Compiling 32-bit app on 64-bit, cannot find -lX11

I'm invoking a command like:

g++ ... -o"myapp" -lX11 ...

And getting

/usr/bin/ld: cannot find -lX11

I've installed the following packages to support multiarch:

ia32-libs, libc6-dev-i386 and g++-multilib

But to no avail. There does not appear to be any X11.so in /usr/lib32, actually there are only 52 libraries in that directory. I'm under the impression you should manually install libraries, but I'm at a loss as to what I should check next.

Any help appreciated.

You need the header files, which are provided by -dev packages, and not in the library packages themselves. So, try installing libx11-dev. That should fix this particular build error, though you may get similar errors about other libraries (and then you can install their -dev packages.)

Even though these packages provide header files rather than binaries, and in general header files account for all supported architectures through the use of preprocessor macros, nonetheless -dev packages in Ubuntu tend to be architecture-specific, and this is the case for libx11-dev (as can be seen here by expanding a release and finding the .deb packages listed for libx11-dev in that release). Since your Ubuntu system is 64-bit and you're compiling a 32-bit program which must link against the 32-bit version of the library, you'll probably need to install the 32-bit version of libx11-dev. If you're installing with apt-get or aptitude, you can specify that by indicating libx11-dev:i386 as the package to install (since multarch is supported and being used).

http://packages.ubuntu.com/ is a good resource for finding the name of the -dev package corresponding to a library package. It's not always the library package's name immediately followed by -dev; sometimes version numbers present in the library package name, especially after a -, are absent in the name of the corresponding -dev package.