As a newbie to c++, coming from python, I'm not sure why c++ doesn't allow statements outside of a function (in the global namespace?). It seems like this could be useful to do some initialization before main() is called or other functions are even declared. (I'm not trying to argue with the compiler, I'd just like to know the thought process behind implementing it this way.)
When you're running a python program, the interpreter runs through it from top to bottom executing as it goes. In C++, that doesn't happen. The compiler builds all your functions into little blobs of machine code and then the linker hooks them up. At runtime, the operating system calls your main function, and everything goes on from there. In that context, code outside of functions is meaningless - when would it run?
This can be thought of as a style difference between C++ and Python. However, there are pretty good reasons for it too. In C and C++, there is a very clear and specific place that the execution of your code starts from, and that's the beginning of the main() function (of course, this is only an approximation of the truth, but we can ignore that for now.) Actually, a C/C++ program starts and ends with the main() function which - in my opinion - helps quite a lot when you want to understand what a program actually does. The high-level flow of the program is clearer. Contrast this with the alternative; with code scattered all through the file and in between functions and whatnot.
Even in a well-organized and non-trivial Python program, you put your main body of code under an if __name__ == "__main__":, don't you?
Now for some stuff that are a little bit more advanced. There are ways for code to run before the main() function is called. Here's one of them:
#include <iostream>
using namespace std;
bool RunBeforeMain ()
{
cout << "Before main()!" << endl;
return true;
}
// This variable here causes the above function to be called
bool ignore_this_variable = RunBeforeMain ();
int main ()
{
cout << "Start of main()" << endl;
return 0;
}
Also, the constructors of all the global variables and all the static members of classes and some platform-dependent stuff are run before main(). Analogously, there are ways for code to run after the main() is finished. Those are usually the destructors for objects constructed before main() began, and functions registered with the atexit() function.
The main() is the access point to the program. So any code you wanna write would be needs to have an order of execution from that point.
Static variables are initiated before the main() is executed, so you can initiate whatever variables you need before that.
But if you want run code that initiates the state of the program, you should do it just in the beginning of the program, and abuse the static variables and do it some constructor.
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Closed 10 years ago.
Possible Duplicate:
Does the program execution always start from main in C?
i want to start the execution of my program which contains 2 functions (excluding main)
void check(void)
void execute(void)
i want to start my execution from check(), is it possible in c/c++?
You can do this with a simple wrapper:
int main()
{
check();
}
You can't portably do it in any other way since the standard explicitly specifies main as the program entry point.
EDIT for comment: Don't ever do this. In C++ you could abuse static initialization to have check called before main during static init, but you still can't call main legally from check. You can just have check run first. As noted in a comment this doesn't work in C because it requires constant initializers.
// At file scope.
bool abuse_the_language = (check(), true);
int main()
{
// No op if desired.
}
Various linkers have various options to specify the entry point. Eg. Microsoft linker uses /ENTRY:function:
The /ENTRY option specifies an entry point function as the starting
address for an .exe file or DLL.
GNU's ld uses the -e or ENTRY() in the command file.
Needles to say, modifying the entry point is a very advanced feature which you must absolutely understand how it works. For one, it may cause skipping the loading the standard libraries initialization.
int main()
{
check();
return 0;
}
Calling check from main seems like the most logical solution, but you could still explore using /ENTRY to define another entry point for your application. See here for more info.
You cannot start in something other than main, although there are ways to have some code execute before main.
Putting code in a static initialization block will have the code run prior to main; however, it won't be 100% controllable. while you can be assured it runs prior to main, you cannot specify the order that two static initialization blocks will run prior to them both executing before main.
Linkers and loaders both have the concept of main held as a shared "understood" start of a C / C++ program; however, there is code that runs prior to main. This code is responsible for "setting up the environment" of the program (things like setting up stdin or cin). By putting code in a static initialization block, you effectively say, "hey you need to do this too to have the right environment". Generally, this should be something small, that can stand independently in execution order of other items.
If you need two or three things to execute in order before main, then make them into proper functions and call them at the beginning of main.
There is a contrived way to achieve that, but it is nothing more than a hack.
