dlopen - Undefined symbol error

Question

I\'m using dlopen to load a shared library at run time

dlopen(\"SharedLibarary1.so\", RTLD_NOW | RTLD_GLOBAL);

In that shared object I refe

Answer 1

When building a shared library, you can link another inside, e.g. like

 gcc -shared -rdynamic lib1*.pic.o -lshared2 -o SharedLibrary1.so  

Then check with ldd SharedLibrary1.so

(look e.g. at the output of ldd on your system's libgtk-3.so.0 for an example)

% ldd /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
linux-vdso.so.1 =>  (0x00007fff6afff000)
libgdk-3.so.0 => /usr/lib/x86_64-linux-gnu/libgdk-3.so.0 (0x00007f0572628000)
libgmodule-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgmodule-2.0.so.0 (0x00007f0572424000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f057221b000)
libpangocairo-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpangocairo-1.0.so.0 (0x00007f057200e000)
libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6 (0x00007f0571cd2000)
libXi.so.6 => /usr/lib/x86_64-linux-gnu/libXi.so.6 (0x00007f0571ac2000)
libXcomposite.so.1 => /usr/lib/x86_64-linux-gnu/libXcomposite.so.1 (0x00007f05718c0000)
libXdamage.so.1 => /usr/lib/x86_64-linux-gnu/libXdamage.so.1 (0x00007f05716be000)
libXfixes.so.3 => /usr/lib/x86_64-linux-gnu/libXfixes.so.3 (0x00007f05714b7000)
libatk-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libatk-1.0.so.0 (0x00007f0571294000)
libcairo-gobject.so.2 => /usr/lib/x86_64-linux-gnu/libcairo-gobject.so.2 (0x00007f057108b000)
libcairo.so.2 => /usr/lib/x86_64-linux-gnu/libcairo.so.2 (0x00007f0570d91000)
libgdk_pixbuf-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgdk_pixbuf-2.0.so.0 (0x00007f0570b71000)
libpangoft2-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpangoft2-1.0.so.0 (0x00007f0570946000)
libpango-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpango-1.0.so.0 (0x00007f05706f8000)
libfreetype.so.6 => /usr/lib/x86_64-linux-gnu/libfreetype.so.6 (0x00007f0570459000)
libfontconfig.so.1 => /usr/lib/x86_64-linux-gnu/libfontconfig.so.1 (0x00007f0570222000)
libgio-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0 (0x00007f056fece000)
libgobject-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0 (0x00007f056fc7e000)
libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007f056f986000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f056f703000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f056f4e7000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f056f160000)
libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6 (0x00007f056ef4c000)
libXinerama.so.1 => /usr/lib/x86_64-linux-gnu/libXinerama.so.1 (0x00007f056ed4a000)
libXrandr.so.2 => /usr/lib/x86_64-linux-gnu/libXrandr.so.2 (0x00007f056eb42000)
libXcursor.so.1 => /usr/lib/x86_64-linux-gnu/libXcursor.so.1 (0x00007f056e937000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f056e733000)
/lib64/ld-linux-x86-64.so.2 (0x00007f0572f57000)
libxcb.so.1 => /usr/lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f056e512000)
libpixman-1.so.0 => /usr/lib/x86_64-linux-gnu/libpixman-1.so.0 (0x00007f056e28c000)
libpng12.so.0 => /lib/x86_64-linux-gnu/libpng12.so.0 (0x00007f056e064000)
libxcb-shm.so.0 => /usr/lib/x86_64-linux-gnu/libxcb-shm.so.0 (0x00007f056de61000)
libxcb-render.so.0 => /usr/lib/x86_64-linux-gnu/libxcb-render.so.0 (0x00007f056dc57000)
libXrender.so.1 => /usr/lib/x86_64-linux-gnu/libXrender.so.1 (0x00007f056da4d000)
libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f056d836000)
libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f056d60b000)
libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f056d3eb000)
libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f056d1d5000)
libgthread-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgthread-2.0.so.0 (0x00007f056cfd2000)
libffi.so.5 => /usr/lib/x86_64-linux-gnu/libffi.so.5 (0x00007f056cdc5000)
libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f056cb87000)
libXau.so.6 => /usr/lib/x86_64-linux-gnu/libXau.so.6 (0x00007f056c983000)
libXdmcp.so.6 => /usr/lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f056c77e000)

Answer 2

If possible, you should link each shared library you dlopen() with the linker arguments to force it to resolve all it's symbols when the library is created. This way you can be more sure that you know where all your symbols will be coming from. ("-z defs" for example, under Linux x86)

That way, if you add a new source file, but forget to add the object to the link, you won't be scratching your head because of a runtime error from dlopen/dlsym.