Linux c++ error: undefined reference to 'dlopen'

169

votes

I work in Linux with C++ (Eclipse), and want to use a library. Eclipse shows me an error:

undefined reference to 'dlopen'

Do you know a solution?

Here is my code:

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

int main(int argc, char **argv) {
    void *handle;
    double (*desk)(char*);
    char *error;

    handle = dlopen ("/lib/CEDD_LIB.so.6", RTLD_LAZY);
    if (!handle) {
        fputs (dlerror(), stderr);
        exit(1);
    }

    desk= dlsym(handle, "Apply");

    if ((error = dlerror()) != NULL)  {
        fputs(error, stderr);
        exit(1);
    }

    dlclose(handle);
}

c++linuxeclipse

280

votes

You have to link against libdl, add

-ldl

to your linker options

88

votes

@Masci is correct, but in case you're using C (and the gcc compiler) take in account that this doesn't work:

gcc -ldl dlopentest.c

But this does:

gcc dlopentest.c -ldl

Took me a bit to figure out...

11

votes

this doesn't work:
gcc -ldl dlopentest.c
But this does:
gcc dlopentest.c -ldl
That's one annoying "feature" for sure

I was struggling with it when writing heredoc syntax and found some interesting facts. With CC=Clang, this works:

$CC -ldl -x c -o app.exe - << EOF
#include <dlfcn.h>
#include <stdio.h>
int main(void)
{
  if(dlopen("libc.so.6", RTLD_LAZY | RTLD_GLOBAL))
    printf("libc.so.6 loading succeeded\n");
  else
    printf("libc.so.6 loading failed\n");
  return 0;
}
EOF

./app.exe

as well as all of these:

$CC -ldl -x c -o app.exe - << EOF
$CC -x c -ldl -o app.exe - << EOF
$CC -x c -o app.exe -ldl - << EOF
$CC -x c -o app.exe - -ldl << EOF

However, with CC=gcc, only the last variant works; -ldl after - (the stdin argument symbol).

8

votes

The topic is quite old, yet I struggled with the same issue today while compiling cegui 0.7.1 (openVibe prerequisite).

What worked for me was to set: LDFLAGS="-Wl,--no-as-needed" in the Makefile.

I've also tried -ldl for LDFLAGS but to no avail.

7

votes

I was using CMake to compile my project and I've found the same problem.

The solution described here works like a charm, simply add ${CMAKE_DL_LIBS} to the target_link_libraries() call

5

votes

you can try to add this

LIBS=-ldl CFLAGS=-fno-strict-aliasing

to the configure options

4

votes

You needed to do something like this for the makefile:

LDFLAGS='-ldl'
make install

That'll pass the linker flags from make through to the linker. Doesn't matter that the makefile was autogenerated.

1

votes

I met the same problem even using -ldl.

Besides this option, source files need to be placed before libraries, see undefined reference to `dlopen'.

1

votes

In order to use dl functions you need to use the -ldl flag for the linker.

how you do it in eclipse ?

Press Project --> Properties --> C/C++ build --> Settings --> GCC C++ Linker -->
Libraries --> in the "Libraries(-l)" box press the "+" sign --> write "dl" (without the quotes)-> press ok --> clean & rebuild your project.

1

votes

 $gcc -o program program.c -l <library_to_resolve_program.c's_unresolved_symbols>

A good description of why the placement of -l dl matters

But there's also a pretty succinct explanation in the docs From $man gcc

   -llibrary
   -l library
       Search the library named library when linking.  (The second
       alternative with the library as a separate argument is only for POSIX
       compliance and is not recommended.)

       It makes a difference where in the command you write this option; the
       linker searches and processes libraries and object files in the order
       they are specified.  Thus, foo.o -lz bar.o searches library z after
       file foo.o but before bar.o.  If bar.o refers to functions in z,
       those functions may not be loaded.

0

votes

Try to rebuild openssl (if you are linking with it) with flag no-threads.

Then try to link like this:

target_link_libraries(${project_name} dl pthread crypt m ${CMAKE_DL_LIBS})

Linux c++ error: undefined reference to 'dlopen'

11 Answers