How to use functions in a library

Hello
I created a library that includes a function.

#include "AT91SAM7x256.H"

void ASD(void)
{
        AT91C_BASE_PMC->PMC_PCER=1<<2;
}

int main()
{

}

I added the lib file to an executable project, and when I use ASD(); , I take two errors:

Rebuild target 'Target 1'
assembling SAM7.s...
compiling Main_Exec.c...
Main_Exec.c(6): warning:  #223-D: function "ASD" declared implicitly
  ASD();
Main_Exec.c: 1 warning, 0 errors
linking...
.\Executable_P.axf: Error: L6200E: Symbol __ARM_use_no_argv multiply defined (by main_lib.o and main_exec.o).
.\Executable_P.axf: Error: L6200E: Symbol main multiply defined (by main_lib.o and main_exec.o).
Not enough information to list image symbols.
Not enough information to list the image map.
Finished: 2 information, 0 warning and 2 error messages.
".\Executable_P.axf" - 2 Error(s), 1 Warning(s).
Target not created

What's wrong with the procedure? :(
I forgot something? or
what configurations should I apply?

Thanks