how do we call assembly function in c?

Hello :) i got a serious problem to call my assembly function in my c program, it said that it's undefined reference.

here my assembly code :

[GLOBAL gdt_flush]gdt_flush:    mov eax, [esp+4]  ; Get the pointer to the GDT, passed as a parameter.    lgdt [eax]        ; Load the new GDT pointer    mov ax, 0x10      ; 0x10 is the offset in the GDT to our data segment    mov ds, ax        ; Load all data segment selectors    mov es, ax    mov fs, ax    mov gs, ax    mov ss, ax    jmp 0x08:.flush   ; 0x08 is the offset to our code segment: Far jump!.flush:    ret

then my c code :my file where i put the call of the function :

#include <k/types.h>#include "io.h"#include "init_tables.h"#include "events.h"extern void gdt_flush(u32 gdt_ptr);/* 1. SERIAL PORT */void init_uart(void)                //Initialize uart 16550 for 38400 bps{    u8 ier = inb(SERIAL_PORT + 1);    u8 fcr = inb(SERIAL_PORT + 2);    u8 lcr = inb(SERIAL_PORT + 3);    u16 rate = 38400;    outb(SERIAL_PORT + 3, lcr | 1 << 7);    outb(SERIAL_PORT + 0, (115200 / rate) & 255); // = 0x3    outb(SERIAL_PORT + 1, (115200 / rate) >> 8);  // = 0x0    outb(SERIAL_PORT + 3, lcr & ~(1 << 7));    outb(SERIAL_PORT + 2, fcr | 1 << 1);    outb(SERIAL_PORT + 2, fcr | 1 << 2);    outb(SERIAL_PORT + 2, fcr | 1 << 6);    outb(SERIAL_PORT + 2, fcr | 1 << 7);    outb(SERIAL_PORT + 1, ier | 1 << 1);}/* 2. MEMORY MANAGER */static struct gdt_entry gdt[5];static struct gdt_r gdt_r;static void set_gdt_entry(unsigned index, u32 base_adress, u32 limit, u16 access, u16 flags)/* Function to initialize an index of the gtd_entry structure */{    if (index >= SIZE_ARRAY(gdt))        printf("ARRAY SIZE INCORRECT\n");    gdt[index].seg_lim_15_00 = (limit & 0xFFFF); // set segment limit of 00-15    gdt[index].granularity = (limit >> 16) & 0x0F; // set segment of 16-19     gdt[index].granularity |= flags & 0xF0; // granularity, operand size, zero and avl  -> flags    gdt[index].access_byte = access; //set up type, s, dpl, p     gdt[index].base_15_00 = (base_adress & 0xFFFF); //set up base 00-15 bit    gdt[index].base_23_16 = (base_adress >> 16) & 0xFF; //set up base 16-23 bit    gdt[index].base_31_24 = (base_adress >> 24) & 0xFF; //set up base 24-31 bit}static void gdt_load(void)        /* Function to load our gtd and reload segment selectors*/{    __asm__ volatile ("lgdt %[gdt_r]\n""mov  %%cr0, %%eax\n""or      $1, %%eax\n""mov  %%eax, %%cr0\n""mov  %[ds], %%ax\n""mov   %%ax, %%ds\n""mov   %%ax, %%es\n""mov   %%ax, %%fs\n""mov   %%ax, %%gs\n"                      //"mov   %%ax, %%ss\n""ljmp %[cs], $set_cs\n""set_cs:\n"                      :                      : [gdt_r]"m"(gdt_r), [cs]"i"(0x8), [ds]"i"(0x10)                      : "%eax");    /*asm volatile(" lgdt %0\n" : : "m"(gdt_r) : "memory"); //LOAD GTD    asm volatile ("movl %cr0, %eax\n\t" // activate protected mode"or %al, 1\n\t""movl %eax, %cr0\n\t""movw $0x10, %ax\n\t""movw %ax, %ds\n\t""movw %ax, %es\n\t""movw %ax, %fs\n\t""movw %ax, %gs\n\t""movw %ax, %ss\n\t""ljmp $0x08, $1f\n\t""1:\n\t");*/}static void init_gdt(void){    gdt_r.limit = sizeof(gdt) - 1;    gdt_r.base  = (u32)&gdt;    set_gdt_entry(0, 0, 0, 0, 0);                /* Null segment */    set_gdt_entry(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); /* Code segment */    set_gdt_entry(2, 0, 0xFFFFFFFF, 0x92, 0xCF); /* Data segment */    //set_gdt_entry(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); /* User mode code segment */    //set_gdt_entry(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); /* User mode data segment */    gdt_flush((u32)&gdt_r);    //gdt_load();}void init_kernel(void){    init_uart();    init_gdt();    //init_interrupt();    //load_idt();    //print_stack();}

then my Makefile :

## Copyright (c) LSE# All rights reserved.## Redistribution and use in source and binary forms, with or without# modification, are permitted provided that the following conditions are met:#     * Redistributions of source code must retain the above copyright#       notice, this list of conditions and the following disclaimer.#     * Redistributions in binary form must reproduce the above copyright#       notice, this list of conditions and the following disclaimer in the#       documentation and/or other materials provided with the distribution.## THIS SOFTWARE IS PROVIDED BY LSE AS IS AND ANY# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE# DISCLAIMED. IN NO EVENT SHALL LSE BE LIABLE FOR ANY# DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.#include ../config.mkTARGET  = kOBJS    = \      crt0.o \      k.o \      libvga.o \      list.o \      memory.o \      io.o \      init_tables.o \      #events.o      #isr.oDEPS = $(OBJS:.o=.d)all: $(TARGET)$(TARGET): CPPFLAGS += -MMD -Iinclude -I ../libs/libc/include/$(TARGET): CFLAGS += $(K_EXTRA_CFLAGS)$(TARGET): LDFLAGS += -Wl,-Tk.lds$(TARGET): LDLIBS = -L../libs/libc -lc$(TARGET): $(OBJS) init_tables.sinstall: $(TARGET)    $(INSTALL) $(TARGET) $(INSTALL_ROOT)/$(TARGET)clean:    $(RM) $(OBJS) $(DEPS) $(TARGET)-include $(DEPS)

Right now i put a gas file to be execute at the loading so i can't understand why it doesnt work and why i need to link these files.I was sure that the link is doing it automaticallyCan somebody help me ?Thanks a lot :)