C++ call dlopen function_c++ dlopen function-优快云博客

本文介绍了一个 C++ 程序如何使用动态链接库 (DLL) 的示例。通过创建一个简单的库文件 libmylib.so 和调用程序 mypgm，演示了如何在运行时加载和调用库中的函数 myFun。文章还讨论了 C++ 符号名称修饰的问题及解决方法。

Description

This is a sample of how C++ call a dlopen function.

1. mypgm.cpp : the main program

2. mylib.cpp: the main library

mypgm.cpp

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

typedef char * (* myFun_t)(const char *, int &);

const char * LIB = "libmylib.so";
const char * FUN = "myFun";
//const char * FUN = "_Z5myFunPKcRi";

int main(int argc, char * argv[]) {
    void * handle = dlopen(LIB, RTLD_NOW);
    if (handle == NULL) {
        printf("Cannot load library: %s\n", LIB);
        return -1;
    }

    myFun_t myFun = (myFun_t) dlsym(handle, FUN);
    if (myFun == NULL) {
        printf("Cannot get function: %s\n", FUN);
        return -1;
    }

    for (int i = 0; i < 3; i++) {
        int length = i;
        char * buffer = myFun("any", length);
        if (buffer == NULL) {
            printf("Failed to call myFun\n");
            return -1;
        }
        else {
            printf("CALLER: myFun() return [%d],[%s]\n", length, buffer);
        }
    }

    int ret = dlclose(handle);
    if (ret != 0) {
        printf("Cannot call dlclose\n");
        return -1;
    }

    return 0;
}

mylib.cpp

#include <stdio.h>

//extern "C" char * myFun(const char * filename, int & len);
//extern "C" { char * myFun(const char * filename, int & len); }

extern "C" 
char * myFun(const char * filename, int & len) {
    static int count = 0;
    printf("CALLEE: myFun(len=[%d]), count=[%d]\n", len,  ++count);

    len = count;
    char * ret = "aaa";

    return ret;
}

makefile

     1
     2  TARGET=mypgm libmylib.so
     3
     4  all: $(TARGET)
     5
     6  mypgm: mypgm.cpp
     7          g++ -o $@ $< -ldl
     8
     9  libmylib.so: mylib.cpp
    10          g++ -fPIC -shared -o $@ $<
    11
    12  .PHONY: clean
    13
    14  clean:
    15          -rm -rf $(TARGET)

* Execute

$ make
g++ -o mypgm mypgm.cpp -ldl
g++ -fPIC -shared -o libmylib.so mylib.cpp
$ ./mypgm 
CALLEE: myFun(len=[0]), count=[1]
CALLER: myFun() return [1],[aaa]
CALLEE: myFun(len=[1]), count=[2]
CALLER: myFun() return [2],[aaa]
CALLEE: myFun(len=[2]), count=[3]
CALLER: myFun() return [3],[aaa]

C++ mangle

The function myFun defined in mylib.cpp must be declared with "extern C", otherwise, the caller could not find the symbol, because C++ will mangle the symbol name.

Take above example: if there is no "extern C", the myFun function will be given a name as "_Z5myFunPKcRi", so when caller (mypgm) use

myFun_t myFun = (myFun_t) dlsym(handle, "myFun") to find the function symbol, it will not get the expected result, instead the caller can use

myFun_t myFun = (myFun_t) dlsym(handle, "_Z5myFunPKcRi"); to get the correct function.

The mangled symbol name can be get through:

$ nm libmylib.so | grep myFun

00000000000005ac T _Z5myFunPKcRi

0000000000200914 b _ZZ5myFunPKcRiE5count