Checks with read-only library directory (the directory where packages are installed). Packages should never attempt to write into the directory where they have been installed. This restrictions is primarily for security reasons and for transparency when the library is to be shared by multiple users. Packages sometimes violate this restriction and attempt to save test results or download data into their installation directory. This is undetected on personal installations of R where the library is usually writable, but leads to crashes on installations where the library is mounted read-only. These checks are just regular package checks run with the library mounted read-only. There are no false alarms (check outputs are searched for error messages related to writing to a read-only file systems), but in principle a package may be reported while its dependency is at fault. Also, some true errors may be missed.