Why am I not able to use my function that is declared in .so library?

Question

I'm trying to write a program to call functions inside my .so library in Android.

main.cpp:

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

typedef int (*test_ptr)(int, int);

int main()
{
    int rc;
    void *handle;
    const char *error;

    handle = dlopen("./testdll.so", RTLD_LAZY);
    if(!handle){
        printf("dlopen %s\n", dlerror());
        return -1;
        }
    printf("dlopen success.\n");

    test_ptr test = (test_ptr)dlsym(handle, "max");
    error = dlerror();
    if (error)  {
        printf("error %s\n", error);
        dlclose(handle);
        return -1;
    }
    rc = test(10, 20);
    printf("rc = %d\n", rc);

    dlclose(handle);
    return 1;
}

testdll.cpp:

#include<stdio.h>

int max(int x,int y){
    return x>y?x:y;
}

Android.mk:

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES:= \
    testdll.cpp
LOCAL_MODULE_TAGS := optional
LOCAL_PRELINK_MODULE := true
LOCAL_LDFLAGS := -Wl,--export-dynamic
LOCAL_MODULE:= testdll
include $(BUILD_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_SRC_FILES:= \
    main.cpp
LOCAL_LDLIBS := -ldl
LOCAL_SHARED_LIBRARIES := \
libcutils   \
libdl
LOCAL_MODULE_TAGS := optional
LOCAL_PRELINK_MODULE := true
LOCAL_MODULE := mytest
LOCAL_MODULE_PATH := $(TARGET_OUT_OPTIONAL_EXECUTABLES)
include $(BUILD_EXECUTABLE)

and I use mmm to make file and push testdll.so in to /system/lib and mytest to /data/data.

Then I run ./mytest, it can open testdll.so successed, but can't connect my function.

error is:

Symbol not found:

Anybody know how to solve this problem?

==========================================================================================

Thanks everybody!

I call my functions successed use extern "C".

But now, I change to use C++ to write the .so library and got the same error message.

testdll.cpp:

#include <sys/types.h>
#include <unistd.h>
#include <grp.h>

#include <binder/IPCThreadState.h>
#include <binder/ProcessState.h>
#include <binder/IServiceManager.h>
#include <utils/Log.h>

#include <private/android_filesystem_config.h>

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

using namespace android;
extern "C" {
    int TestClass::max(int x,int y){
        return x>y?x:y;
    }
}

testdll.h:

using namespace android;

class TestClass
{
    public:
        int max(int, int);
};

main.h:

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

typedef int (*test_ptr)(int, int);

int main()
{
    int rc;
    void *handle;
    const char *error;

    handle = dlopen("./testdll.so", RTLD_LAZY);
    if(!handle){
        printf("dlopen %s\n", dlerror());
        return -1;
    }
    printf("dlopen success.\n");

    test_ptr test = (test_ptr)dlsym(handle, "max");
    error = dlerror();
    if (error)  {
        printf("error %s\n", error);
        dlclose(handle);
        return -1;
    }
    rc = test(10, 20);
    printf("rc = %d\n", rc);

    dlclose(handle);
    return 1;
}

Then I got the same error message: Symbol not found:

I use objdump -TC to show:

DYNAMIC SYMBOL TABLE:
000003f5 g    DF .text  0000000a TestClass::max(int, int)
00000000      DF *UND*  00000000 __aeabi_unwind_cpp_pr0
00000000      DF *UND*  00000000 __cxa_finalize
000010f0 g    D  .bss   00000000 __dso_handle
00001000 g    D  .init_array    00000000 __INIT_ARRAY__
00001008 g    D  .fini_array    00000000 __FINI_ARRAY__
00000410 g    D  *ABS*  00000000 __exidx_end
00000400 g    D  *ABS*  00000000 __exidx_start
000010ec g    D  *ABS*  00000000 __data_start
000010ec g    D  *ABS*  00000000 _edata
000010ec g    D  *ABS*  00000000 __bss_start
000010ec g    D  *ABS*  00000000 __bss_start__
00001100 g    D  *ABS*  00000000 _bss_end__
00001100 g    D  *ABS*  00000000 __bss_end__
00001100 g    D  *ABS*  00000000 __end__
00001100 g    D  *ABS*  00000000 _end
00080000 g    D  *ABS*  00000000 _stack

How do I modify?

Thanks again!

you may have to add __declspec(dllexport) in front of max () to export the symbol, i.e :-__declspec(dllexport) int max(int x,int y){..} , check this question to see all your exported symbol http://stackoverflow.com/questions/34732/how-do-i-list-the-symbols-in-a-so-file — Sudar Nimalan, Jul 12 '12 at 02:05
While your problem appears to be name mangling, you really should be checking the return value of dlsym() first and only then check dlerror(). — Chris Stratton, Jul 12 '12 at 03:02

score 2 · Accepted Answer · answered Jul 12 '12 at 04:26

TestClass::max(int, int) in your symbol table - does that mean you max function is a method of a class?

Well, if so, you need to create a C wrap to your TestClass, for example

extern "C"
{
    int max (void *pHandle, int x, int y) 
    {
      TestClass *tcObject = (TestClass *)pHandle;
      return tcObject->max(x, y);
    }
}

score 1 · Answer 2 · answered Jul 12 '12 at 02:28

1

Any function you plan to access via dlopen must be declared extern "C". The reason is name mangling.

answered Jul 12 '12 at 02:28

n. m. could be an AI

112,515
14
128
243

It's not really that you need to use `extern "C"`. It's really that you have to use the name as it's known in the external module, which isn't necessarily how it's known in the source code. The *easy* way is `extern "C"` so that both the names are the same, but it's not strictly necessary. You can import using the mangled name if you want. – Rob Kennedy Jul 12 '12 at 02:48
Yes, it could be imported by the mangled name, but then the mangled name would have to be passed to dlsym(). On the other hand, if an executable was generated with hard-coded dependence on the library, that would probably be taken care of automatically, since dlsym() would not be called manually. – Chris Stratton Jul 12 '12 at 03:02
Thanks, it work! now I want to use C++ to write, and got the same error message. – Tony Jul 12 '12 at 03:57

Why am I not able to use my function that is declared in .so library?

2 Answers2