The idea is to create a static library containing the main function, and make it call your "check" function.
The linker will resolve the symbol when linking against your "program", and your "program" code will indeed not have a main by itself.
This is NOT recommended, unless you have very specific needs (an example that pops to mind is Windows Screensavers, as the helper library that comes with the Windows SDK has a main function that performs specific initialization like parsing the command line).
It may be supportted by the compiler. For example, gcc, you can use -nostartfiles and --entry=xxx to set the entry point of the program. The default entry point is _start, which will call the function main.
You can "intercept" the call to main by creating an object before the main starts. The constructor needs to execute your function.
#include <iostream>
void foo()
{
// do stuff
std::cout<<"exiting from foo" <<std::endl;
}
struct A
{
A(){ foo(); };
};
static A a;
int main()
{
// something
std::cout<<"starting main()" <<std::endl;
}
I have found solution to my own question.
we can simply use
#pragma startup function-name <priority>
#pragma exit function-name <priority>
These two pragmas allow the program to specify function(s) that should be called either upon program startup (before the main function is called), or program exit (just before the program terminates through _exit).
The specified function-name must be a previously declared function taking no arguments and returning void; in other words, it should be declared as:
void func(void);
The optional priority parameter should be an integer in the range 64 to 255. The highest priority is 0. Functions with higher priorities are called first at startup and last at exit. If you don't specify a priority, it defaults to 100.
thanks!
I keep getting this question asked in interviews:
Write a program without using main() function?
One of my friends showed me some code using Macros, but i could not understand it.
So the question is:
Is it really possible to write and compile a program without main()?
No you cannot unless you are writing a program in a freestanding environment (embedded environment OS kernel etc.) where the starting point need not be main(). As per the C++ standard main() is the starting point of any program in a hosted environment.
As per the:
C++03 standard 3.6.1 Main function
1 A program shall contain a global function called main, which is the designated start of the program. It is implementation-defined whether a program in a freestanding environment is required to define a main function. [ Note: In a freestanding environment, start-up and
termination is implementation-defined; startup contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration.
What is freestanding Environment & What is Hosted Environment?
There are two kinds of conforming implementations defined in the C++ standard; hosted and freestanding.
A freestanding implementation is one that is designed for programs that are executed without the benefit of an operating system.
For Ex: An OS kernel or Embedded environment would be a freestanding environment.
A program using the facilities of an operating system would normally be in a hosted implementation.
From the C++03 Standard Section 1.4/7:
A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries.
Further,
Section: 17.4.1.3.2 Freestanding implementations quotes:
A freestanding implementation has an implementation-defined set of headers. This set shall include at least the following headers, as shown in Table:
18.1 Types <cstddef>
18.2 Implementation properties <limits>
18.3 Start and termination <cstdlib>
18.4 Dynamic memory management <new>
18.5 Type identification <typeinfo>
18.6 Exception handling <exception>
18.7 Other runtime support <cstdarg>
Within standard C++ a main function is required, so the question does not make sense for standard C++.
Outside of standard C++ you can for example write a Windows specific program and use one of Microsoft's custom startup functions (wMain, winMain, wWinmain). In Windows you can also write the program as a DLL and use rundll32 to run it.
Apart from that you can make your own little runtime library. At one time that was a common sport.
Finally, you can get clever and retort that according to the standard's ODR rule main isn't "used", so any program qualifies. Bah! Although unless the interviewers have unusual good sense of humor (and they wouldn't have asked the question if they had) they'll not think that that's a good answer.
Sample program without a visible main function.
/*
7050925.c
$ gcc -o 7050925 7050925.c
*/
#include <stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf("How mainless!\n");
}
From: http://learnhacking.in/c-program-without-main-function/
main means an entry point, a point from which your code will start executing. although main is not the first function to run. There are some more code which runs before main and prepares the environment to make your code run. This code then calls main . You can change the name of the main function by recompiling the code of the startup file crt0.c and changing the name of the main function. Or you can do the following:
#include <stdio.h>
extern void _exit (register int code);
_start()
{
int retval;
retval = my_main ();
_exit(retval);
}
int my_main(void)
{
printf("Hello\n");
return 0;
}
Compile the code with:
gcc -o no_main no_main.c -nostartfiles
The -nostartfiles will not include the default startup file. You point to the main entry file with the _start .
main is nothing but a predefined entrypoint for the user code. Therefore you can name it whatever, but at the end of the day you do need an entry point. In C/C++ and other languages the name is selected as main if you make another language or hack the sources of these language compilers then you can change the name of main to pain but it will bring pain, as it will violate the standards.
