How is main() called?

Hi,
Was wondering how to locate resources on the net that tell me the internals of C, how the main() is called? I tried googling with "C internals", "how main() is called" and the like but in vain.

Any pointers on the answer and some resources on the net will be useful.

Thanks,
Anoop.

What do you mean, "how main() is called"?

If you mean how you need to write it in code, use either:-

Code:

int main(void)
{
//code goes here
}

or

Code:

int main(int argc, char **argv)
{
//code goes here
}

Depending on whether you need any information from the command line or not.

If you mean "how/why does an operating system call main()", then what do you expect it to do? You could argue that it should just run programs starting at the first bytes in the file, but C/C++ are structured, function-based languages and the OS also needs to know certain information about the program before running it, i.e. how much memory it thinks it will need, which parts of it are code and which are data, etc.

Under Windows, main(...) (Or WinMain(...) for GUI-based programs) is exposed to the OS via a table usually towards the beignning of the program file known as the "export table", which tells Windows exactly where in the program to begin execution.

Older, small programs referred to as .COM files usually ran from the beginning of the file, but certain assumptions were made on how they operate and thus they were only suitable for small utilities.

Main is called by the assembly startup file created by your compiler. This assembly startup inits the needed variables and prepares your program to be run.

Then:

call main

You can disassemble the startup.obj file with a disassembler or with the one that came with your dev environment.

For HP Unix:

C compilers automatically link a _start module that is found in crt0.o -
_start calls _crt_open to create stdout for example. There are some other specialty
startup modules that you can link to instead of crt0.o.

How the compiler does this on your system is installation dependent.

You could cause to main() produce a backtrace and see for your self who called it. For a glibc based system it could look like this:

Code:

  #include <stdlib.h>
      #include <execinfo.h>
      
      int main(int argc, char *argv[]) {
        void *apRaw[16];
        char **pasReadable;
        size_t iSize;
        size_t i;
       
        iSize= backtrace(apRaw, 16);
        pasReadable= backtrace_symbols(apRaw, iSize);
      
        printf("Backtrace, %zd calls:\n", iSize);
        for (i= 0; i < iSize; i++)
      	printf("%s\n", pasReadable[i]);
       
        free(pasReadable);
  return 0;
      }

edit: I don't know why the the board messes up the code like this. sorry.

Originally Posted by Nyda

edit: I don't know why the the board messes up the code like this. sorry.

Probably because you use tabs instead of spaces.

Or, if you have a recent copy of Visual Studio, have a look in \Vc7\crt\src.

crt0.c is responsible for calling your application's entry point, while the other crt*.c files contain various support functions.

Originally Posted by itsme86

Probably because you use tabs instead of spaces.

No, I don't ever use tabs except for Makefiles. Actually every time I edit a post, code blocks get worse, even when I don't change anything in the code block. Maybe that java component editor is broken on *nix boxes?

The code box code is broken in that aspect. Any edit of your post will cause the code to be left-justified which is quite annoying. It has nothing to do with tabs or spaces.

Thanks a lot.

I wanted to know how is main() called by the compiler. I guess I should have mentioned the OS I am using. Fedore Core 2. I will try the piece of code Nyda has given. Thanks a lot for that.

Any pointers on any resources which can help me understand it more and better, on the net?

thanks,
Anoop.

I wanted to know how is main() called by the compiler.

Read above.

That was a reply to SMurf who wanted t o know what exactly I had meant by that. As I said, I would try out Nyda's code. I did. Now I am hunting for resources on the net on those functions, backtrace() and backtrace_symbols(). I did look it up in the execinfo.h but I am sure I can locate detailed info on it.


The one answer that I have not got is, if I can get any info on the net over the internals of calling mechanisms. Or should I objdump the executable and try and learn more about it? Is that the right way to proceed?


Thanks a lot for any pointers.

Anoop.

Main is called from the startup module. Its just a simple call main. In assembly a call statement signals the CPU to do several things and then jump to a new section of code.

For specifications about the calling mechanism consult the Intel IA32 and IA64 tech refs.
They break the whole process down step by step.

Thanks a lot