How to work with a C library in Linux

I’ll talk about how to get started and play with one of the many C libraries out there on a Linux machine.

Let’s say you want to write and run C code on your Linux machine. I’ll talk about how to install it, get GCC to recognize it, and then use a makefile to simplify building. I’m using Ubuntu 14.04, but this should work in any distro.

The library I’ve used here is libnl – a library for userspace code to use generic netlink sockets, a mechanism which is often used to interact with KLM’s (kernel loadable modules).

First, we need the libraries on our machine.

sudo apt-get install libnl-3-dev libnl-genl-3-dev

libnl-3 is the core library for netlink sockets, and libnl-genl-3 is the library for generic netlink sockets, which is a special type of netlink socket.

If you want to install from the source, use the standard configure and make commands, as listed here.

Now that we have the libnl library with the headers with us, let’s start with a simple piece of C code-

/* userspace.c */
#include <stdio.h>
#include <netlink/netlink.h>
static struct nl_sock *sk;

int main() {
printf("Hello, world.\n");
/* Allocate a new socket */
sk = nl_socket_alloc();
/* Destroy the socket */
return 0;

This uses the netlink library for the API’s to allocate and destroy the netlink socket.

Build it using gcc.

$ gcc userspace.c -o userspace -llibnl3 -g -Wall -O3
userspace.c:2:29: fatal error: netlink/netlink.h: No such file or directory
 #include <netlink/netlink.h>

This failed because the gcc compiler was unable to find the include file netlink/netlink.h.

We need to add the header file path to the include path for GCC to pick it up.

$ gcc userspace.c -o userspace -I/usr/include/libnl3 -g -Wall -O3

However, just running this will give still give errors.

 /tmp/ccTtP6p3.o: In function `main':
 /home/saksbhat/userspace.c:10: undefined reference to `nl_socket_alloc'
 /home/saksbhat/userspace.c:11: undefined reference to `nl_socket_free'

GCC knows where to get the header files from, where nl_socket_alloc() and nl_socket_free() are defined, but what about the actual libraries? We also need to add the shared object to the linker.

In Ubuntu 14.04, a quick search (sudo find . -name “*genl*”) showed me the location of the libraries. We need to tell GCC to add this directory path using -L, and link the specific object file using -l.

$ gcc userspace.c -o userspace -I/usr/include/libnl3 -L/lib/x86_64-linux-
gnu/ -lnl-3 -g -Wall -O3

Notice that I used the flag -lnl. This is because the prefix lib and the suffix .a (for static) and .so (for shared) is automatically understood and expected by the -l flag.

GCC first compiled the userspace.c source file into userspace.o object file (which is an assembly code translation), and then linked it, by resolving the dependencies on libnl library and generating an executable.

Instead of giving these command line arguments each time, we can use a makefile that does the same thing. Create a file named makefile in the same folder and write this-

P = userspace
CFLAGS = -g -Wall -O3 -I/usr/include/libnl3
LDLIBS = -L/lib/x86_64-linux-gnu/ -lnl-3
CC = c99
$(P): $(OBJECTS)

When you do a make, it will run these 6 instructions to generate the executable.

The first 5 lines define variables that the makefile uses to build. Notice that unlike bash scripts, makefile uses $() to dereference variables.

$(P) points to the userspace.c file. $(OBJECTS) talks about object files userspace.c is dependent on, when linking. These object files are linked first, and then userspace.o is processed.

$(CFLAGS) lists the compiler flags. $(LDLIBS) lists the linker arguments. $(CC) represents the C compiler.

The last line is of the rule that invokes it all. target : dependencies. Our target is the executable userspace and, in this case we don’t have any additional object files apart from userspace.o. We don’t need to list that though, “make” understands that by default.

Let’s add a clean procedure to the makefile.

    rm $(P)

As earlier, the first line is the target, and the dependencies is blank. The second line is the “recipe”, which in this case is removing the executable. Note that there must be a <TAB> before the recipe.

Run make clean to delete the executable. For more details on makefiles, click here.


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