Does `main()` Always Mark the True Beginning of a C Program's Execution?

Is main() Truly the Starting Point of a C Program?

The C Standard, in section 3.6.1/1, declares that a program must possess a global function named main, which serves as the designated starting point. However, further analysis and an illustrative code example have brought into question the validity of this statement.

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 achieves its intended purpose: it outputs the squares of integers from 0 to 9. However, the puzzling aspect lies in the fact that the function user_main() executes prior to main(), which is supposedly the starting point of the program. Compiling this code with the -pedantic option using GCC 4.5.0 raises no errors or warnings.

This observation prompts the question: does this code conform to the Standard?

To answer this question, we must delve deeper into the meaning of the phrase "start of the program." The Standard defines this term specifically for the context of the Standard itself. It does not assert that no code can execute before main(). Instead, it establishes that the beginning of the program is the point at which main() begins.

In the case of this example code, user_main() does run before the program "starts," according to the definition provided by the Standard. Hence, this code is fully compliant. By design, significant code often executes before main() is called, including this example.

Therefore, the misconception arises from misinterpreting the Standard's definition. For the sake of the Standard's discussions, user_main() is executed before the program "starts," and this behavior remains entirely compliant with the Standard.

The above is the detailed content of Does `main()` Always Mark the True Beginning of a C Program's Execution?. For more information, please follow other related articles on the PHP Chinese website!