How to understand "main function's prototype cannot be supplied by the program"?

I read main function, and came across following words:

The main function has several special properties:

1. A prototype for this function cannot be supplied by the program.

Then I wrote a simple program:

# cat foo.c
int main(void);

int main(void)
{
    return 0;
}

And compiled it:

# gcc -Wall -Wextra -Wpedantic -Werror foo.c
#

All seems OK! So I am little confused about how to understand "A prototype for this function cannot be supplied by the program". Anyone can give some insights?

>Solution :

The C standard (5.1.2.2.1) just says that the compiler (for hosted systems like PC etc) will not provide a prototype for the main function. So cppreference.com isn’t really correct, the C standard doesn’t prohibit the application programmer from writing a prototype, although doing so is probably meaningless practice in hosted systems.

In freestanding systems (embedded systems etc), it might be meaningful to declare a prototype for main in case it needs to be called from a reset ISR or from the "C runtime" (CRT).

What’s important to realize no matter system is that the compiler specifies which forms of main that are valid. Never the programmer.