All C programs begin execution with the function entitled 'main', and end execution when this function terminates. The arguments and return value of this function are assigned and evaluated in an operating system-dependent manner.
Unix systems allow command-line arguments to be provided when a program is executed. These arguments are usually assigned to argv and argc parameters which the main() function may optionally access.
If the main() function provides a return statement, the value returned may be evaluated by the operating system to provide feedback to the user.
Think C for the Macintosh provides the ccommand() function to simulate the use of command-line arguments. Generally, the main() function in Macintosh programs uses neither arguments nor a return value.
