An odd little one: I was compiling some older code on gcc 4.6 / Ubuntu 11.10 and it failed to make, with the traditional error:

undefined reference to `sqrt'

My Makefile was trying to do something like this:

gcc -c -o eg.o eg.c
gcc -lm -o eg eg.o

… yep, there’s -lm so what’s the problem?

It turns out that gcc 4.6 has become fussier about the order of its options now, and wants the libs on the end when linking, like this:

gcc -c -o eg.o eg.c
gcc -o eg eg.o -lm

This will catch you out if you set up your Makefile like this:

LDFLAGS=-lm

Because the libs always appear up front. Instead, put the libs on the end where they belong by doing this in the Makefile:

LDLIBS=-lm

If you’re still having trouble with the order of libraries, it might help to add this as well:

LDFLAGS=-Xlinker --no-as-needed

Which will pass the --no-as-needed flag to ld, to relax the constraints on the order which things are evaluated by the linker, or something.