Reputation: 7079

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

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);
}

Upvotes: 205

Answers (13)

Seim2k20

Reputation: 31

using cmake for me it helped to put -ldl after ssl to the target:

target_link_libraries(ssl_test -WL,--start-group ssl -ldl crypto -WL,--end-group )

Upvotes: 0

Gopesh Bharadwaj

Reputation: 170

In earlier versions(~2.7) of GNU tool chain, glibc did not have direct interface to link loader(dlopen and dlsym functions), so you had to provide -ldl(libdl) at compile time. You don't have to do that anymore with latest glibc version. Just include <dlcfn.h> and you are good to go.

Upvotes: 1

James

Reputation: 129

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})

Upvotes: 0

Lucas Coelho

Reputation: 603

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

Upvotes: 17

vulcan raven

Reputation: 33582

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).

Upvotes: 17

Rob

Reputation: 4434

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.

Upvotes: 4

flerb

Reputation: 743

 $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.

Upvotes: 1

Amitk

Reputation: 332

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.