An entry point is the starting point of a program, module, or function, identified by a memory address. In linear programming, it is the first line of code, while in modular programming, it is the initial module or the “Main()” function in C. Older programs may have multiple entry points, but modern ones have a single entry point. The concept of entry points was created to identify the starting point of processing in non-linear programs. In C, the main() function is the entry point, while in other languages, it is identified by the first line of code following variable declarations or subroutines.
In computers, an entry point is the point in a program, module, or function where the code begins; in particular, the memory address where it starts. In a program, it is the first module of code, or even the first line of code that is executed. In a linear program, such as BASIC or COBOL, the entry point is literally the first line of code. The code then follows in sequence until some sort of jump or call is made. In modular programming, like all variants of the C programming language, the entry point is the initial module. In C, this is the “Main()” function.
In older programs and operating systems, a program might have multiple entry points, depending on the function or system it is running. In most modern operating systems, including Windows® and Unix, programs are written in languages that support a single entry point. Even in the early days of modern computing, languages like BASIC, while having a single entry point to the program, could support “long jumps”, where the program could “jump” from one module to the entry point of another module indiscriminately . This encouraged was what was called the “spaghetti code,” which was difficult to maintain or change.
An entry point may not even refer to the entry point of a program, but it can be the entry point of a dynamic link library (DLL), which are sort of mini-programs shared between other programs. An example is a DLL that controls keyboard input. Regardless of the type of program, module or function, the entry point is the only place where processing in that piece of code takes place. The key to understanding any language or modifying and supporting any program is understanding how the entry point into that specific program or operating system is identified.
The concept of entry points was implemented when computer programming left the completely linear methods of the early days. Back then, a computer program started on the first line of code and proceeded one line at a time until the process was finished. Soon, programming constructs like loops, conditional branching, recursion, and others created programs that were functionally more efficient, but processing bounced all over the place within the code. The nature of these programs means that there may be many lines or sections of code before the point where processing actually begins. For this reason the entry point was created and identified.
In C, this has become the main() function. Regardless of where this function existed in the code, that’s where the processing started. In other languages, entry points are identified not so much by where they are, but by where they are not. The beginning of the program might contain sections of declarations of variables and subroutines. The first line of code following these areas or functions, by default, becomes the entry point.
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