Nạp thư viện lúc chạy

#include <dlfcn.h>

void *dlopen(const char *duong_dan, int co);
void *dlsym(void *tay_cam, const char *ten);
int   dlclose(void *tay_cam);
char *dlerror(void);

/* Dich voi -ldl tren Linux. Tren macOS va cac he BSD moi thi
   khong can, chung nam san trong libc. */
gcc main.c -ldl -o app

#include <stdio.h>
#include <dlfcn.h>

int main(void) {
    dlerror();                          /* xoa loi cu, xem chu thich duoi */

    void *h = dlopen("./libplugin.so", RTLD_NOW);
    if (h == NULL) {
        fprintf(stderr, "dlopen: %s\n", dlerror());
        return 1;
    }

    int (*xu_ly)(const char *);
    *(void **)(&xu_ly) = dlsym(h, "xu_ly");

    const char *loi = dlerror();
    if (loi != NULL) {
        fprintf(stderr, "dlsym: %s\n", loi);
        dlclose(h);
        return 1;
    }

    xu_ly("du lieu");

    dlclose(h);
    return 0;
}

# Ba duong dan dac biet cua dlopen

dlopen("libmath.so.1", RTLD_NOW);   /* tim theo quy tac thong thuong:
                                       rpath, LD_LIBRARY_PATH, ld.so.cache */

dlopen("./libmath.so", RTLD_NOW);   /* co dau gach cheo -> DUONG DAN,
                                       khong tim o dau khac */

dlopen(NULL, RTLD_NOW);             /* tay cam toi CHINH CHUONG TRINH.
                                       Cho phep dlsym tim ky hieu trong
                                       ban thuc thi va moi thu vien da nap */

/* Cach cuoi rat huu ich: */
void *ta = dlopen(NULL, RTLD_NOW);
void *f  = dlsym(ta, "ham_nguoi_dung_dinh_nghia");
if (f != NULL) { /* nguoi dung co cung cap ban thay the */ }

#include <windows.h>

HMODULE LoadLibraryA(const char *ten);
FARPROC GetProcAddress(HMODULE h, const char *ten);
BOOL    FreeLibrary(HMODULE h);
DWORD   GetLastError(void);

/* Khong can co dich them: chung nam trong kernel32, luon co san. */

#include <stdio.h>
#include <windows.h>

int main(void) {
    HMODULE h = LoadLibraryA("./libplugin.dll");
    if (h == NULL) {
        fprintf(stderr, "LoadLibrary that bai, loi %lu\n",
                (unsigned long)GetLastError());
        return 1;
    }

    int (*xu_ly)(const char *);
    xu_ly = (int (*)(const char *))(void *)GetProcAddress(h, "xu_ly");

    if (xu_ly == NULL) {
        fprintf(stderr, "khong co ham xu_ly, loi %lu\n",
                (unsigned long)GetLastError());
        FreeLibrary(h);
        return 1;
    }

    xu_ly("du lieu");

    FreeLibrary(h);
    return 0;
}

/* 1. Macro, cach hay dung nhat */
#define NAP_HAM(bien, tay_cam, ten) \
    (*(void **)(&(bien)) = dlsym((tay_cam), (ten)))

int (*xu_ly)(const char *);
NAP_HAM(xu_ly, h, "xu_ly");

/* 2. Union, ro rang hon voi nguoi doc */
union { void *du_lieu; int (*ham)(const char *); } u;
u.du_lieu = dlsym(h, "xu_ly");
int (*xu_ly)(const char *) = u.ham;

/* 3. memcpy, khong the che trach gi ve mat chuan */
void *p = dlsym(h, "xu_ly");
int (*xu_ly)(const char *);
memcpy(&xu_ly, &p, sizeof xu_ly);

/* Ca ba deu cho ra cung mot ma may. Chon cach nao la chuyen
   phong cach, nhung dung dung phep ep truc tiep. */

#ifndef THUVIEN_H
#define THUVIEN_H

typedef struct TayCam TayCam;

/* Nap mot thu vien. Tra ve NULL neu that bai. */
TayCam *tv_nap(const char *duong_dan);

/* Tim mot ham. Tra ve NULL neu khong co. */
void *tv_ham(TayCam *h, const char *ten);

/* Dong. Chap nhan NULL. */
void tv_dong(TayCam *h);

/* Thong bao loi cua lan goi that bai gan nhat. Khong bao gio NULL. */
const char *tv_loi(void);

/* Macro nap ham an toan ve kieu */
#define TV_HAM(bien, tay_cam, ten) \
    (*(void **)(&(bien)) = tv_ham((tay_cam), (ten)))

