> In fact, main() is just a convention of libc.

No, it's not just a convention. 'main' is defined as the execution entrypoint in at least the C11 [0], C90 [1] standards. Both have both these forms defined:

    int main(void) {}

    int main(int argc, char* argv[]) {}

You don't have to follow that convention, but then it becomes implementation-defined behaviour.

C without libc can still expect to have a main. One doesn't imply the other. It's just without a main, you also have to manually link to _start as well.

[0] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf