Want to execute program with out main

eshwar_ind · 03-01-2004, 06:51 AM

Hi!
I have a doubt, That is Can we write a C program with out main?
I think its possible. If possible I think we have to configure the loader such that it loads PC with the address of our newmain at the end of loading.
If that is right method, Can anybody explain me how to change or configure that loader (on linux)?

Strike · 03-01-2004, 07:57 AM

I'm pretty sure you can, but why? If nothing else, just call whatever you want to be your entry point from main.

Code:

int main(int argc, char **argv)
{
    return some_other_function(argc, argv);
}

deiussum · 03-01-2004, 08:37 AM

From 'info ld'

Quote:

`-e ENTRY'
`--entry=ENTRY'
Use ENTRY as the explicit symbol for beginning execution of your
program, rather than the default entry point. If there is no
symbol named ENTRY, the linker will try to parse ENTRY as a number,
and use that as the entry address (the number will be interpreted
in base 10; you may use a leading `0x' for base 16, or a leading
`0' for base 8). *Note Entry Point::, for a discussion of defaults
and other ways of specifying the entry point.

worldmagic · 03-01-2004, 09:14 AM

There's some problems with even compiling a C program without a mail.

[shadow@alien tmp]$ gcc test.c
/usr/lib/gcc-lib/i386-redhat-linux/3.3.2/../../../crt1.o(.text+0x18): In function `_start':
: undefined reference to `main'
collect2: ld returned 1 exit status

crt1.o is the startup code for the program, its automaticly linked with your binary when you.. erh, link your binary =)

running "nm" will on crt.o will show the following symbols (and missing ones).

00000000 D __data_start
00000000 W data_start
00000000 R _fp_hw
00000004 R _IO_stdin_used
U __libc_csu_fini
U __libc_csu_init
U __libc_start_main
U main
00000000 T _start

Im not totaly sure who actulay calls _start() and __libc_start_main(). But I do know that _start calls your main() in your program. So if you not intrested in replaying crt.o its hard to do a (executable) program thats missing the main symbol.

You can tell GCC not to include crt.o when compiling (using the flag: -mno-crt0)

Why do you want to skip main?

deiussum · 03-01-2004, 10:03 AM

Hmmm... according to the info for LD, you can use the following is used to determine what the starting point is.

Quote:

File: ld.info, Node: Entry Point, Next: File Commands, Up: Simple Commands

Setting the Entry Point
-----------------------

The first instruction to execute in a program is called the "entry
point". You can use the `ENTRY' linker script command to set the entry
point. The argument is a symbol name:
ENTRY(SYMBOL)

There are several ways to set the entry point. The linker will set
the entry point by trying each of the following methods in order, and
stopping when one of them succeeds:
* the `-e' ENTRY command-line option;

* the `ENTRY(SYMBOL)' command in a linker script;

* the value of the symbol `start', if defined;

* the address of the first byte of the `.text' section, if present;

* The address `0'.

However, when I try the following:

Code:

// blah.c
#include <stdio.h>

int MyMain()
{
    printf("Hello\n");

    return 0;
}

$ gcc -o blah -eMyMain blah.c
/usr/lib/gcc-lib/i486-slackware-linux/3.2.3/../../../crt1.o(.text+0x18): In func
tion `_start':
: undefined reference to `main'
collect2: ld returned 1 exit status

Or I can get it to compile like so:

Code:

$ gcc -o blah.o -c blah.c
$ ld -o blah blah.o -lc -eMyMain
$ ./blah
-bash: ./blah: No such file or directory

$ ls -l blah
-rwxr-xr-x    1 deiussum users          2040 Mar  1 09:56 blah*

I tried the -mno-crt0 option and got this:

Code:

$ gcc -o blah -eMyMain blah.c -mno-crt0
cc1: invalid option `no-crt0'

So, I'm not sure anymore. VC++ has an option like /entryPoint:SomeEntryPoint, but with Windows programming you often have either a main() or a WinMain() or a DLLMain()...