Entry point

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For the Judas Priest album, see Point of Entry.

In computer programming, an entry point is where control enters a program or piece of code.

Usage customs

Contemporary

In most of today's popular computer systems, such as Microsoft Windows and Unix, a computer program usually only has a single entry point.

In C and C++ programs this is the main() function.

Historical

Historically, and in some contemporary legacy systems, such as VMS and OS/400, computer programs have a multitude of entry points, each corresponding to the different functionalities of the program. The usual way to denote entry points, as used system-wide in VMS and in PL/I and MACRO programs, is to append them at the end of the name of the executable image, delimited by a dollar sign ($), e.g. directory.exe$make. The Apple 1 computer also used this to some degree. For example an alternative entry point in Apple 1 BASIC would keep the BASIC program useful when the reset button was accidentally pushed.

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