Why Does Recursively Calling `main()` in C Lead to Undefined Behavior?

Recursive Calls of Main in C : A Journey into Undefined Territory

The perplexing code snippet presented below has sparked curiosity within the programming community:

int main () { 
    return main(); 
}

To the astonishment of many, this code compiles successfully. However, its execution reveals an intriguing behavior: it runs indefinitely. This peculiar observation has prompted questions about the underlying mechanics.

The Dilemma of Calling Main

The C standard explicitly declares in [basic.start.main] p3 that "The function main shall not be named by an expression." This prohibition stems from the nature of main as the program's entry point, which the runtime library invokes once. Any subsequent calls to main are not permitted, hence the lack of named expression.

Compiler Behavior and Undefined Behavior

According to [intro.compliance.general], a compiler's obligation is to generate executable code from well-formed programs. While it may provide warnings or errors, it is not required to enforce all rules. In this particular case, compilers have the discretion to respond differently to the violation of calling main.

Embrace the Unknown

Ultimately, the decisive verdict on the behavior of this code snippet lies within the realm of undefined behavior. As per the standard's definition, anything can happen when undefined behavior occurs. Attempts to rationalize such outcomes are futile, as the consequences are entirely unpredictable.

In conclusion, while the syntax of the presented code is valid, its execution results in undefined behavior. It is best to heed the standard's prohibition against named expressions calling main, lest your programs embark on unpredictable journeys into the unknown.

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