How to change the entry point main() in c langauge

[maheshgupta024]

Can anyone give me the solution, for changing the entry point instead of main function.

[r035198x]

Can anyone give me the solution, for changing the entry point instead of main function.

And why would you want to do that?

[weaknessforcats]

Check your linker options. There is usually an option where you can specify the entry point function. main() is just the default.

If you are using Visual Studio.NET you select project properties->Configuration Properties->Linker->Advanced and on the right pane of the dialog is a place for the name of the entry point function.

[maheshgupta024]

Check your linker options. There is usually an option where you can specify the entry point function. main() is just the default.

If you are using Visual Studio.NET you select project properties->Configuration Properties->Linker->Advanced and on the right pane of the dialog is a place for the name of the entry point function.

sorry for delayed reply.. Im working as developer, we develop applications in Linux OS, so i want to know in linux

[JosAH]

For gcc try the '-e <symbol>' flag to make your program start at <symbol>
instead of the default 'main' entry point.

kind regards,

Jos

[AmberJain]

OK, I went thorugh the complete thread but there are some doubts---->

1. Why would one like to do something like this? (could not find appropriate explanation about this on GOOGLE) What are it's practical uses?


2. Even if it's possible with some practical uses, this contradicts the standard i.e.
According to Ritchie and Kernigham's "The C programming language" book (page10-second edition-ANSI C)-
"........your program begins executing at the beginning of main. This means that every program must have a main somewhere"

So if program begins executin at main, how's this possible? Doesn't it contradicts the standard?

_NOTE_I may be wrong here as I am a newbie
PLUS I don't own a latest edition of The C programming language BOOK by Dennis Ritchie and Brian Kernigham
PLUS I have just started reading the above mentioned book for the first time.
So please guide me....


THANKS TO EVERY ONE IN ADVANCE...............

============
AmbrNewlearner
============

[Laharl]

K&R C hasn't been standard for nearly 20 years now, though it's still a great book. As a beginner, yes, everything starts with main. This is because when you begin execution of a compiled C program, there is a function that calls main. By changing the entry point, you change what function that calls (I think). I'm not entirely sure why it would be useful, but I'm sure there are uses.

[Plater]

Only use I could come up with is to create "different" programs from the same source code.
For example I suppose you could compile with "DebugMain" as your entry point to perform lots of other little things or something? And then compile with "ReleaseMain" as entry point when you are satisfied.
A quick change in the tool chain could do these things automatically, perhaps easier for developing?

[oler1s]

Why would one like to do something like this?

Writing a non-standard program. An example would be a Windows program, which uses WinMain, not main. Another would be the Boost.Test main for testing.

Even if it's possible with some practical uses, this contradicts the standard

Yep. The implications of writing OS specific or API specific programs.

So if program begins executin at main

Technically, main is not the entry point. It’s compiler implemented. That compiler implementation calls main(). From a standards perspective, the compiler implementation is irrelevant.

[Banfa]

Technically, main is not the entry point. It’s compiler implemented. That compiler implementation calls main(). From a standards perspective, the compiler implementation is irrelevant.

This becomes very obvious on some embedded platforms like the one I am using now.

The processor has a start-up mechanism as follows

1. Jump to the location stored in program address 0
2. Execute from there

This code all has to be implemented in assembler and includes initialising the C data segments as required as well as other platform related stuff like initialising the memory management unit.

A default implementation of this code is provided with the tool chain and the last thing it does is

bsr $main

If you don't know bsr is the assembler directive to branch to subroutine, basically start running from main. It could be anything though.

All programs written in C have this sort of start-up code in them somewhere, something has to initialise the C data segments (stack, heap, global data etc). But on some platforms, PCs for instance this is hidden from you by the tool chain. On many embedded platforms this is available for editing but a default implementation is provided.

[maheshgupta024]

For gcc try the '-e <symbol>' flag to make your program start at <symbol>
instead of the default 'main' entry point.

kind regards,

Jos

This is the test program I have written and compiled with
$>cc -e test test.c -o test
#include <stdio.h>

int test(void)
{
printf("Hai\n");
main();
return 1;
}


main()
{
printf("OK\n");
}
The output is as follows :
Hai
OK
Segmentation fault

Can anyone tell what's the reason for the Segmentation fault

Thanks in advance

[Banfa]

you have not declared the return type of main

int main()
{
}
 

[JosAH]

you have not declared the return type of main

int main()
{
}
 

That's not it: the -e flag changes the address of the startup code. Startup code
has the responsibility of returning to the OS again so it can perform its process
cleanup code and other bookkeeping stuff. The test() function simply returns;
but there is no place to return to and hence the segmentation fault.

That function should call the exit() function instead; maybe that'll work ...

kind regards,

Jos

[r035198x]

An exit status should do the trick.
@OP You'd need to include stdlib for that of course.

[AmberJain]

For gcc try the '-e <symbol>' flag to make your program start at <symbol>
instead of the default 'main' entry point.

kind regards,

Jos

I'm using Bloodshed Dev c++ 4.9.9.2 with Mingw port of GCC (GNU Compiler Collection) as its compiler.

Well, I wrote a simple code in C. It is given below--->

#include <stdio.h>
 
int test(void);
 
int main()
{
    int a=1;
    printf("\nHELLO main %d",a);
    printf("\n\nPRESS ANY KEY TO EXIT");
    getch();
    return(0);
}
 
int test(void)
{
   int val; 
   printf("\nHELLO test()");
   val=main();
   return(0);
}
 

Now, what I want is that test() must be entry point instead of main(). What should I do?

THANKS IN ADVANCE.............

============
AmbrNewlearner
============

[JosAH]

Now, what I want is that test() must be entry point instead of main(). What should I do?

