On Thu, 09 Dec 2021 at 15:24:27 +0100, Alexander Traud wrote: > if the header included another header, > and that header included further headers but was not in the root but in > a subfolder, an -I flag *might* be required. For example, the package > 'libopusfile-dev' has its header file in '/usr/include/opus/opusfile.h' > with #include <opus_multistream.h> which is part of the package > 'libopus-dev' and is not within '/usr/include/' but within > '/usr/include/opus/'. Therefore, > $ pkg-config --cflags opusfile > -I/usr/include/opus
This is deliberate, and is a very large part of the purpose of pkg-config. Users of libraries should #include the library's headers in whatever way the library maintainer has documented them to be used. The correct #include pattern is part of the library's API. For example, for GTK, the documentation says to always #include <gtk/gtk.h>, and using <gtk-3.0/gtk/gtk.h> would be wrong. For SDL2, it's correctly <SDL.h>, not <SDL2/SDL.h>. GTK has multiple major versions, which are incompatible and parallel-installable. If your program uses GTK 3, it needs to see GTK 3 headers: using GTK 2 or 4 headers would not work. This is implemented by keeping the GTK 2, 3 and 4 headers in different non-default directories, so that you only get the version of <gtk/gtk.h> you asked for, and not the others. SDL 1.2 and 2 are similar, and there are many other libraries with a similar design. The canonical text on this is https://ometer.com/parallel.html (and you can tell how old it is because it talks about GNOME 2 as though that was a new thing - but it's still relevant). Not doing that would give us the situation we have for things like libjpeg, libcurl and libopenssl, where you can only have one version of the development files installed system-wide at a time: if you have a program that needs OpenSSL 1.1, and another program that needs OpenSSL 1.0, you have to choose which one you can compile (by installing libssl-dev or libssl1.0-dev), and you cannot compile the other one. Conversely, if the old and new versions of library are *mostly* compatible (like the way some SDL programs such as src:quakespasm can compile against either SDL 1.2 or 2 with a few #ifdefs), we don't want to have to change every source file to do something like this: #ifdef USING_MYLIB_3 #include <mylib-3/mylib.h> #else #include <mylib-2/mylib.h> #endif which explains why GTK and SDL have chosen to document <gtk/gtk.h> and <SDL.h> respectively, instead of a versioned name appearing in the source code. For libraries with many smaller headers (popular in C++, e.g. Qt, because parsing C++ is more expensive than parsing C), instead of one big meta-header like the one in GTK, you certainly wouldn't want to do: #ifdef USING_MYLIB_3 #include <mylib-3/MyFrobnicator> #include <mylib-3/MyTemplate> #include <mylib-3/MyWidget> ... #else #include <mylib-2/MyFrobnicator> #include <mylib-2/MyTemplate> #include <mylib-2/MyWidget> ... #endif The other major part of the purpose of pkg-config is that it lets you use dependencies that are *not* installed system-wide, without having to make source code changes: if I set the PKG_CONFIG_PATH correctly, it will find a GTK installation in /opt/gnome-42/{include,lib} and use that, with my source code still using #include <gtk/gtk.h> as usual. smcv
