Q:

How can I use one function from main application and bootloader? (embedded)

First of all I need to say I develop application for embedded device based on cortex m4.
I have functions that are common for bootloader and main application. For now I compile source files 2 times once for bootloader and app. But I am running short on space for dual bank dfu and I would like to have those functions only once in ROM. Any idea how can I achieve this? 
EDIT:
Using functions pointers maybe danger in some cases, check on my problems - Using pointer functions - 2 seperate applications on 1 device

A:

This is only a partial answer and assumes you can jump into your bootloader from your main code using the same address space. Then, a common technique is to provide your "bootloader API" as a table of function pointers.
Say for example you have the following functions in your bootloader:
static int do_something(void)
{
    return 42;
}

static int do_something_else(int arg)
{
    return arg+5;
}

Then you would declare your API in a header like this:
struct bootloaderApi
{
    int (*do_something)(void);
    int (*do_something_else)(int arg);
};

In the implementation of your bootloader, you define this table in its own section:
// this is GCC syntax, use whatever your compiler provides to specify the section
struct bootloaderApi api __attribute__((section("API"))) = {
    do_something,
    do_something_else
};

Then when building the bootloader, make sure your section is placed at a suitable fixed address. When e.g. using the GNU linker, you could have something like this in your linker script:
SECTIONS {
  // standard sections, e.g.:
  .text : { *(.text) }
  .data : { *(.data) } 
  .bss :  { *(.bss)  *(COMMON) }

  // your API table:
  .API 0x10000 : { *(.API) }
}

This now assumes your API table will be placed at 0x10000. Then you could do the following to access the API from your main code:
struct bootloaderApi *api = (struct bootloaderApi *)0x10000;

api->do_something();

All of this is just a sketch to give you an idea how to do this in a sensible way. It will heavily depend on your target platform and the toolchain you're using.