But manipulating the entry function name is useful for kernel code, the first function to run in the kernel, or code written for embedded systems.
They may refer to a program written for a freestanding implementation. The C++ Standard defines two sorts of implementations. One is a hosted implementation. Programs written for those implementations are required to have a main function. But otherwise, no main function is required if the freestanding implementation doesn't require one. This is useful for operation system kernels or embedded system programs that don't run under an operation system.
Yes
$ cat > hwa.S
write = 0x04
exit = 0xfc
.text
_start:
movl $1, %ebx
lea str, %ecx
movl $len, %edx
movl $write, %eax
int $0x80
xorl %ebx, %ebx
movl $exit, %eax
int $0x80
.data
str: .ascii "Hello, world!\n"
len = . -str
.globl _start
$ as -o hwa.o hwa.S
$ ld hwa.o
$ ./a.out
Hello, world!
The kernel that really runs an executable knows nothing about internal symbols, it just transfers to an entry point specified in binary in the executable image header.
The reason you need a main is because normally your "main program" is really just another module. The entry point is in library-provided startup code written in some combination of C and assembly and that library code just happens to call main so you normally need to provide one. But run the linker directly and you don't.
To include a C module1...
Mac:~/so$ cat > nomain.S
.text
.globl start
start:
call _notmain
Mac:~/so$ as -o nomain.o nomain.S
Mac:~/so$ cat > notmain.c
#include <unistd.h>
void notmain(void) {
write(1, "hi\n", 3);
_exit(0);
}
Mac:~/so$ cc -c notmain.c
Mac:~/so$ ld -w nomain.o notmain.o -lc
Mac:~/so$ ./a.out
hi
1. And I'm also switching to x86-64 here.
Yes it possible to compile with out main but you cannot pass the linking phase though.
g++ -c noMain.cpp -o noMain.o
"Without using main" might also mean that no logic is allowed within main, but the main itself exists. I can imagine the question had this cleared out, but since it's not cleared here, this is another possible answer:
struct MainSub
{
MainSub()
{
// do some stuff
}
};
MainSub mainSub;
int main(int argc, char *argv[]) { return 0; }
What will happen here is that the stuff in MainSub's constructor will execute before the unusable main is executed, and you can place the program's logic there. This of course requires C++, and not C (also not clear from the question).
As long as you are using g++ you could change your entry point with linker option -e, so the following code and compile command may let you create a program without a main() function:
#import <iostream>
class NoMain
{
public:
NoMain()
{
std::cout << "Hello World!" << std::endl;
exit(0);
}
} mainClass;
I gave file name as noname.cpp, and the compile option is:
g++ nomain.cpp -Wl,-e,_mainClass -v
To tell the truth, I didn't fully understand why the code can works fine. I suspect that the address of global variable mainClass is the same to constructor of NoMain class. However, I also have several reasons that I could tell my guess may not correct.
I think the macro reference was to renaming the main function, the following is not my code, and demonstrates this. The compiler still sees a main function though, but technically there's no main from a source point of view. I got it here http://www.exforsys.com/forum/c-and-c/96849-without-main-function-how-post412181.html#post412181
#include<stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf(" hello ");
}
Disregarding specific language standards, most linking loader provides some means to declare a function name (entry point) which must be executed when the binary is loaded is loaded into memory.
For old school c language, default was something like 'start' or '_start', defined in so-called crt (c runtime?), which does several householding jobs needed for c standard functions, such as preparing memory heap, initialising static variable areas, parsing command line into argc/argv, etc.
Possibly you could override the entry point function if you take enough care not to use the standard functions which requires those household things (e.g. malloc(), free(), printf(), any class definitions have custom constructor, ...)
