main() in C, C++, Java, C#

Question

Is main() (or Main()) in C, C++, Java or C#, a user-defined function or a built-in function?

Answer 1

Quote from the C Standard (emphasis is mine):

5.1.2.1 Freestanding environment

1. In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

main(), in a freestanding environment, is very much a user-defined function.