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

As Paul mentioned, use realpath(). Please note though, that since many file systems in Linux support hard links, any given directory can have a number of different absolute paths.

Running on RedHat 5.3, realpath doesn't exist but readlink is installed. You can use it on relative paths and symlinks, plus it will resolve symlinks recursively for you. It's thus a better option that realpath in my opinion

readlink -f .