Main function

In many programming languages, the main function is where a program starts execution. It is responsible for the high-level organization of the program's functionality, and typically has access to the command arguments given to the program when it was executed.

The main function is generally the first programmer-written function that runs when a program starts, and is invoked directly from the system-specific initialization contained in crt0 or equivalent. However, some languages can execute user-written functions before main runs, such as the constructors of C++ global objects.

Variants

C and C++

In C and C++, the function prototype of the main function looks like one of the following:

int main(void);
int main();

int main(int argc, char **argv);
int main(int argc, char *argv[]);

The parameters argc, argument count, and argv, argument vector, respectively give the number and values of the program's command-line arguments. The names of argc and argv may be any valid identifier in C, but it is common convention to use these names. In C++, the names are to be taken literally, and the "void" in the parameter list is to be omitted, if strict conformance is desired. Other platform-dependent formats are also allowed by the C and C++ standards, except that in C++ the return type must always be int; for example, Unix (though not POSIX.1) and Microsoft Windows have a third argument giving the program's environment, otherwise accessible through getenv in stdlib.h:

int main(int argc, char **argv, char **envp);

Mac OS X and Darwin have a fourth parameter containing arbitrary OS-supplied information, such as the path to the executing binary:

int main(int argc, char **argv, char **envp, char **apple);

The value returned from the main function becomes the exit status of the process, though the C standard only ascribes specific meaning to two values: EXIT_SUCCESS (traditionally 0) and EXIT_FAILURE. The meaning of other possible return values is implementation-defined. In case a return value is not defined by the programmer, an implicit return 0; at the end of the main() function is inserted by the compiler; this behavior is required by the C++ standard.

It is guaranteed that argc is non-negative and that argv[argc] is a null pointer. By convention, the command-line arguments specified by argc and argv include the name of the program as the first element if argc is greater than 0; if a user types a command of "rm file", the shell will initialise the rm process with argc = 2 and argv = ["rm", "file", NULL]. As argv[0] is the name that processes appear under in ps, top etc., some programs, such as daemons or those running within an interpreter or virtual machine (where argv[0] would be the name of the host executable), may choose to alter their argv to give a more descriptive argv[0], usually by means of the exec system call.

The main() function is special; normally every C and C++ program must define it exactly once.

If declared, main() must be declared as if it has external linkage; it cannot be declared static or inline.

In C++, main() must be in the global namespace (i.e. ::main), cannot be overloaded, and cannot be a member function, although the name is not otherwise reserved, and may be used for member functions, classes, enumerations, or non-member functions in other namespaces. In C++ (unlike C) main() cannot be called recursively and cannot have its address taken.

C#

When executing a program written in C#, the CLR searches for a static method marked with the .entrypoint IL directive, which takes either no arguments, or a single argument of type string[], and has a return type of void or int, and executes it.

static void Main();
static void Main(string[] args);
int Main();
static int Main(string[] args);

Command-line arguments are passed in args, similar to how it is done in Java. For versions of Main() returning an integer, similar to both C and C++, it is passed back to the environment as the exit status of the process.

D

In D, the function prototype of the main function looks like one of the following:

void main();
void main(string[] args);
int main();
int main(string[] args);

Command-line arguments are passed in args, similar to how it is done in C# or Java. For versions of main() returning an integer, similar to both C and C++, it is passed back to the environment as the exit status of the process.

Go

In Go programming language, program execution starts with the main function of the package main

package main

import "fmt"

func main() {
	fmt.Println("Hello, World!")
}

Haskell

A Haskell program must contain a name called main bound to a value of type IO t, for some type t; which is usually IO (). IO is a monad, which organizes side-effects in terms of purely functional code. The main value represents the side-effects-ful computation done by the program. The result of the computation represented by main is discarded; that is why main usually has type IO (), which indicates that the type of the result of the computation is (), the unit type, which contains no information.

main :: IO()
main =
     putStrLn "Hello, World!"

Command line arguments are not given to main; they must be fetched using another IO action, such as System.Environment.getArgs.

Java

Java programs start executing at the main method, which has the following method heading:

public static void main(String[] args)
public static void main(String... args)
public static void main(String args[])

Command-line arguments are passed in args. As in C and C++, the name "main()" is special. Java's main methods do not return a value directly, but one can be passed by using the System.exit() method.

Unlike C, the name of the program is not included in args, because the name of the program is exactly the name of the class that contains the main method called, so it is already known. Also unlike C, the number of arguments need not be included, since the array class in Java has an attribute that keeps track of how many elements there are.

Pike

In Pike syntax is similar to that of C and C++. The execution begins at main. The "argc" variable keeps the number of arguments passed to the program. The "argv" variable holds the value associated with the arguments passed to the program.

Example:

int main(int argc, array(string) argv)

REALbasic

In REALbasic, there are two different project types, each with a different main entry point. Desktop (GUI) applications start with the App.Open event of the project's Application object. Console applications start with the App.Run event of the project's ConsoleApplication object. In both instances, the main function is automatically generated, and cannot be removed from the project.

Visual Basic

In Visual Basic, when a project contains no forms, the startup object may be the Main() procedure. The Command$ function can be optionally used to access the argument portion of the command line used to launch the program:

Sub Main()
    Debug.Print "Hello World!"
    MsgBox "Arguments if any are: " & Command$
End Sub

External links

• How main method works in Java.