Ubuntu 26.04 upgrade breaks all the man pages

I upgraded from Ubuntu 24.04 to Ubuntu 26.04 using sudo do-release-upgrade -d and now man (manual) pages aren't working. I tried man atoi and I get No manual entry for atoi although compiling that with gcc worked fine, but the absence of man pages is bothering me.

Based on the output of printenv | grep MAN it appears that you have set MANPATH=/opt/testwell/ctc_v10.3.0/man in your environment. Setting MANPATH in this way causes man to ignore the default manpath.config and only search the given path:

$ MANPATH=/opt/testwell/ctc_v10.3.0/man manpath
manpath: warning: $MANPATH set, ignoring /etc/manpath.config
/opt/testwell/ctc_v10.3.0/man

This is likely related to your installation of the third-party Testwell CTC++ tool rather than anything to do with the Ubuntu release upgrade - although the Linux installation instructions only mention the environment variables PATH and CTCHOME. If you want to append to the default manual search path, you should change the assignment to MANPATH=$MANPATH:/opt/testwell/ctc_v10.3.0/man :

$ MANPATH=$MANPATH:/opt/testwell/ctc_v10.3.0/man manpath
manpath: warning: $MANPATH set, prepending /etc/manpath.config
/usr/local/man:/usr/local/share/man:/usr/share/man:/opt/testwell/ctc_v10.3.0/man

Note that on an out-of-the-box Ubuntu installation, MANPATH is unset, so this is usually equivalent to setting MANPATH=:/opt/testwell/ctc_v10.3.0/man . See also How to set MANPATH without overriding defaults?