在一些编程语言里，主函式（main function）是程序开始运行的地方。

在C语言或C++编程语言，main的函式原型如下所示：

main的两个环境参数：argc（argument count）与argv（argument vector），两者个别自程序的指令列给予参数的数量与参数数组的指标地址。argc与argv这两个参数的名称，在编程语言的命名规则底下，虽然可以依照用户的喜好自行定义，但是一般在使用上，还是会以现有的名称argc与argv来进行程序的编写。其他具有相依平台的格式也可以被C与C++标准所接受；例如，UNIX（非POSIX.1）与Microsoft Visual C++有第三个参数，是用来接收程序的环境变量，利用其他方法访问环境变量，可以使用stdlib.h头文件定义的getenv函式：

In computer programming, an entry point is where control is transferred from the operating system to a computer program, at which place the processor enters a program or a code fragment and execution begins. In some operating systems or programming languages, the initial entry is not part of the program but of the runtime library, in which case the runtime library initializes the program and then the runtime library enters the program. In other cases, the program may call the runtime library before doing anything when it is entered for the first time, and, after the runtime library returns, the actual code of the program begins to execute. This marks the transition from load time (and dynamic link time, if present) to run time.