How to get the absolute path for a given relative path programmatically in Linux?

Question

How to get the absolute path for a given relative path programmatically in Linux?

Incase of Windows we have the _fullpath() API. In other words, I mean

Answer 1

The is also another useful way, like "readlink -m $filename"

First of all, it works without requirement for target file to exist. Secondly, it will handle symlinks and get really real path.