Quite restrictive but not impossible if you use functions provided by o/s, not by standard c runtime.
For example, you can make a simple helloworld using write() function on descriptor 1.
When C or C++ code runs, it executes at a known start address, the code here initialises the run-time environment, initialises the stack pointer, performs data initialisation, calls static constructors, then jumps to main().
The code that does this is linked with your code at build time by the linker. In GCC it is usually in crt0.s, with a commercial compiler it is unlikely that this code will be available to you.
In the end, it has to start somewhere and main() is just a symbolic name for that location. It is specified by the language standard so that developers know what to call it, otherwise code would not be portable from one tool chain to another.
If you are writing code for a 'bare-metal' system with no OS or at least no OS in the sense of a process loader (embedded systems often include an RTOS kernel that is started after main()) , then you can in theory call the C code entry point whatever you wish since you usually have complete control over run-time start-up code. But do do so would be foolish and somewhat perverse.
Some RTOS environments such as VxWorks, and most application frameworks in general include main() )or its equivalent) within their own library code so that it runs before the user application code. For example VxWorks applications start from usrAppInit(), and Win32 applications start from WinMain().
Write a class and print your name in the constructor of that class and declare a GLOBAL OBJECT of that class. So the class' constructor gets executed before main. So you can leave the main empty and still print your name.
class MyClass
{
myClass()
{
cout << "printing my name..." <<endl;
}
};
MyClass gObj; // this will trigger the constructor.
int main()
{
// nothing here...
}
I realize this is an old question, but I just found this out and had to share. It will probably not work with all linkers, but it's at least possible to trick ld (I'm running version 2.24.51.20140918) into thinking there is a main-function by doing this:
int main[] {};
or even just
int main;
You can then apply one of the aforementioned tricks to let the program execute some code, e.g. through the use of a constructor:
struct Main
{
Main()
{
cout << "Hello World!\n";
exit(0);
}
} main_;
The exit(0) is to prevent the array from being "called". Good fun :-)
Yes you can do it by changing the entry point of the C language from main() to _start
Here is the code :
#include<stdio.h>
#include<stdlib.h>
int sid()
{
printf("Hallo World\n");
exit(0);
}
Then run the code using gcc complier.Let's assume that you have saved the file with the name of test.c then.
gcc -nostartfiles test.c
./a.out
Maybe it's possible to compile a .data section and fill it with code?
It depends what they mean.
Did they mean:
Write a program with no main() function.
Then generally speaking no.
But there are ways to cheat.
You can use the pre-processor to hide main() in plain sight.
Most compiler allow you to specify the entry point into your code.
By default it is main(int,char*[])
Or did they mean:
Write a program that runs code without using main (to run your code).
This is a relatively simple trick. All objects in the global namespace run their constructors before main() is entered and destruction after main() exits. Thus all you need to do is define a class with a constructor that runs the code you want, then create an object in the global namespace.
Note: The compiler is allowed to optimize these objects for delayed load (but usually does not) but to be safe just put the global in the same file as the main function (that can be empty).
Function main is only default label for address where program will start execution. So technically yes it`s possible, but you have to set name of function that will start execution in your environment.
1) Using a macro that defines main
#include<stdio.h>
#define fun main
int fun(void)
{
printf("stackoverfow");
return 0;
}
Output:
stackoverflow
2) Using Token-Pasting Operator
The above solution has word ‘main’ in it. If we are not allowed to even write main, we ca use token-pasting operator (see this for details)
#include<stdio.h>
#define fun m##a##i##n
int fun()
{
printf("stackoverflow");
return 0;
}
yes it is possible to write a program without main().
But it uses main() indirectly.
following program will help you to understand..
#include<stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,r,e)
int begin()
{
printf(” you are inside main() which is hidden“);
}
The ‘##‘ operator is called the token pasting or token merging operator. That is we can merge two or more characters with it.