My question is: why do you want to do that? It doesn't buy you much and you
have to take care that your test() function returns to the OS properly; simply
returning doesn't do it. That -e flag tells the linker the name of the startup code.
Normally that'd be c0 or crt0 or similar that does all the nitty gritty work for you.
Why do it the difficult way when it has been made easy for you?

Since you're a beginner, writing a proper startup (and exit) functionality is way
over your head; don't go there now but write proper C instead.

kind regards,

Jos

[maheshgupta024]

An exit status should do the trick.
@OP You'd need to include stdlib for that of course.

Thanks for the reply.. it is working..
But my question is.. if the exit is so required for C program.. then even if we dont give the exit in main function.. it doesnt give any seg. fault..
What is the reason behind this????

[r035198x]

Thanks for the reply.. it is working..
But my question is.. if the exit is so required for C program.. then even if we dont give the exit in main function.. it doesnt give any seg. fault..
What is the reason behind this????

Changing your entry point from main also means that your exit point has changed. You don't get the seg fault if you put the exit status in the entry point function.

[AmberJain]

My question is: why do you want to do that? It doesn't buy you much and you
have to take care that your test() function returns to the OS properly; simply
returning doesn't do it. That -e flag tells the linker the name of the startup code.
Normally that'd be c0 or crt0 or similar that does all the nitty gritty work for you.
Why do it the difficult way when it has been made easy for you?

Since you're a beginner, writing a proper startup (and exit) functionality is way
over your head; don't go there now but write proper C instead.

kind regards,

Jos

OK, for the time being (till I get A LITTLE expertise in C) I will stick to as you advised.

Thanks JosAH.....

============
AmbrNewlearner
============

[JosAH]

OK, for the time being (till I get A LITTLE expertise in C) I will stick to as you advised.

Thanks JosAH.....

You're welcome of course. Just to lift a tip of the curtain, the following code shows
(conceptually) what startup code must do; the next function is the target of any
-e flag:

void startup() {
   int argc= system_dependent_argc();
   char** argv= system_dependent_argv();
 
   exit(main(argc, argv)); /* call main and exit */
}
 
void exit(int exit_code) {
   system_dependent_exit(exit_code);
}
 

You figure out the system_dependent_* functions and you're in business.

kind regards,

Jos

ps. As a side effect the above code snippet perfectly show why main() is supposed
to return an int and not void as Microsoft keeps suggesting.

[Banfa]

You're welcome of course. Just to lift a tip of the curtain, the following code shows
(conceptually) what startup code must do;

Don't you think that the start-up code would also include initialising all the data-segments that are expected to exist, heap, stack, initialised global data, uninitialised global data, constant data, etc. plus processor registers (some/many processors have stack registers) for the given platform?

[JosAH]

Don't you think that the start-up code would also include initialising all the data-segments that are expected to exist, heap, stack, initialised global data, uninitialised global data, constant data, etc. plus processor registers (some/many processors have stack registers) for the given platform?

No, I consider that a job of the 'binder' i.e. the loader-editor that is supposed to
resolve symbolic or relative addresses to virtual addresses. I know that embedded
systems don't have a proper binder (a unixism?) but nevertheless, I know about
all that crapola ;-)

kind regards,

Jos

[AmberJain]

You're welcome of course. Just to lift a tip of the curtain, the following code shows
(conceptually) what startup code must do; the next function is the target of any
-e flag:

void startup() {
   int argc= system_dependent_argc();
   char** argv= system_dependent_argv();
 
   exit(main(argc, argv)); /* call main and exit */
}
 
void exit(int exit_code) {
   system_dependent_exit(exit_code);
}
 

You figure out the system_dependent_* functions and you're in business.

kind regards,

Jos

ps. As a side effect the above code snippet perfectly show why main() is supposed
to return an int and not void as Microsoft keeps suggesting.

:=0
HUH.....I accept it's a lot advanced C programming. So what I have decided is that I will stick to PROPER C CODE (as you advised me in REPLY #16) till I get a good nerve of C. And then I will try all this advanced stuff.

BTW, thanks for your help.....

============
AmbrNewlearner
============

[lipu]

I'm using Bloodshed Dev c++ 4.9.9.2 with Mingw port of GCC (GNU Compiler Collection) as its compiler.

Well, I wrote a simple code in C. It is given below--->

#include <stdio.h>
 
int test(void);
 
int main()
{
    int a=1;
    printf("\nHELLO main %d",a);
    printf("\n\nPRESS ANY KEY TO EXIT");
    getch();
    return(0);
}
 
int test(void)
{
   int val; 
   printf("\nHELLO test()");
   val=main();
   return(0);
}
 

Now, what I want is that test() must be entry point instead of main(). What should I do?

THANKS IN ADVANCE.............

============
AmbrNewlearner
============

do 1 thing ...just delete
val=main() becoz....val cannot call the function main
and rewrite the declaration int val( )
as u have called the main function above
here u also have to call..
see it will work..

[JosAH]

do 1 thing ...just delete
val=main() becoz....val cannot call the function main
and rewrite the declaration int val( )
as u have called the main function above
here u also have to call..
see it will work..

Please read up on a bit of C programming because what you just wrote doesn't
make any sense at all. main() is a function that returns an int so the assignment
makes perfect sense.

kind regards,

Jos

[AmberJain]

Please read up on a bit of C programming because what you just wrote doesn't
make any sense at all. main() is a function that returns an int so the assignment
makes perfect sense.

kind regards,

Jos

Thans JosAh.......Always guiding towards the right way and in time.
When I read reply posted by lipu in REPLY #24 above, I was just amazed by what lipu said. But then you came to rescue and that's where you are perfect.

[JosAh]------>"A man always there to guide me..................."

THANKS.................for your plenty of guidance.

=============
AmbrNewlearner
=============