Does C 's `main()` Function Truly Mark the *Beginning* of Program Execution?

Is Main() the True Beginning of a C Program? Analyzing a Counterintuitive Code Snippet

In the realm of C , the main() function is often heralded as the commencement point of program execution. However, a peculiar code sample challenges this assumption.

Consider the following code:

int square(int i) { return i*i; }
int user_main()
{
    for ( int i = 0 ; i < 10 ; ++i )
           std::cout << square(i) << endl;
    return 0;
}
int main_ret= user_main();
int main() 
{
        return main_ret;
}

This code seemingly prints the squares of integers from 0 to 9 before entering the ostensibly starting function, main(). Furthermore, it compiles without errors or warnings under the -pedantic flag in GCC 4.5.0.

This raises the question: Does this code adhere to the C Standard, which stipulates in section 3.6.1/1 that "A program shall contain a global function called main, which is the designated start of the program"?

Unraveling the Standard's Definition

To answer this question, we must delve into the semantics of the Standard's definition. The Standard is defining the term "start" within the context of its own usage. It does not proclaim that no code precedes the invocation of main. Rather, it designates the main function as the point from which the program commences its execution.

In this regard, the example code complies with the Standard. While user_main() executes prior to the initiation of main(), this occurs before the program "starts" as per the Standard's definition.

Conclusion

The unusual sequence of execution in this code snippet does not invalidate the Standard's assertion that main() marks the start of the program. The Standard's definition of "start" allows for prologue code to execute before main(), as exemplified by this intriguing code sample. Therefore, the program remains fully compliant despite its counterintuitive behavior.

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