dlopen on new binary with same name returns old handle

Question

I'm using dlopen to load dynamically generated code. The program calls the compiler on the code and generates a .so file which is then loaded by the program to extend itself.

The problem is that if I use the same name for the generated code, the dlopen returns a handle to the old object, not the new one.

The code looks like:

…generate code into test.cpp
system("gcc <args> test.cpp -o test.so");
void *handle = dlopen("test.so");
void *sym = dlsym(handle, "run");
(*sym)();
dlclose(handle);
…Do other work
…generate different code to test.cpp
system("gcc <args> test.cpp -o test.so");
void *handle = dlopen("test.so");
void *sym = dlsym(handle, "run");
(*sym)();
<crash here because the code isn't what was expected>

Is this a basic flaw in dlopen's cache code or something well known and not well documented in dlopen?

Answer 1

Most probly dlclose failed to unload the library. This usually happens when it contains GNU_UNIQUE symbols (which tend to sneak in if you link with static libstdc++). This can be verified via

$ readelf -sW --dyn-syms path/to/libxyz.so | grep '\bUNIQUE\b'
...
3808: 0000000000302e78     8 OBJECT  UNIQUE DEFAULT   27 _ZNSt8messagesIcE2idE@@GLIBCXX_3.4

To fix this, you can try one of the following:

• build library with -fvisibility=hidden and __attribute__((visibility("default"))) to hide unique symbols
• build with -Wl,--version-script to achieve the same
• build shlib with toolchain that was configured with --disable-gnu-unique-object (see discussion in GCC list)