In C, main need not be a function?

Question

This code compiles, but no surprises, it fails while linking (no main found):

Listing 1:

void main();

Link error: \mingw\lib\libmingw32.a(main.o):main.c:(.text+0x106) undefined reference to _WinMain@16'

But, the code below compiles and links fine, with a warning:

Listing 2:

void (*main)();

warning: 'main' is usually a function

Questions:

1. In listing 1, linker should have complained for missing "main". Why is it looking for _WinMain@16?

2. The executable generated from listing 2 simply crashes. What is the reason?

Thanks for your time.

Answer 1

True, main doesn't need to be a function. This has been exploited in some obfuscated programs that contain binary program code in an array called main.

The return type of main() must be int (not void). If the linker is looking for WinMain, it thinks that you have a GUI application.

Answer 2

In most C compilation systems, there is no type information associated with symbols that are linked. You could declare main as e.g.:

char main[10];

and the linker would be perfectly happy. As you noted, the program would probably crash, uless you cleverly initialized the contents of the array.

Your first example doesn't define main, it just declares it, hence the linker error.

The second example defines main, but incorrectly.