Thư viện tĩnh chỉ là một kho lưu trữ

mathlib/
  include/
    mathlib.h
  src/
    hinhhoc.c
    sohoc.c
  build/
  main.c

#ifndef MATHLIB_H
#define MATHLIB_H

#define PI 3.14159

double dt_hinh_tron(double r);
double cv_hinh_tron(double r);
long   giai_thua(int n);
int    uoc_chung_lon_nhat(int a, int b);

#endif

#include "mathlib.h"

static int tru(int a, int b) { return a - b; }   /* noi bo, khong xuat ra */

long giai_thua(int n) {
    long t = 1;
    for (int i = 2; i <= n; ++i) t *= i;
    return t;
}

int uoc_chung_lon_nhat(int a, int b) { ... }

CC      = gcc
CFLAGS  = -std=c11 -Iinclude -Wall -Wextra -Wpedantic -O2

NGUON   = $(wildcard src/*.c)
DOI_TUONG = $(patsubst src/%.c,build/%.o,$(NGUON))

build/libmath.a: $(DOI_TUONG)
	ar rcs $@ $^

build/%.o: src/%.c include/mathlib.h | build
	$(CC) $(CFLAGS) -c -o $@ $<

build:
	mkdir -p build

sach:
	rm -rf build

.PHONY: sach

# Chu y "| build": day la phu thuoc thu tu.
# No bao dam thu muc build ton tai truoc khi dich, nhung KHONG
# lam .o phai dich lai moi khi thu muc doi thoi diem sua.

ar t libmath.a          # liet ke ten tep .o
ar tv libmath.a         # kem kich thuoc va thoi diem
ar x libmath.a          # GIAI NEN: lay lai cac tep .o ra thu muc hien tai
ar x libmath.a sohoc.o  # chi lay mot tep
ar d libmath.a sohoc.o  # xoa mot tep khoi thu vien
ar r libmath.a moi.o    # them hoac thay the mot tep

nm libmath.a            # ky hieu cua tung tep .o
nm --defined-only libmath.a   # bo cac ky hieu chua dinh nghia
objdump -a libmath.a    # dau muc cua tung thanh vien

# Cach do: dem so ky hieu that su vao trong chuong trinh

nm app.exe | grep -E 'dt_hinh_tron|cv_hinh_tron|giai_thua'

# Hoac de trinh bien dich chia nho gium:

gcc -ffunction-sections -fdata-sections -c src/*.c
gcc main.o -Wl,--gc-sections -Lbuild -lmath -o app

#  -ffunction-sections   moi ham vao mot phan doan rieng
#  -fdata-sections       moi bien vao mot phan doan rieng
#  --gc-sections         trinh lien ket xoa phan doan khong ai dung
#
# Ba co nay lam viec chia .o thanh nho tu dong. Rat dang bat
# trong ma nhung, noi tung KB deu quy. Bai 46.3 da noi.