Questions about how libraries work in C - c

Question

I am a newbie student learning C and wish to use the gLib library functions for a project: http://www.linuxfromscratch.org/blfs/view/svn/general/glib2.html
(I am using Ubuntu)
I have a couple questions about how libraries work in C and what happens when you install one or want to use one:
When I install this (run ./configure && make && make install inside the folder), what exactly is it doing? From what I learned there are shared libraries in C and static libraries in C. Is it true that it is installing library and include files to /usr/lib/ or somewhere?
When using gcc with external libraries, you have to specify -L and -I flags to specify where to look for library and header files. When I install glib, will I need to specify these flags?
If I want to package my executable for another machine, what would happen if the other machine doesn't have glib? I think if I had static libraries I would be able to include it in the binary, but how would it work for glib?

Answer 1

I am familiar with developing with GTK+ and GLIB. As i'm aware library files reside in /usr/lib and include files are found in /usr/include. Some libraries might be in places such as /usr/local/lib. I will attempt to answer your questions as best as I could.
When installing a library through the source package yes it installs files to the various folders /usr/share /usr/lib /usr/include and etc. It's highly recommended you use your distribution's package manager to install library packages and development headers. Installing from source is always bound to fail as necessary dependencies might be required.
This is where tools such as autogen and makefiles come handy. You don't necessarily need to concern yourself with specifying all that. tools such as pkg-config handle all that work. Most libraries will install a package configuration file into /usr/lib/pkgconfig & /usr/share/pkgconfig directories. This helps anyone developing an application easily link their code to the libraries.
Using package config to get the config:
$ pkg-config --cflags --libs glib-2.0
-I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -lglib-2.0
Linking using GCC & package config:
$gcc example.c `pkg-config --cflags --libs gtk+2.0 glib-2.0` -o example
The above command would link my program with gtk & glib.
Using Makefile to not ever have to enter those long lines again:
Makefile:
OBJS = main.o callbacks.o
CFLAGS = `pkg-config --cflags --libs gtk+-2.0`
program: $(OBJS)
gcc -o program $(OBJS)
main.o: main.c
gcc -c main.c $(FLAGS)
callbacks.o: callbacks.c callbacks.h
gcc -c callbacks.c $(FLAGS)
.PHONY : clean
clean:
rm *.o
rm program
.PHONY : install
install:
cp program /usr/bin
.PHONY : uninstall
uninstall:
rm /usr/bin/program
The above makefile is for a simple GTK+2.0 application as you can tell by what package config is including in CFLAGS to make the program executable all you have to enter in your source directory would be make. pkg-config will only work if you have installed the development packages for the library you are trying to work with. For ubuntu to install GTK+-3.0 and GLIB development files you would enter:
$ apt-get install libgtk-3-dev
I think this is a good concern for portability. No single static library is going to be cross platform. It would have to be compiled for those platforms manually. I reckon to get rid of all the headache you would use Anjuta IDE developed by the GNOME software foundation. It makes developing GLIB & GTK+ apps a breeze supporting both C & C++. It will create the Makefile, configure and other files to make developing code on cross platforms easy and make deployment easy. I could link you some resources, but my reputation on stack overflow is less then 10. So I will just mention the name of some resources below.
Further Reading
Makefile Tutorial
Anjuta IDE (C/C++)): ://anjuta.org/
GTK+-3.0 Hello World with Compiling and linking using pkg-config:

Answer 2

When I install this (run ./configure && make && make install inside
the folder), what exactly is it doing? From what I learned there are
shared libraries in C and static libraries in C. Is it true that it is
installing library and include files to /usr/lib/ or somewhere?
Well it is running first ./configure and then if that succeeds it runs make and if that succeeds it runs make install. configure is a script that takes care of a lot of compatibility issues between systems. They are usually shell scripts as this is the common denominator across systems so the configure script will work across various systems. One of the thing configure does is create a Makefile. The second command make will use the newly created Makefile to build the library or executable. Since you did not specify a target (like you will in the make install) make will build the default target, which is typically the all target. This is just by convention. Makefiles are basically a list of things to build (targets) along with what they depend on (dependencies) and how to build to target (rules). Finally, make install will actually install the necessary components. For libraries this is the library and necessary header files for executables it is just the program. man pages might also be installed. Where you install the libraries depends on where you specify to install them. Typically configure will take the --prefix argument that lets you control where they are installed. If you do not use --prefix you will most likely install in the default location for your system.
When using gcc with external libraries, you have to specify -L and -I
flags to specify where to look for library and header files. When I
install glib, will I need to specify these flags?
Your question is a little unclear, so let me first make sure I understand. Are you asking if after you install glib will you need to use -L and -I to tell gcc where to look for them? If so it depends on where you install them. Typically when you make and install a library you will install the library and header files in the default location or not. If you did then assuming your gcc is configured correctly then no you will not. If you did not then you will most likely have to use -L and -I
If I want to package my executable for another machine, what would
happen if the other machine doesn't have glib? I think if I had static
libraries I would be able to include it in the binary, but how would
it work for glib?
If it doesn't have glib and you used the shared libraries your application will not work. You will need to either have the same version glib libraries on the other machine or build the libraries statically. How to build them statistically depends on the library. This SO question might help.

Answer 3

Regarding configure, make and make install. configure is a shell script that is used to discover (and configure) your development environment. make and make install are convenient way of building your software. Where make would normally involve compiling and linking, where as make install would normally involve copying executables and libraries to standard path and setting up things (also include files if any usually in /usr/include), so that you don't have to explicitly give path before running the executable. What make does can be done by hand, but it's very cumbersome.
For glibc - yes you have to specify those flags. Normally all libraries will come in two flavors on most of the platforms. The binary form are used for dynamic linking when programs are actually loaded. Also - most distributions will have -dev or -devel versions of those libraries. Those are required for building software that makes use of those libraries (configure above can help find out whether devel libraries are installed). Typically when you see a library installed but not it's devel - you are likely to see configure errors. In short you require devel versions if you want to link with those libraries. This step is not needed if you are building libraries also from source using make and make install.
If you want to package your executable for another machine and you are not sure whether another glib is there or you want to be sure that the glib to be installed should be one specific version that you want, you should statically link while building (compiling/linking) the library. this gcc man page has got several details about link options. I believe there should be a way to statically link glib(or glib2). Though normally that may not be required if you have enough applications that are using it already.