Is main() (or Main()) in C, C++, Java or C#, a user-defined function or a built-in function?
It's a user-defined function that is necessary for a program to execute. When you go to run your program in the compiled language, the main function is what is executed. For instance, in Java, if you have a function of the signature public static void main(String ... args) in a class then that class can be executed, as the JVM will execute the contents of that main method.
Example in Java:
public class Test {
public static void main(String ... args) {
System.out.println("Hello World");
}
}
...
javac Test.java
...
java Test
Results in "Hello World" being printed to the console.
I'm not sure what you mean by built-in vs. user defined. Almost no language actually gives your user-defined function the privilege of being the true entry-point into the program. C++, any .NET language, and Java all have hidden (built-in) entry point methods that in turn call your user-defined Main method (or whatever the entrypoint method for that language is called -- in .NET it can be named anything, although C# and VB.NET force it to be called Main).
So yes, virtually every language has a concept of a method that is automatically called, and this method is a user-defined method and usually mandatory. But virtually every language also has a built-in entry point method that actually sets up the framework and/or memory management for the process before invoking your user-defined "entry-point" function.
Quote from the C Standard (emphasis is mine):
5.1.2.1 Freestanding environment
In a freestanding environment (in
which C program execution may take
place without any benefit of an
operating system), the name and
type of the function called at
program startup are
implementation-defined. Any
library facilities available to a
freestanding program, other than the
minimal set required by clause 4,
are implementation-defined.
main(), in a freestanding environment, is very much a user-defined function.
It's a required user defined function (the entry point for executables)...
It is not "built-in" in any language, in a sense that there is no standard implemented-for you main() avialable.
For C/C++/Java, it is a function with a special property, namely, the function that will be called at the start of your program after all the static setup is done. E.g. entire C program's execution path is:
Do some initialization code
Call main()
Exit.
As such, it has a standard declaration (# of parameters passed from command line + array of "strings" - however the language implements that - which are the actual arguments from command line)
In C/C++, it a standard so its built in and reconized.
Java, not sure, no experience
C# - Its a part of a class so its defined by you.
All of these are defined by you -- you tell it what to do.
It's a user defined function that is called by the language's runtime library. For example, a C runtime library will grab the command line arguments and sometimes environment variables from the operating system and pass them into to your main() function.
Different language runtimes usually perform the same operation in one form or another, and will throw some sort of error if the function it tries to call doesn't exist.
It's declaration is built-in. It's definition is user supplied, or in some cases supplied by an application framework that has some other entry point, or in the case of most event-driven GUI frameworks, no single user-defined entry point.
In Java main(String[] args) is the entry point for applications by convention (to make C++ programmers comfortable). For applets or servlets the invocation of code happens differently. Note that a jar may contain any or none of these entry points and that each class may contain a main so a given jar can be invoked in many different ways as an applcation if so desired.
Related
What is a global method in .net? I'm talking about the ones created by the ModuleBuilder.DefineGlobalMethod method.
Can these methods be called from C# or other .net languages?
Hard to talk about this at all in familiar terms, this capability is not at all exposed in the C# or VB.NET languages. The C++/CLI compiler uses it however. The way it is shown in disassemblers like Ildasm.exe or Reflector is also not standardized.
Perhaps the best angle is Reflector, take a look-see at the System.Data.dll assembly. It is in an unnamed namespace ("-" in Reflector), <Module> node. The .cctor you see there is a module initializer. Same kind of animal as a static class constructor but at the module level. It runs when the assembly gets loaded. C++/CLI uses it to initialize the C runtime library.
The ___CxxCallUnwindDtor() method you find there is an example of a "global method". The C++/CLI language doesn't give any way to make these kind of functions public, they are always embedded in the metadata with internal accessibility. And can thus not be called directly from a C# or VB.NET program. I haven't played enough with ModuleBuilder to know if that can be messed with at all beyond what C++/CLI does. This is all very obscure and not actually that useful.
Global method means that the method can be called without fully qualifying it's name ie
Method(Param) instead of Module.Method(Param).
In vb a public method inside a module is global.
In c# a public static method in a static class is global.
I'm a bit new at this so bear with me. I'm currently learning C# and Java and one of their similarities is that the main function needs to be encapsulated within a class. For example
public class HelloWorld {
public static void main(String[] args) {
// Some Code
}
}
Now I understand that main is often the "entry point" when you run the program. So basically, your program will start executing wherever the main function is. But I believe in both languages you can have multiple main functions within multiple classes. So when I compile a project with multiple main functions, where is the "entry point"? How does the compiler know where to start?
In Java, the computer determines the "entry point" when you actually execute the program, not when you compile. For example, from the command-line
java MyClass
searches for main() in MyClass. All other main() functions are ignored.
If you are using an IDE, then you can set which class contains the main() function that you want to use.
In .NET, you can define which class contains the Main method you want to use when you're compiling.
http://msdn.microsoft.com/en-us/library/x3eht538.aspx
In Java, if you're bundling to a jar, you can define your entry point in the jar's manifest.
http://docs.oracle.com/javase/tutorial/deployment/jar/appman.html
In C#, you can use multiple Main methods.
If there are multiple Main methods, the compiler doesn't know which entry point to use and hence it will show you an error.
You need to specify the Main method to be used at compilation:
You can specify which method to be used as a compiler option in the Visual Studio development environment or through the csc compiler.
