ld cannot find an existing library

In Ubuntu, you can install libtool which resolves the libraries automatically.

$ sudo apt-get install libtool

This resolved a problem with ltdl for me, which had been installed as libltdl.so.7 and wasn't found as simply -lltdl in the make.

The problem is the linker is looking for libmagic.so but you only have libmagic.so.1

A quick hack is to symlink libmagic.so.1 to libmagic.so

It is Debian convention to separate shared libraries into their runtime components (libmagic1: /usr/lib/libmagic.so.1 → libmagic.so.1.0.0) and their development components (libmagic-dev: /usr/lib/libmagic.so → …).

Because the library's soname is libmagic.so.1, that's the string that gets embedded into the executable so that's the file that is loaded when the executable is run.

However, because the library is specified as -lmagic to the linker, it looks for libmagic.so, which is why it is needed for development.

See Diego E. Pettenò: Linkers and names for details on how this all works on Linux.

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In short, you should apt-get install libmagic-dev. This will not only give you libmagic.so but also other files necessary for compiling like /usr/include/magic.h.

As just formulated by grepsedawk, the answer lies in the -l option of g++, calling ld. If you look at the man page of this command, you can either do:

• g++ -l:libmagic.so.1 [...]
• or: g++ -lmagic [...] , if you have a symlink named libmagic.so in your libs path