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()?
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.
main. –
Kakaaba 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
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 a freestanding environment & what is a 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 example an OS kernel or embedded system 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>
main, check the quote from the standard. –
Keesee ## operator isn't really changing the name of main - main is still there after the pre-processor translation phases. So any answers suggesting tricks with ## are perhaps more interested in showing off entertaining obfuscation tricks (shamelessly plagiarized from external sources) than to answer the actual question. –
Kellene 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.
main. –
Kakaaba 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/
#define begin() main(), You can just add a lot of lesser known/popular C/C++ constructs and mask it but the main has to be defined for a hosted environment implementation,Adding this comment here because without this the answer might be little misleading to newbies. –
Keesee gcc -E file.c would give you the code which is actually being compiled. –
Aghast 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.
$ 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
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).
MainSub::MainSub() be executed? You don't construct a MainSub object anywhere? –
Consideration int main. –
Billfold int, its value would be undefined, no? –
Consideration int, it would have been initialized to 0. –
Billfold 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.
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().
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 :-)
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.
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...
}
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
Arriving late to the party - there are lot of questionable answers to this old post. A better one to use as reference might be What should main() return in C and C++? for example the answer by yours sincerely there, with relevant quotes from the language standards.
Brief summary:
Both C and C++ make a difference between hosted programs and freestanding programs. Hosted programs are those running on top of an OS. Freestanding programs are either the OS itself or a program written for a RTOS, or a so-called "bare-metal" embedded system with no OS at all.
A compiler + all the standard libs supported is formally called an implementation by the standards. And so a hosted implementation is a compiler + standard libs for an OS environment, whereas a freestanding implementation is an embedded systems compiler. Freestanding implementations are allowed by the standard to implement a minimal subset of the standard library.
For freestanding programs, both C and C++ allow implementation-defined forms of the function first executed. Implementation-defined meaning that the compiler might chose a different name and format for the function first executed and will document that name and format.
It does not mean that the programmer gets to decide the name or format of that function - anyone attempting such is writing a non-conforming/ill-formed program. And so such programs are to regarded as extensions beyond the language - it is something else than C or C++.
C++ is a bit pickier than C since it forces freestanding implementations that want to use the name main for the implementation-defined form to also use the format int main(). This is quite inconvenient for bare metal programs since it sometimes creates a bit of useless overhead where the "CRT" calls main. C is more relaxed and has no such requirement, and so the most common implementation-defined form in freestanding systems is void main (void).
Implementation-defined forms are also (arguably) allowed for hosted systems. Those familiar with Windows programming may recognize WinMain, for example. C allows such implementation-defined forms, but it is debatable if C++ does or if such forms are just non-conforming/ill-formed.
Once again, implementation-defined forms are only allowed if the compiler supports them and document them. The programmer cannot go ahead and invent their own forms.
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.
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).
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;
}
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(){}
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.
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-
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 –
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…
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