libelf was not found in the pkg-config search path

Building from source under CentOS the user may stumble on some compilation errors and most of them are for missing -devel packages. Here is such example with not so easy to find the name of a missing library:

[/tmp/netdata-libbpf-El77Ld/libbpf-0.0.9_netdata-1/src]# env CFLAGS=-fPIC CXXFLAGS= LDFLAGS= BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=.. make install 
Package libelf was not found in the pkg-config search path.
Perhaps you should add the directory containing `libelf.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libelf' found
mkdir -p build/staticobjs
cc -I. -I../include -I../include/uapi -DCOMPAT_NEED_REALLOCARRAY -fPIC -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64   -c bpf.c -o build/staticobjs/bpf.o
cc -I. -I../include -I../include/uapi -DCOMPAT_NEED_REALLOCARRAY -fPIC -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64   -c btf.c -o build/staticobjs/btf.o
btf.c:17:18: fatal error: gelf.h: No such file or directory
 #include <gelf.h>
compilation terminated.
make: *** [build/staticobjs/btf.o] Error 1

The missing development library file is with the name: elfutils-libelf-devel. Installing the package with yum or dnf will resolve the above error:

yum install -y elfutils-libelf-devel

Or for CentOS 8 and newer Fedora versions:

dnf install -y elfutils-libelf-devel

removing the default kernel in CentOS 8 – remove elrepo kernel

Removing the default kernel aka the loaded kernel in CentOS 8 maybe challenging because the package is protected and cannot be removed by the yum or dnf.
Here is the case: an elrepo kernel-ml loaded and the dnf prints it cannot remove the package, because it is protected:

[root@srv ~]# dnf remove kernel-ml kernel-ml-core kernel-ml-modules
 Problem: The operation would result in removing the following protected packages: kernel-ml-core
(try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)
[root@srv ~]# uname -a
Linux srv.localhost 5.10.4-1.el8.elrepo.x86_64 #1 SMP Tue Dec 29 11:04:23 EST 2020 x86_64 x86_64 x86_64 GNU/Linux
[root@srv ~]# grubby --default-kernel

The system is booted up with the kernel we are trying to remove, which is impossible.

The solution is to set a new default kernel and load it. Then dnf will be able to remove the first kernel.

For CentOS 7, just use the yum instead of dnf command.
Using grubby is really easy and straightforward:

STEP 1) List all installed and available to boot kernels

[root@srv ~]# grubby --info=ALL |grep ^kernel

STEP 2) Select the kernel to load the next time

[root@srv ~]# grubby --set-default "/boot/vmlinuz-4.18.0-259.el8.x86_64"
The default is /boot/loader/entries/45e12f0814fd4947b99cbdcb88950361-4.18.0-259.el8.x86_64.conf with index 1 and kernel /boot/vmlinuz-4.18.0-259.el8.x86_64

Keep on reading!

CentOS 7 – Dependency Resolution – Error – Requires: dkms – missing epel repository

Quick not note for those not familiar with the CentOS 7 peculiarity and especially the repository peculiarity.
Receiving the follwoing error:

--> Finished Dependency Resolution
Error: Package: 3:kmod-nvidia-latest-dkms-418.87.00-2.el7.x86_64 (cuda)
           Requires: dkms
 You could try using --skip-broken to work around the problem
 You could try running: rpm -Va --nofiles --nodigest

It means you need a package (or meta-package, which might pull multiple packages and dependencies offering a big framework, for example), which could not be found in the existing repositories. In this very case, we need the DKMS (Dynamic Kernel Module Support) –

The DKMS is offered in epel repository and it could not be found in the CentOS 7 official repositories. Just add the epel repository.

[root@localhost ~]# yum install -y epel-release
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base:
 * extras:
 * updates:
Resolving Dependencies
--> Running transaction check
---> Package epel-release.noarch 0:7-11 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

 Package                                       Arch                                    Version                                Repository                               Size
 epel-release                                  noarch                                  7-11                                   extras                                   15 k

Transaction Summary
Install  1 Package

Total download size: 15 k
Installed size: 24 k
Downloading packages:
epel-release-7-11.noarch.rpm                                                                                                                         |  15 kB  00:00:00     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : epel-release-7-11.noarch                                                                                                                                 1/1 
  Verifying  : epel-release-7-11.noarch                                                                                                                                 1/1 

  epel-release.noarch 0:7-11                                                                                                                                                


And rerun your first install yum line. Now you won’t receive the DKMS error.