Đánh số phiên bản và SONAME

gcc -shared -Wl,-soname,libmath.so.1 *.o -o libmath.so.1.2.3

ln -sf libmath.so.1.2.3  libmath.so.1     # SONAME
ln -sf libmath.so.1      libmath.so       # ten khi lien ket

$ ls -l
lrwxrwxrwx  libmath.so     -> libmath.so.1
lrwxrwxrwx  libmath.so.1   -> libmath.so.1.2.3
-rwxr-xr-x  libmath.so.1.2.3

$ objdump -p libmath.so.1.2.3 | grep SONAME
  SONAME               libmath.so.1

$ gcc app.c -L. -lmath -o app          # -lmath tim thay libmath.so
$ objdump -p app | grep NEEDED
  NEEDED               libmath.so.1     # KHONG phai libmath.so

/* Trinh lien ket doc SONAME tu thu vien va ghi CHUOI DO vao app.
   Nen luc chay, app tim libmath.so.1, khong phai libmath.so.

   Do la ly do ban co the xoa lien ket tuong trung libmath.so
   sau khi dich xong ma chuong trinh van chay. Cac ban phan phoi
   Linux dua vao dieu nay: goi "libmath1" chua tep .so.1, con goi
   "libmath-dev" moi chua lien ket .so va cac header. */

TEN     = math
CHINH   = 1
PHU     = 2
VA      = 3

SO_DAY  = lib$(TEN).so.$(CHINH).$(PHU).$(VA)
SO_NAME = lib$(TEN).so.$(CHINH)
SO_LINK = lib$(TEN).so

$(SO_DAY): $(DOI_TUONG)
	$(CC) -shared -Wl,-soname,$(SO_NAME) $^ -o $@
	ln -sf $(SO_DAY)  $(SO_NAME)
	ln -sf $(SO_NAME) $(SO_LINK)

cai: $(SO_DAY)
	install -D -m 755 $(SO_DAY) $(DESTDIR)/usr/local/lib/$(SO_DAY)
	ln -sf $(SO_DAY)  $(DESTDIR)/usr/local/lib/$(SO_NAME)
	ln -sf $(SO_NAME) $(DESTDIR)/usr/local/lib/$(SO_LINK)
	install -D -m 644 include/$(TEN).h $(DESTDIR)/usr/local/include/$(TEN).h
	ldconfig

.PHONY: cai

# Cach dat ten co so phien ban tren MinGW:

gcc -shared *.o -o libmath1.dll -Wl,--out-implib,libmath.dll.a

#   DLL co so 1 trong ten
#   Thu vien nhap KHONG co so, nen nguoi dung van viet -lmath
#
# Khi len ban 2, ban tao libmath2.dll va mot thu vien nhap moi.
# Chuong trinh cu van tim libmath1.dll, ban ay van con tren dia.

# Va thu tu tim DLL cua Windows:
#   1. Thu muc chua tep exe          <- quan trong nhat
#   2. Thu muc he thong (System32)
#   3. Thu muc Windows
#   4. Thu muc hien tai
#   5. Cac thu muc trong PATH
#
# Tu Windows 7 co che do an toan bo buoc 4 di, vi no la lo hong
# bao mat: ke tan cong dat mot DLL gia trong thu muc hien tai.

/* mathlib.h */

#define ML_PHIEN_BAN_CHINH  1
#define ML_PHIEN_BAN_PHU    2
#define ML_PHIEN_BAN_VA     3

#define ML_PHIEN_BAN \
    (ML_PHIEN_BAN_CHINH * 10000 + ML_PHIEN_BAN_PHU * 100 + ML_PHIEN_BAN_VA)

#define ML_PHIEN_BAN_CHUOI  "1.2.3"

/* Va MOT HAM, vi macro o tren la phien ban LUC DICH: */
unsigned    ml_phien_ban(void);        /* phien ban LUC CHAY */
const char *ml_phien_ban_chuoi(void);

#include <stdio.h>
#include "mathlib.h"

int main(void) {
    unsigned luc_dich = ML_PHIEN_BAN;
    unsigned luc_chay = ml_phien_ban();

    printf("dich voi %s, chay voi %s\n",
           ML_PHIEN_BAN_CHUOI, ml_phien_ban_chuoi());

    if (luc_chay / 10000 != luc_dich / 10000) {
        fprintf(stderr, "sai ban CHINH: can %u, co %u\n",
                luc_dich / 10000, luc_chay / 10000);
        return 1;
    }

    if (luc_chay < luc_dich) {
        fprintf(stderr, "thu vien qua cu\n");
        return 1;
    }

    ...
}