How can a program with a global variable called main instead of a main function work?

Question

Consider following program:

#include <iostream> int main = ( std::cout << "C++ is excellent!\n", 195 );  

Using g++ 4.8.1 (mingw64) on Windows 7 OS, the program compiles and runs fine, printing:

C++ is excellent!

to the console. main appears to be a global variable rather than a function; how can this program execute without the function main()? Does this code conform to the C++ standard? Is the behavior of the program is well defined? I have also used the -pedantic-errors option but the program still compiles and runs.

Answer 1

Before going into the meat of the question about what is going on, it is important to point out that program is ill-formed as per defect report 1886: Language linkage for main():

[...] A program that declares a variable main at global scope or that declares the name main with C language linkage (in any namespace) is ill-formed. [...]

The most recent versions of clang and gcc makes this an error and the program will not compile (see gcc live example):

error: cannot declare '::main' to be a global variable int main = ( std::cout << "C++ is excellent!\n", 195 );      ^ 

So why was there no diagnostic in older versions of gcc and clang? This defect report did not even have a proposed resolution until late 2014 and so this case was only very recently explicitly ill-formed, which requires a diagnostic.

Prior to this, it seems like this would be undefined behavior since we are violating a shall requirement of the draft C++ standard from section 3.6.1 [basic.start.main]:

A program shall contain a global function called main, which is the designated start of the program. [...]

Undefined behavior is unpredictable and does not require a diagnostic. The inconsistency we see with reproducing the behavior is typical undefined behavior.

So what is the code actually doing and why in some cases does it produce results? Let's see what we have:

declarator   |        initializer---------------------------------- |        |                                           | v        v                                           v int main = ( std::cout << "C++ is excellent!\n", 195 );      ^      ^                                   ^     |      |                                   |     |      |                                   comma operator     |      primary expression global variable of type int 

We have main which is an int declared in the global namespace and is being initialized, the variable has static storage duration. It is implementation defined whether the initialization will take place before an attempt to call main is made but it appears gcc does do this before calling main.

The code uses the comma operator, the left operand is a discarded value expression and is used here solely for the side effect of calling std::cout. The result of the comma operator is the right operand which in this case is the prvalue 195 which is assigned to the variable main.

We can see sergej points out the generated assembly shows that cout is called during static initialization. Although the more interesting point for discussion see live godbolt session would be this:

main: .zero   4 

and the subsequent:

movl    $195, main(%rip) 

The likely scenario is that the program jumps to the symbol main expecting valid code to be there and in some cases will seg-fault. So if that is the case we would expect storing valid machine code in the variable main could lead to workable program, assuming we are located in a segment that allows code execution. We can see this 1984 IOCCC entry does just that.

It appears we can get gcc to do this in C using (see it live):

const int main = 195 ; 

It seg-faults if the variable main is not const presumably because it is not located in an executable location, Hat Tip to this comment here which gave me this idea.

Also see FUZxxl answer here to a C specific version of this question.