Referring symbols from linker file

Hey there,
Simply, i tried to use symbols from Keil's linker script but keil outputs "identifier __bss_end__ is undefined"

I found many symbols defined in gcc_arm.ld file and i want to use one of them within other files (C or Assembly).
Can anyone tell what makes me use these symbols within other source codes ???

/* A simple C code */
void init(void)
{
        char *ptr = &__bss_end__;



}

/* Linker script to configure memory regions. */
MEMORY
{
  FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 0x40000   /* 256k */
  RAM (rwx)  : ORIGIN = 0x20000000, LENGTH = 0x08000   /*  32k */
}

/* Library configurations */
GROUP(libgcc.a libc.a libm.a libnosys.a)

/* Linker script to place sections and symbol values. Should be used together
 * with other linker script that defines memory regions FLASH and RAM.
 * It references following symbols, which must be defined in code:
 *   Reset_Handler : Entry of reset handler
 *
 * It defines following symbols, which code can use without definition:
 *   __exidx_start
 *   __exidx_end
 *   __copy_table_start__
 *   __copy_table_end__
 *   __zero_table_start__
 *   __zero_table_end__
 *   __etext
 *   __data_start__
 *   __preinit_array_start
 *   __preinit_array_end
 *   __init_array_start
 *   __init_array_end
 *   __fini_array_start
 *   __fini_array_end
 *   __data_end__
 *   __bss_start__
 *   __bss_end__
 *   __end__
 *   end
 *   __HeapLimit
 *   __StackLimit
 *   __StackTop
 *   __stack
 */
ENTRY(Reset_Handler)