error at dlsym(): ./dynlibrary: undefined symbol: method

Question

I've got an issue I am not able to solve. Looked up everything I found so far. My problem is, I create a dyn library in my program a want to dlopen it and dlsym a method out of that lib.

It seems that dlopen works but dlsym return me the error "undefined symbol: method"

where "method" is the name of the method I passed to dlsym.

Here is how I create the library:

execl("/usr/bin/gcc", "gcc", "-fPIC", "-Wall", "-g", "-c", "userinput.c", NULL);

and:

execl("/usr/bin/gcc", "gcc", "-ggdb3", "-shared", 
      "-Wl,-soname,libuserinput.so.1", "-o", "libuserinput.so.1.0",
  "userinput.o", "-lc", NULL);

This should work as there is a library after running my code.

I open the library like this:

static void *my_load_dyn (const char *lib) {
static void *handle;
handle = dlopen ("./libuserinput.so.1.0", RTLD_NOW | RTLD_DEEPBIND);
if (handle == NULL) {
  printf ("error at dlopen(): %s\n", dlerror ());
  exit (EXIT_FAILURE);
}
return handle;
}

/* load func from dyn lib"*/
static void *my_load_func (void *handle, char *func) {
  void *funcptr = dlsym (handle, func);
  if (funcptr == NULL) {
    printf ("error at dlsym(): %s\n", dlerror ());
    exit (EXIT_FAILURE);
  }
  return funcptr;
}

and call those functions like this:

void *libhandle;
void (*userMethod) (unsigned char *d);

libhandle = my_load_dyn(LIBUSERINPUT);
userMethod = my_load_func(libhandle, "testMethod");
(*userMethod)(d);

EDIT: here is the code from the userinput.c:

#include <stdio.h>
#include <unistd.h>
void testMethod(unsigned char *d)
{
d[0] = 'Z';
}

It is generated in my programm and also compiled and linked in the running programm

Do `nm -D libuserinput.so.1.0 | grep testMethod`, what do you see? — n. m. could be an AI, Dec 19 '13 at 14:25
nothing. Yes i should say that as well. I tried nm on that lib and the function name is not listet in the symbol table. sorry for not mentioning that. Now my problem is: Why is the Method not in there? — LongRon, Dec 19 '13 at 14:28
maybe my forking in the rest code is wrong. If i compile/link my self it seems to work — LongRon, Dec 19 '13 at 15:22

score 0 · Answer 1 · answered Dec 19 '13 at 14:29

0

i can see two possible problems

there's a problem in the code

are you sure that you declare your method as a public function in userinput.c?

e.g. if your method is declared static, then it won't be accessible from "outside". also there are other ways to hide functions from being seen outside of the library, but it's impossible to tell whether you are having that problem without seeing any code.
your compilation/linking of the library is broken

e.g. you seem to be linking in a header-file (userinput.h) into the resulting libray?

i'd suggest to use a proper build system until the problem is solved, and switch to on-the-fly compilation later (e.g. using make)

answered Dec 19 '13 at 14:29

umläute

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i want to compile/link in my programm. Ill post the code of the userinput.c above in my question. Yes the userinput.h is not necessary. Ill remove that one – LongRon Dec 19 '13 at 14:33
i'm suggesting that you *first* compile/link outside of your program, and once that works, move to doing it within your program. if you are having troubles with complicated setups, it's always a good idea to lower the complexity as a first step... – umläute Dec 19 '13 at 14:52
with compiling/linking myself it works. maybe my forking is bad – LongRon Dec 19 '13 at 15:23

error at dlsym(): ./dynlibrary: undefined symbol: method

1 Answers1