main() is the user-level
root of the application. It requires the execution environment to
be initialized and input/output functions to be capable of being
called. While in
main() the program might perform
one of the following actions that calls user-customizable functions
in the C library:
Extend the stack or heap.
Call library functions that require a callout to a user-defined function, for example
Call library functions that use
Perform floating-point calculations that require the floating-point unit or floating-point library.
Input or output directly through low-level functions, for example
putc(), or indirectly through high-level input/output functions and input/output support functions, for example,
Raise an error or other signal, for example