Why Calling Main() Is Not Allowed in C++

Why calling main() is not allowed in C++

In addition to the other answers: The c++ spec guarantees that all static initialization has happened before main is called.

If code could call main then some static scoped object could call main, in which case a fundamental guarantee is violated.

The spec can't say "static scoped objects should not call main()" because many objects are not written specifically to be instantiated at static scope always. It also can't say that constructors should not call main() - because its very difficult to audit and prove that a constructor isn't calling a method, calling a method, that might sometimes, call main().

Does int main() need a declaration on C++?

A definition of a function is also a declaration of a function.

The purpose of a declaring a function is to make it known to the compiler. Declaring a function without defining it allows a function to be used in places where it is inconvenient to define it. For example:

If a function is used in a source file (A) other than the one it is defined in (B), we need to declare it in A (usually via a header that A includes, such as B.h).
If two or more functions may call each other, then we cannot define all those functions before the others—one of them has to be first. So declarations can be provided first, with definitions coming afterward.
Many people prefer to put “higher level” routines earlier in a source file and subroutines later. Since those “higher level” routines call various subroutines, the subroutines must be declared earlier.

In C++, a user program never calls main, so it never needs a declaration before the definition. (Note that you could provide one if you wished. There is nothing special about a declaration of main in this regard.) In C, a program can call main. In that case, it does require that a declaration be visible before the call.

Note that main does need to be known to the code that calls it. This is special code in what is typically called the C++ runtime startup code. The linker includes that code for you automatically when you are linking a C++ program with the appropriate linker options. Whatever language that code is written in, it has whatever declaration of main it needs in order to call it properly.

Can main function call itself in C++?

TLDR: Calling main results in undefined behavior.

There seems to be confusion about the terminology used in the standard, and the implications that has for the programmer and compiler.

Firstly, the standard alone determines everything about the C++ language. If your particular version of a particular compiler allows some particular action, that has no bearing on whether or not that action is legal. For the remainder of the post, I'm referring to the ISO03 standard.

So to quote once again, the standard says in §3.6.1.3:

The function main shall not be used within a program.

Additionally, §3.2 defines "used" as:

An object or non-overloaded function is used if its name appears in a potentially-evaluated expression.

This means that once the program begins executing, main should never be entered again. That means programmers cannot call main, that means the compiler cannot insert another call to main (why it would, who knows), you cannot take the address of main and call that, etc. You cannot even have the potential of calling main.

The only call to main should be by the run-time library the program is running on; all other calls invoke undefined behavior. (Which means anything could happen!)

Now onto compiler behavior:

A diagnosable rule is defined as (§1.4.1):

The set of diagnosable rules consists of all syntactic and semantic rules in this International Standard except for those rules containing an explicit notation that “no diagnostic is required” or which are described as resulting in “undefined behavior.”

In our case, §3.6.1.3 defines a diagnosable rule. Here's what compilers should do according to §1.4.2:

— If a program contains no violations of the rules in this International Standard, a conforming implementation shall, within its resource limits, accept and correctly execute3) that program.

— If a program contains a violation of any diagnosable rule, a conforming implementation shall issue at least one diagnostic message, except that

— If a program contains a violation of a rule for which no diagnostic is required, this International Standard places no requirement on implementations with respect to that program.

So compilers are not required to enforce rules. All compilers have to do is take well-formed programs (§1.3.14) and turn them into an executable program. A compiler is free to warn, error, etc. however it likes, as long as it does not conflict with the language. It is required to display a message in our particular case, according to the second clause.

For this particular problem, on gcc the -pedantic option will warn about the illegality of calling main within the program. Visual Studio will not warn about calling main, but on any warning level (greater than 0) it will warn about the recursive nature of the program.

What does all this mean in terms of the answers you should expect? It means it's completely meaningless to try and define with certainty what the code snippet posted will do. Calling main results in undefined behavior, and trying to define undefined behavior is obviously a lost cause. The only honest answer anyone can give to "what happens when I call main?" is "Anything."

I hope this clears things up.

Why Calling Main() Is Not Allowed in C++