Load shared lib in c++ causes segmentation fault - c++

Question

I am learning c++, and am experimenting with loading a shared lib on linux (.so).
I get a segmentation fault when I run the below code.
When I try to run the console app using valgrind, I get the following:
valgrind ./TestLoadSo --leak-check=full -v
==26828== Memcheck, a memory error detector
==26828== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==26828== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==26828== Command: ./TestLoadSo --leak-check=full -v
==26828==
!!!Hello World!!!
==26828== Jump to the invalid address stated on the next line
==26828== at 0x0: ???
==26828== by 0x53E63F0: (below main) (libc-start.c:291)
==26828== Address 0x0 is not stack'd, malloc'd or (recently) free'd
==26828==
==26828==
==26828== Process terminating with default action of signal 11 (SIGSEGV)
==26828== Bad permissions for mapped region at address 0x0
==26828== at 0x0: ???
==26828== by 0x53E63F0: (below main) (libc-start.c:291)
==26828==
==26828== HEAP SUMMARY:
==26828== in use at exit: 3,126 bytes in 9 blocks
==26828== total heap usage: 13 allocs, 4 frees, 76,998 bytes allocated
==26828==
==26828== LEAK SUMMARY:
==26828== definitely lost: 0 bytes in 0 blocks
==26828== indirectly lost: 0 bytes in 0 blocks
==26828== possibly lost: 0 bytes in 0 blocks
==26828== still reachable: 3,126 bytes in 9 blocks
==26828== suppressed: 0 bytes in 0 blocks
==26828== Rerun with --leak-check=full to see details of leaked memory
==26828==
==26828== For counts of detected and suppressed errors, rerun with: -v
==26828== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
[1] 26828 segmentation fault (core dumped) valgrind ./TestLoadSo --leak-check=full -v
C++ Main class
extern "C" typedef char* (*helloWorld_t)();
int main() {
void* handle = dlopen("./libMyLib.dll.so", RTLD_LAZY);
if (!handle) {
cerr << "Cannot open library: " << dlerror() << '\n';
return 1;
}
helloWorld_t hello = (helloWorld_t)dlsym( handle, "helloWorld" );
const char * tmp = hello();
printf("\n%s",tmp);
return 0;
}
The extern function is:
extern "C++" char* helloWorld() {
char str[25];
strcpy(str, "HelloWorld");
}
If I use extern "C" I get a compilation error:
error: conflicting declaration of ‘char* helloWorld()’ with ‘C’ linkage
extern "C" char* helloWorld() {
Its really not clear to me where I am going wrong.

Answer 1

A function cannot have both C and C++ linkage, and a function pointer type must match its target function's linkage.
You cannot dlsym an extern "C++" function by its unadorned name. You have to either use extern "C" in both cases (recommended), or use extern "C++" throughout and replace the string in dlsym(handle, "helloWorld") with the mangled name of your function (not recommended).
Always check the result of dlsym and report an error if it returns a null pointer (use dlerror() like you've done for dlopen).
Don't use character arrays or pointers to represent strings. There is a type for string, called std::string.
Last but not least, always compile with -Wall -Werror so things like a non-void function that doesn't actually return a value will be caught.

Answer 2

Many problems here:
extern "C++" char* helloWorld() {
char str[25];
strcpy(str, "HelloWorld");
}
It should use "C" linkage. And it should return something. And it copies the string to local variable, so value gets lost when it returns. So probably
extern "C" char* helloWorld() {
static char str[25]; // will keep its value accross calls, not thread safe
return strcpy(str, "HelloWorld"); // return pointer to start of str
}
Note that multiple calls all return same static buffer. If you need copies, you need to let caller provide a buffer, or return buffer allocated with malloc.