C++ how to create a 'global' library

Question

I'm working on a project that has the line: #include <fftw3.h>

Now I can simply download and make fftw. But how do I make it a 'global' library?

I have tried adding it to $PKG_CONFIG_PATH but this doesn't seem to work. I'm constantly ending up with this error: main.c:30:10: fatal error: 'fftw3.h' file not found

So my question is: where do I need to make the fftw library (or any library for that matter) to make it globally accessible?

note: I might use the term global incorrectly, in that case, please let me know what the correct term would be.

Update: The command is:

gcc -Wall -march=native -O3 -ffast-math `pkg-config --cflags fftw3` `pkg-config --cflags sndfile` -c -o main.o main.c

main.c contains the line

#include <fftw3.h>

Answer 1

I don't think there is a precise answer the way you put it - there is no "global" library, because "fftw3.h"is a header file.

There are 3 things here: 1) Where you find header files 2) Where you find static (or stub/import) libraries 3) Where you find dynamic libraries (at runtime)

For 1), the answer is "depends on the configuration of your build tool". So is 2). For 3, it is usually "in the path, in the same folder as the executable, in the current directory, or in a platform specific directory". It would be slightly different for Windows, Linux, or another platform.

In short, you will need to configure your build tool for it.

EDIT: Now that we know what your tool is, I would suggest reading this short page explaining how GCC looks for headers (your .h file). You can either put your header file in one of the search paths (sloppy!, not recommended), or put your "extra" headers in a directory of your choosing and add it to the compilation command with the "-I" parameter.

https://gcc.gnu.org/onlinedocs/cpp/Search-Path.html

Answer 2

beign able to include files using <file> is generally bad practise, it increase the risk of files with same name became ambiguos (especially on big projects).

You can achieve that by specifing additional global include directories using the -I compiler option. However there is no best way to achieve that (it depends on how project is organized and how it is big).

Include by absolute path:

#include <file>
#include <somepath/file2>

Pros:

• Source code structure is more flexibile (you can move your files without warrying about breaking compilation)

Cons:

• More chance of name clash between files

Include by path relative to source file:

#include "../file"
#include "../somepath2/file2"

Pros:

• No chance for name clash, there exist only 1 file given any relative path

Cons:

• You cannot move files around otherwise relative paths needs to be changed (however refactoring tools of your IDE may help with that)

Definitely I prefer using absolute path for standard library stuff and eventually FEW other dependencies, while otherwise I use only relative paths. You should note that when the compiler search for global include files, it will search in all provided -I directories, so if you abuse that feature it is likely you will increase compile time.

Of course even when using global includes, I prefer adding just 1 extra directory and organize that so that the compiler doesn't have to guess paths

Include
  +FolderNameSpace1
  +FolderNameSpace2

So I have just to add the namespace when including

#include <FolderNameSpace1/File1>

Some open source projects, had this problem in mind, however since not all people is willing to spend too much time organizing folder structure, their developers used the more simple strategy to prepend a suffix on Header file names (to avoid name clash).. In example in Ogre you have to do:

#include <OgreFileName>
//or (depending on how your project is organized)
#include "../Deps/OgreFileName"

---

So after some premise, to answer your question,

you have to add as "global include directory" the directory that has all include files you want to use, so Given you are using GCC and your project is organized like that:

Project
  +FFWT
    +includes
  +yourfolder
    +yoour files

you have just to do:

gcc -Wall -march=native -O3 -ffast-math -I../FFWT/includes c -o main.o main.c

(I am assuming you are calling GCC from inside "yourfolder")

How to add additional global include directories:

• GCC/Clang GUIDE
• MSVC GUIDE