make a file with unique name (mktemp). It was omitted for clarity.