The main class is a special class for only one reason: when you run the Java Virtual Machine, that function is what the JVM calls. It is essentially like any other function, and in fact you can call one class's main function from another class.
When you compile a project with multiple classes, you tell the JVM to run the class with the main class you want to use, like so:
java SomeClass
and it will run the main method of SomeClass, assuming that SomeClass is compiled and that the appropriate compiled file is in your classpath. (That is something you'll have to work out with your particular OS, but I think it's fairly usual for the -cp option to specify a classpath). So this:
java -cp /home/MyName Someclass
will run the main function of SomeClass in the directory /home/MyName
In C#, you specify the entry point using the /main: compiler option.
Consider the following code containing two main() functions:
namespace Application {
class ClassOne {
static void main () {
// Code here
}
}
class ClassTwo {
static void main () {
// Code here
}
}
}
To use ClassOne.main() as your entry point, you would specify the following when compiling:
csc /main:Application.ClassOne hello.cs
For multiple main functions entry point can be declared by :
To set this compiler option in the Visual Studio development environment
Open the project's Properties page.
Click the Application property page.
Modify the Startup object property.
reference : http://msdn.microsoft.com/en-us/library/x3eht538.aspx
In Java, as others pointed out, you define the class containing your main function when you run the java command.
But you could also build an executable jar, which can be run with java -jar my.jar. In this case, you need a manifest file called MANIFEST.MF in the folder META-INF in the jar. In this file, you specify the class containing the main function using the statement: Main-Class: YourClass.
The main method is static, which means it belongs to the class rather than the object. So the object won't have another main method inside it at all. It won't have an additional main-method, as main is static. So it's once per class.
If you have multiple main-methods in your project, you will specify which one to launch when starting your application
In fact, in binary file, for example, PE format in windows and ELF format in Linux or any other system, The header of binary file will specify where is the start address and there can be only one.
Which one should be the entry point? It depends on the linker. Just like #SetFreeByTruth said that you can specify it with command line parameters.
Many linkers support specifying entry point with command line parameters. for example, gnu's gld can specify entry point with parameter -e.
I don't know the behavior of Java because it is loaded by Java virtual machine.
In Visual Studio, you select the project that you want to be the entry point, right click and Set as StartUp Project.
I have a (C#) function similar to the following.
private static bool SpecialCase = false;
public void Foo()
{
if (SpecialCase)
{
InternalMethod();
return;
}
Console.WriteLine();
}
[MethodImpl(MethodImplOptions.InternalCall)]
private static extern void InternalMethod();
When I execute this with the .NET Framework 4 inside the debugger, the method successfully prints blank line to the console and returns. When I execute it outside the debugger, it throws an exception with the following message:
System.Security.SecurityException: ECall methods must be packaged into a system module.
It appears the exception is thrown when the JIT compiler compiles the method as opposed to when (if) InternalMethod is called. Is there anything I can do (e.g. attributes) to tell the CLI to either not throw the SecurityException, or delay the exception until the method is actually called?
Side note on the use case: The SpecialCase field is effectively false when running with Microsoft .NET Framework, and true when running under a different (specific) implementation of the CLI. When running under the Microsoft .NET Framework, the call to InternalMethod is effectively unreachable.
Add attribute [ComImport] to your class declaration
You may want to check the compiler directives as a possible option.
Unlike using a run-time "if", this will determine whether the call is included in the compiled code at all, rather than always compiling it into the code and trying to determine whether to call it at run-time (which is too late, based on your analysis).
Your use-case seems like a testing/validation scenario, which means that you don't need it compiled into the code except when the internal call will actually be made.
Note that if your use-case involves a non-.NET runtime, you should provide more information since that could drastically change the correct answer.
Why we put main() method always inside the class in C# while in c++ it always placed outside of the class.
The C++ language designers followed the lead of C and so the main function is a plain function.
The C# language designers made the choice, when designing the language, that all methods must be part of classes.
For historical reasons. C++ evolved from C, which had a global main() function. C# is much younger and was designed from scratch. One of the design features of C# is the absence of global functions, so the main function has to belong to a class.
Because in .NET you can place methods only inside types. You cannot have them floating around in the empty space. C++ has its legacy from C which is not an OOP language so you could define functions anywhere.
You cannot place method outside class/struct in C#. Each method must be in class/struct
It is a convention. Which is in line with Java (also follows the semantic of having a method inside class).
C# is complete object oriented language where everything is considered as objects. Hence, Main() is kept inside class.
With python, I can use if __name__ == "__main__": for using the module both as a library and a program.
Can I mimic this feature in C#?
I see a class in C# can have a 'static void Main()', but I'm not sure if every class can have a Main() without a problem.
ADDED
/m:CLASS_NAME is a way to specify the class to run the Main().
You can put a Main method in as many classes as you like, although only one can be an entry point for an application. (For talks, I often have a main method in every class, and use a helper library to present all of those pseudo-entry-points when I run the project.)
Likewise you can definitely add a reference to a .exe assembly and treat it like a library. For example, you could make a unit testing assembly work like a class library in most cases, but also write a main method so that you could just run it to execute the tests without a GUI or whatever.
You can compile a C# project as a program (executable) with a Main() method, and you'd still be able to use it as a library. No special syntax required.
You could add a Main() method to every class, but I doubt it's useful.
.NET applications usually have different structures than Python ones; trying to fit the same programming model is unlikely to get you good results.
C# project files specify a startup object when multiple entry points are available.
See this article for more info.