How to call C functions from ARM Assembly?

Question

I'm writing code targeting ARM Cortex-A on Android devices (using GNU assembler and compiler), and I'm trying to interface between Assembly and C. In particular, I'm interested in calling functions written in C from Assembly. I tried many things, including the .extern directive, declaring C functions with asm and __asm__ and so on, but none of them worked, so I'm looking for a minimal example of doing so. A reference to such example would be just as welcome.