#endif

#include <stdio.h>
#include <string.h>
#include "thuvien.h"

static char g_loi[512];

#if defined(_WIN32)

#include <windows.h>

struct TayCam { HMODULE h; };

static void ghi_loi(const char *viec) {
    DWORD ma = GetLastError();
    char *tin = NULL;

    FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER
                 | FORMAT_MESSAGE_FROM_SYSTEM
                 | FORMAT_MESSAGE_IGNORE_INSERTS,
                   NULL, ma, 0, (char *)&tin, 0, NULL);

    snprintf(g_loi, sizeof g_loi, "%s: loi %lu: %s",
             viec, (unsigned long)ma, tin ? tin : "(khong ro)");
    LocalFree(tin);
}

TayCam *tv_nap(const char *duong_dan) {
    HMODULE h = LoadLibraryA(duong_dan);
    if (h == NULL) { ghi_loi("nap"); return NULL; }

    TayCam *t = malloc(sizeof *t);
    if (t == NULL) { FreeLibrary(h); return NULL; }

    t->h = h;
    return t;
}

void *tv_ham(TayCam *t, const char *ten) {
    if (t == NULL) return NULL;

    void *p = (void *)GetProcAddress(t->h, ten);
    if (p == NULL) ghi_loi("tim ham");
    return p;
}

void tv_dong(TayCam *t) {
    if (t == NULL) return;
    FreeLibrary(t->h);
    free(t);
}

#else

#include <dlfcn.h>
#include <stdlib.h>

struct TayCam { void *h; };

TayCam *tv_nap(const char *duong_dan) {
    dlerror();

    void *h = dlopen(duong_dan, RTLD_NOW | RTLD_LOCAL);
    if (h == NULL) {
        snprintf(g_loi, sizeof g_loi, "nap: %s", dlerror());
        return NULL;
    }

    TayCam *t = malloc(sizeof *t);
    if (t == NULL) { dlclose(h); return NULL; }

    t->h = h;
    return t;
}

void *tv_ham(TayCam *t, const char *ten) {
    if (t == NULL) return NULL;

    dlerror();
    void *p = dlsym(t->h, ten);

    const char *loi = dlerror();
    if (loi != NULL) {
        snprintf(g_loi, sizeof g_loi, "tim ham %s: %s", ten, loi);
        return NULL;
    }
    return p;
}

void tv_dong(TayCam *t) {
    if (t == NULL) return;
    dlclose(t->h);
    free(t);
}

#endif

const char *tv_loi(void) { return g_loi[0] ? g_loi : "khong co loi"; }

/* Chuong trinh chay duoc khong can thu vien nay, nhung co thi tot hon */

static int (*nen_du_lieu)(const void *, size_t, void *, size_t *);
static TayCam *h_nen;

void khoi_tao_nen(void) {
    h_nen = tv_nap("libzstd.so.1");
    if (h_nen == NULL) {
        printf("khong co libzstd, tat tinh nang nen\n");
        return;
    }
    TV_HAM(nen_du_lieu, h_nen, "ZSTD_compress");
}

int luu(const void *d, size_t n) {
    if (nen_du_lieu != NULL) return luu_co_nen(d, n);
    return luu_khong_nen(d, n);
}

/* Uu diem so voi lien ket luc dich: goi cua ban khong PHU THUOC
   vao libzstd, nen cai duoc tren may khong co no. */

/* Nap ban toi uu cho CPU dang co */

const char *ban;
if (co_avx512())      ban = "libtoan_avx512.so";
else if (co_avx2())   ban = "libtoan_avx2.so";
else                  ban = "libtoan_sse2.so";

TayCam *h = tv_nap(ban);

/* Cach nay duoc dung trong thu vien xu ly anh, mat ma va hoc may.
   C con co mot co che khac cho viec nay, la ifunc cua GNU, nhung
   dlopen de hieu va de go loi hon nhieu. */

/* Sua ma game, dich lai, va thay ngay ma khong khoi dong lai:

     while (dang_chay) {
         if (tep_da_doi("game.so")) {
             luu_trang_thai(&tt);
             tv_dong(h);
             h = tv_nap("game.so");
             TV_HAM(cap_nhat, h, "cap_nhat");
             khoi_phuc_trang_thai(&tt);
         }
         cap_nhat(&tt, dt);
     }

   Ky thuat nay ten la hot reload. Dieu kien: MOI trang thai phai
   nam trong mot struct do CHUONG TRINH CHINH so huu, khong phai
   trong bien toan cuc cua thu vien. Vi thu vien bi go va nap lai,
   moi bien toan cuc cua no bi dat lai. */