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I am familiar with very simple g++ commands that compile and link programs, however when working with GTK+ I found the tutorial indicate that I should use pkg-config --cflags gtk+-2.0

Now when I type in pkg-config --cflags gtk+-2.0 to the terminal I get a list of libraries and files like this...

-pthread -I/usr/include/gtk-2.0 -I/usr/lib/x86_64-linux-gnu/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/harfbuzz

So my question is, what does the -I mean in front of a directory? For example -I/usr/include/libpng12

Matthew
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4 Answers4

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The -I flag is used to add directories to the list of directories to search for finding files in #include <> statements.

In your case, when a file is included by using #include, /usr/include/libpng12 will be one of the directories in which the pre-processor will search for the file.

R Sahu
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  • Thank you! I will have to experiment by making my own simple library containing 1 or two files... So when I download a package like the one that is helping me with GTK, I am getting header files and then compiled binary of the .cpp files right? – Matthew Jul 30 '14 at 04:51
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Please read the manual. All command line options are present there :-)

-I set the search path for libraries.

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

Klaus
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-I is simply adding to the paths to be searched for header files. Note that these are searched before system paths.

You can find pretty much all the options for gcc/g++ here. In this case, you want the Directory Search options specifically; see here. From that page:

-Idir

Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that).

If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after. If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

Yuushi
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As the answers from @R Sahu @Klaus and @Yuushi explain, the -I dir tells the compiler where to look for the #include header.h file. When the program is compiled, it will be linked. You will probably also need to tell the linker where to find the programs that support the functions in the #included headers. This is done with the -llibrary flag. That is a lower case l not a one (1). The pkg-config command references a set of files and will supply the compiler flags --cflags and link flags --ldflags if you supply the package name.

Put the returned -Includes before the source name(s) and the -l's after in your compile command request.

For other than casual, one-off programs, you should use make. The cflag and ldflag info can be put in variables in the Makefile and referenced throughout your make script.

You can get a complete list of packages configured to pkg-config on your Ubuntu system with locate *.pc

cbcalvin
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