man memcmp is available on my Arch install.
I did not just run "apropos", if I run "man" I get the memcmp manual page.
[rocketmouse@archlinux ~]$ apropos memcmp
TIFFmemory (3tiff) - memory management-related functions for use with TIFF files
gnutls_memcmp (3) - API function
memcmp (3) - compare memory areas
memcmp (3p) - compare bytes in memory
wmemcmp (3) - compare two arrays of wide-characters
wmemcmp (3p) - compare wide characters in memory
[rocketmouse@archlinux ~]$ sudo pacman -Syu
[snip]
:: Starting full system upgrade...
warning: fluidsynth: ignoring package upgrade (1.1.11-3 => 2.0.4-1)
warning: gvfs: local (2013.08.18-1) is newer than extra (1.38.1+9+gd4dab113-2)
warning: ignoring package replacement (hunspell-en-7.1-3 => hunspell-en_AU-2018.04.16-5)
warning: ignoring package replacement (hunspell-en-7.1-3 => hunspell-en_CA-2018.04.16-5)
warning: ignoring package replacement (hunspell-en-7.1-3 => hunspell-en_GB-2018.04.16-5)
warning: ignoring package replacement (hunspell-en-7.1-3 => hunspell-en_US-2018.04.16-5)
warning: hunspell-en_US: local (2018:06.29-1) is newer than extra (2018.04.16-5)
warning: linux: ignoring package upgrade (4.20.11.arch2-1 => 4.20.12.arch1-1)
warning: linux-headers: ignoring package upgrade (4.20.11.arch2-1 => 4.20.12.arch1-1)
warning: mate-calc: ignoring package upgrade (1.8.0-2 => 1.20.3-1)
warning: pulseaudio: local (2013.08.18-1) is newer than extra (12.2-2)
warning: pulseaudio-bluetooth: local (2017.12.19-1) is newer than extra (12.2-2)
warning: x42-plugins: ignoring package upgrade (20161230-1 => 20190206-1)
resolving dependencies...
looking for conflicting packages...
Packages (31) brltty-6.0-1 dpf-plugins-1.1-3 getmail-5.13-1 git-2.21.0-1 glm-0.9.9.3-1 lib32-mesa-18.3.4-1 lib32-vulkan-icd-loader-1.1.101-1 libnfs-4.0.0-3 libnm-1.14.6-1
libnm-glib-1.14.6-1 libssh-0.8.7-1 libxkbcommon-0.8.4-1 libxkbcommon-x11-0.8.4-1 linux-docs-4.20.12.arch1-1 mariadb-10.3.13-1 mariadb-clients-10.3.13-1
mariadb-libs-10.3.13-1 mesa-18.3.4-1 networkmanager-1.14.6-1 projectm-3.1.0-4 python-anytree-2.6.0-2 python-atspi-2.30.0-2 python-markupsafe-1.1.1-1
python2-markupsafe-1.1.1-1 qemu-3.1.0-2 qt5-quickcontrols-5.12.1-2 tar-1.32-1 tracker-2.2.0-3 vlc-3.0.6-4 vulkan-icd-loader-1.1.101-1
xf86-video-intel-1:2.99.917+860+g3a2dec17-1
[snip]
[rocketmouse@archlinux ~]$ apropos memcmp
TIFFmemory (3tiff) - memory management-related functions for use with TIFF files
gnutls_memcmp (3) - API function
memcmp (3) - compare memory areas
memcmp (3p) - compare bytes in memory
wmemcmp (3) - compare two arrays of wide-characters
wmemcmp (3p) - compare wide characters in memory
[rocketmouse@archlinux ~]$
