We use #include for a few header files. Unlike in user space 'C' application development, these are kernel headers (as mentioned in the Technical requirements section). Recall from Chapter 3, Building the 5.x Linux Kernel from Source – Part 2, that kernel modules were installed under a specific root-writeable branch. Let's check it out again (here, we're running on our guest x86_64 Ubuntu VM with the 5.0.0-36-generic distro kernel):
$ ls -l /lib/modules/$(uname -r)/
total 5552
lrwxrwxrwx 1 root root 39 Nov 12 16:16 build -> /usr/src/linux-headers-5.0.0-36-generic/
drwxr-xr-x 2 root root 4096 Nov 28 08:49 initrd/
[...]
Notice the symbolic or soft link named build. It points to the location of the kernel headers on the system. In the preceding code, it's under /usr/src/linux-headers-5.0.0-36-generic/! As you shall see, we will supply this information to the Makefile used...
