libnm is fairly simple to use from C. It's based on glib and GObject. If your project uses these already you'll find integration libnm with your project rather convenient. In fact, the nmcli tool shipped with NetworkManager is based on libnm.

libnm should be also the way to go if your project does something non-trivial with NetworkManager, such as manipulating the connection profiles. That is, if you're writing a specialized networking control tool or a desktop environment, libnm is probably the right choice. The popular desktop environments in fact all use libnm directly or with nm-applet and nm-connection-editor that are all based on libnm.

An alternative to use of libnm is the use of the D-Bus API directly. This gives you larger flexibility and reduces the overhead of linking with the libnm library. This makes sense if your task is simple and you have a good D-Bus library at your disposal. Activating a particular connection profile from a Python script is a good example of a task that is perfectly simple without using libnm.

You can use the libnm's C API directly. To do so, all libnm programs need to include NetworkManager.h that provides necessary definitions. The rest of the API is documented in the reference manual.

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#include <glib.h>
#include <NetworkManager.h>

int
main (int argc, char *argv[])
{
    NMClient *client;

    client = nm_client_new (NULL, NULL);
    if (client)
        g_print ("NetworkManager version: %s\n", nm_client_get_version (client));
}

Use pkg-config for libnm to discover the necessary compiler flags.

$ cc $(pkg-config --libs --cflags libnm) -o hello-nm hello-nm.c
  $ ./hello-nm
  NetworkManager version: 1.12.6

  $ 

Utilize the PKG_CHECK_MODULES macro to integrate with an autoconf-based build system. It's also recommended to use NM_VERSION_MIN_REQUIRED and NM_VERSION_MAX_ALLOWED macros to tell libnm headers which API version does your application need to work with. If you use them, the compiler will warn you when you use functionality that is not available in the versions you specified.

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PKG_CHECK_MODULES(LIBNM, libnm >= 1.8)
LIBNM_CFLAGS="$LIBNM_CFLAGS -DNM_VERSION_MIN_REQUIRED=NM_VERSION_1_8"
LIBNM_CFLAGS="$LIBNM_CFLAGS -DNM_VERSION_MAX_ALLOWED=NM_VERSION_1_8"

You can use libnm from other languages than C with the use of GObject introspection. This includes Perl, Python, Javascript, Lua, Ruby and more. The example below shows what the typical libnm use in Python would look like.

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import gi
gi.require_version('NM', '1.0')
from gi.repository import NM

client = NM.Client.new(None)
print ("NetworkManager version " + client.get_version())

There's NM-1.0 Python API Reference maintained a third party that is generated from the introspection metadata.

In general, the C API documentation applies to the use GObject introspection from other languages, with the calling convention respecting the language's customs. Consult the source tree for some examples.