In the 2nd line of the program-
define decode(s,t,u,m,p,e,d) m##s##u##t
What is the preprocessor doing here. The macro decode(s,t,u,m,p,e,d) is being expanded as “msut” (The ## operator merges m,s,u & t into msut). The logic is when you pass (s,t,u,m,p,e,d) as argument it merges the 4th,1st,3rd & the 2nd characters(tokens)
Now look at the third line of the program –
define begin decode(a,n,i,m,a,r,e)
Here the preprocessor replaces the macro “begin” with the expansion decode(a,n,i,m,a,r,e). According to the macro definition in the previous line the argument must be expanded so that the 4th,1st,3rd & the 2nd characters must be merged. In the argument(a,n,i,m,a,r,e) 4th,1st,3rd & the 2nd characters are ‘m’,’a’,’i’ & ‘n’.
so it will replace begin by main() by the preprocessor before the program is passed on for the compiler. That’s it…
By using C++ constructors you write a C++ program without the main function with nothing it it. Let's say for example we can print a hello world without writing anything in the main function as follows:
class printMe{
private:
//
public:
printMe(){
cout<<"Hello Wold! "<<endl;
}
protected:
//
}obj;
int main(){}
According to standards, main() is required and the starting point of hosted environments. That's why you've to use tricks to hide the obvious looking main, like the trick posted above.
#include <stdio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf(" hello ");
}
Here, main is written by the macro tricks. It might not be clear at once, but it eventually leads to main. If this is the valid answer for your question, this could be done much easily, like this.
# include <stdio.h>
# define m main
int m()
{
printf("Hell0");
}
Can I write a program in C or in C++ with two main functions?
No. All programs have a single main(), that's how the compiler and linker generate an executable that start somewhere sensible.
You basically have two options:
Have the main() interpret some command line arguments to decide what actual main to call. The drawback is that you are going to have an executable with both programs.
Create a library out of the shared code and compile each main file against that library. You'll end up with two executables.
You can have two functions called main. The name is not special in any way and it's not reserved. What's special is the function, and it happens to have that name. The function is global. So if you write a main function in some other namespace, you will have a second main function.
namespace kuppusamy {
int main() { return 0; }
}
int main() { kuppusamy::main(); }
The first main function is not special - notice how you have to return explicitly.
Yes; however, it's platform specific instead of standard C, and if you ask about what you really want to achieve (instead of this attempted solution to that problem), then you'll likely receive answers which are more helpful for you.
No, a program can have just 1 entry point(which is main()). In fact, more generally, you can only have one function of a given name in C.
If one is static and resides in a different source file I don't see any problem.
No, main() defines the entry point to your program and you must only one main() function(entry point) in your program.
Frankly speaking your question doesn't make much sense to me.
What do you mean by "main function"? If you mean the first function to execute when the program starts, then you can have only one. (You can only have one first!)
If you want to have your application do different things on start up, you can write a main function which reads the command line (for example) and then decides which other function to call.
In some very special architecture, you can. This is the case of the Cell Processor where you have a main program for the main processor (64-bit PowerPC Processors Element called PPE) and one or many main program for the 8 different co-processor (32-bit Synergistic Processing Element called SPE).
No, you cannot have more than one main() function in C language. In standard C language, the main() function is a special function that is defined as the entry point of the program. There cannot be more than one copy of ANY function you create in C language, or in any other language for that matter - unless you specify different signatures. But in case of main(), i think you got no choice ;)
No,The main() is the entry point to your program,since u can't have two entry points you cant have two main().
You can write it, and it'll compile, but it won't link (unless your linker is non-comformant)
The idiom is to dispatch on the value of argv[0]. With hardlinks (POSIX) you don't even lose diskspace.
Standard C doesn’t allow nested functions but GCC allows them.
void main()
{
void main()
{
printf(“stackoverflow”);
}
printf(“hii”);
}
The o/p for this code will be -hii
if you use GCC compiler.
There is a simple trick if you want to use 2 main() in your program such that both are successfully executed;you can use define.Example-
void main()
{
printf("In 1st main\n");
func1();
}
#define main func1
void main()
{
printf("In 2nd main\n");
}
Here the o/p will be:
In 1st main
In 2nd main
NOTE:here warning conflicting types of func1 will be generated.
And yes don’t change the place of define.