Well to be pedantic, the CRT invokes a callback called 'main' with preprocessed (command line) parameters, in the 'main callback' you're expected to do something with the parameters that have been passed to you (for instance read data from stdin, process it and write back to stdout), and then return from the 'main callback' with a success or failure code.

Ok, I'm throwing together the CRT and standard lib here, but in the end both are parts of the "UNIX command line tools framework" which is supposed to provide a programming environment to easily extend the UNIX shell with your own commands.