Note that the sed used in this installation only works if the line break is placed exactly where it is in the book. Don't try and put it all on one line otherwise it won't work. Install File by running the following commands: cp readelf.h readelf.h.backup && sed '/#define __/a \    #include <stdint.h>' readelf.h.backup > readelf.h && ./configure --prefix=/usr --datadir=/usr/share/misc && make && make install File uses magic numbers to determine a file type. These magic numbers come with File in a plain text file. File internally compiles this database each time it is run. This is not the normal type of operation for File since compiling a plain text file each time is not the fastest way to do it. File offers an option "-C" to compile this magic number file. The reason this isn't done automatically is that some people like to work on the magic numbers. On the other hand many people didn't get it that they should compile the magic numbers, so the author of File added a warning when the plain text magic file is used. As we usually won't work on the plain text magic file, we compile this file, because it's faster, fixes that annoying warning and is how it was meant to be: file -C