主程式

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在一些程式設計語言裡，主函式是程式開始執行的地方。

當程式一開始時，會先從這個由使用者所編寫的函式開始執行(通常由系統指定的軟體會在進入主函式之前先執行)，雖然有些程式語言(值得注意的是 C++ 的全域物件包含建構子)允許在進入主函式之前先執行子程序。通常處於最上層的主函式會組織並執行其他程式的功能。基本上，主函式具備可存取程式的命令列指令。

[编辑] C and C++

在 C語言和 C++ 裡，主函式的函式原型有以下幾種：

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

argc 和 argv 這 2 個參數分別代表命令列指令的數目和值。其他相依平台的格式也允許接受 C 和 C++ 的標準；舉例來說，Unix (不含 POSIX.1) 和 Microsoft Visual C++ 給定了以下 3 個程式的環境參數：

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

主函式的名稱 main 是程式的關鍵字；正常來說，每一個 C 和 C++ 程式嚴格要求只能有一個以 main 為名的函式。

[编辑] C#

執行用 C# 所寫的程式時，虛擬機器會搜尋一個類別並執行它，而這個類別必須內含名稱為 Main 靜態(不需加 public 關鍵字)方法，而這個 Main 方法可以不加環境參數、或是內含 string[] 型態的環境參數，也可以把它宣告成 void 或是 int 型態。

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

args 代表命令列的環境參數，與 JAVA 語言相類似。當 Main 傳回整數值時，整數值會被作業系統接收。

[编辑] GNAT

Using GNAT, the programmer is not required to write a function called main; a source file containing a single subprogram can be compiled to an executable. The binder will however create a package ada_main, which will contain and export a C-style main function.

[编辑] Haskell

In Haskell, there may be a main name bound to a value of type IO (). IO is a monad, which is used to separate side-effects from purely functional areas of the program. Grossly simplified and analogised, the main value serves as the program's entry point.

Command line arguments are not given to main; they must be fetched using another IO action.

[编辑] Java

Java 程式語言是以 main 方法來當做程式開始的起點，方法如下：

public static void main(String[] args)

命令列指定參數是 args。如同 C 和 C++ 一樣，"main" 也是唯一的。Java 的 main 方法無任何的傳回值。

[编辑] Pascal

In Pascal, the main procedure is the only unnamed procedure in the program. Because Pascal programs have the procedures and functions in a more rigorous top-down order than C, C++ or Java programs, the main procedure is usually the last procedure in the program. Pascal does not have a special meaning for the name "main" or any similar name.

Example:

procedure hello() begin
  writeln('Hello world')
end;
begin
  hello()
 end.

[编辑] Pike

Pike 與 C/C++ 的語法相類似。從 main 開始執行。"argc" 代表環境參數的個數。"argv" 代表環境參數的值。

舉例如下：

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

[编辑] Python

In Python a function called main doesn't have any special significance. However, it is common practice to organize a program's main functionality in a function called main and call it with code similar to the following:

def main():
    <main program>

if __name__=="__main__":
    main()

When a Python program is executed directly (as opposed to being imported from another program), the special global variable __name__ has the value "__main__".

[编辑] 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 for you, and cannot be removed from your project.