Blog move, original address: Learning dl Library in https://langzi989.github.io/2017/10/16/Unix/

In Linux, static link library and dynamic link library are two ways of code sharing between processes. Linux provides system calls to load and process dynamic connection libraries in the < dlfnc. H > library, which is very convenient to use. The specific usage is as follows:

Function description in dlfcn Library

The dlfcn library mainly includes four functions:

#include <dlfcn.h>

void* dlopen(const char*, int flag);

char* dlerror();

void* dlsym(void* handler, char* symbol);

int dlclose(void* handler);

• dlopen: open a dynamic connection library, and return a handler of type void *, and flag is the open mode. There are two optional modes
  • RTLD ﹣ lazy postpone decision, and solve the symbol when necessary
  • RTLD? Now immediately decides to release all undecided symbols before returning.
• dlerror: returns the error of dl operation. If no error occurs, NUlL will be returned. Otherwise, the error information will be printed
• dlsym: find the symbol symbol in the dynamic link library and return the address of the symbol
• dlclose: close DLL handle

Using examples

Dynamic link library cal.cpp

//cal.cpp
extern "C" {
int add(int a, int b) {
	return a + b;
}

int sub(int a, int b) {
	return a - b;
}

int mul(int a, int b) {
	return a * b;
}

int div(int a, int b) {
	return a / b;
}
}

Generate dynamic link library libcal.so

g++ -shared -fPIC cal.cpp libcal.so

//main.cpp
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
#define LIB_LIBRARY_PATH_1 "./libcal.so"

typedef int (*CAC_FUNC)(int ,int);

int main() {
	void* handler = NULL;
	char* error = NULL;
	CAC_FUNC cac_func = NULL;

	handler = dlopen(LIB_LIBRARY_PATH_1, RTLD_LAZY);
	if (!handler) {
		fprintf(stderr, "err:%s\n", dlerror());
		exit(1);
	}

	dlerror();

  //Here is the corresponding function address,
	*(void **) (&cac_func) = dlsym(handler, "add");
	if ((error = dlerror()) != NULL) {
		fprintf(stderr, "err:%s", error);
		exit(1);
	}

	printf("add:%d\n", cac_func(1,2));

	cac_func = (CAC_FUNC)dlsym(handler, "sub");
	printf("sub:%d\n", cac_func(1,2));

	cac_func = (CAC_FUNC)dlsym(handler, "mul");
	printf("mul:%d\n", cac_func(1,2));

	cac_func = (CAC_FUNC)dlsym(handler, "div");
	printf("div:%d\n", cac_func(1,2));

	dlclose(handler);
	return 0;
}

Compile function main:

g++ main.cpp -rdynamic -ldl

Execution result:

add:3
sub:-1
mul:2
div:0

Pay attention to problems

In particular, if you use C + + to compile a dynamic link library, you must add extern "C" to the symbol you need to use, otherwise there will be a problem that the symbol cannot be found. undefined symbol