Is `main()` Really the First Line of Execution in a C Program?

Is Main() truly the Commencement of a C Program?

The C Standard states in section $3.6.1/1 that every program must include a global function named main, which serves as its designated starting point. However, a scenario arises where this assertion appears to be challenged.

Consider the following code sample:

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 successfully performs its intended purpose by printing the squares of integers from 0 to 9 before entering the main() function. Intriguingly, the code compiles without errors or warnings when using the -pedantic flag in GCC 4.5.0.

This observation raises questions about the Standard's claim that main() is the program's starting point. The code appears to execute user-defined functions prior to entering the designated starting point.

The Definition of "Start"

The dispute hinges on the precise definition of "start of the program." The Standard does not explicitly define this term, leaving room for interpretation. Some may argue that "start" implies the initial execution of code, which in this case occurs in user_main(). Others may contend that "start" refers to the designated starting point as defined by the language specifications.

The key distinction lies in the interpretation of the C Standard's sentence:

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

The Standard is essentially defining the term "start" in the context of the standard itself. It does not assert that no code is executed before main() is called. Instead, it establishes main() as the program's designated starting point for all subsequent discussions and definitions.

Based on this interpretation, the code sample provided remains compliant with the C Standard. The function user_main() executes prior to the "start" of the program as defined by the Standard. This prior execution does not invalidate the designated starting point of the program at main().

Therefore, the conclusion is that main() retains its designation as the starting point of a C program, even though user-defined code may execute before this designated starting point is reached. The Standard's definition of "start" clarifies this understanding.

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