Definition of main() in C [duplicate]

Answer 1

   int main() {}
   this is the standard prior to the c99 standard of main method.

   int main(void){}
   this is the standard coined by ANSI.

   int main(int argc, char* argv[]) {}     
   This is the another version of main which provides the user to pass the command line
   argument to the main method.

Answer 2

In C, there's a difference between the declarations int main(); and int main(void); (the former declares a function with an unspecified number of arguments, and the latter is actually called a proto­type). However, in the function definition, both main() and main(void) define a function that takes no arguments.

The other signature, main(int, char**), is an alternative form. Conforming implementations must accept either form, but may also accept other implementation-defined signatures for main(). Any given program may of course only contain one single function called main.

Answer 3

int main() and any other function declaration like this, it takes an unknown number of arguments, so this is absolutely wrong for the main function. int main(void) it takes no arguments.

char* argv[] is the argument vector. When you write your argument on the command line you will find arguments in this vector of strings. Sometimes you can also find char **argv but it is the same. The parentheses [] are empty because we don't know how many arguments come from the user; the int argc argument count exists for this purpose: it counts how many arguments are in argv (though the list is terminated with argv[argc] == NULL as a sentinel value too).

Read also this link for the difference between a generic foo() and foo(void)

Answer 4

If the implementation explicitly documents int main() (with no arguments) as a valid signature, then as of C99 everything's fine (§5.1.2.2.1 ¶1, "... or in some other implementation-defined manner.").

If the implementation doesn't document it, then strictly speaking the behavior is undefined (§4 ¶2), but the odds of it leading to behavior significantly different from int main(void) are, in my experience, pretty darned low.