Loading and unloading shared libraries can be tricky sometimes. The biggest problem is when you use the g++ compiler (as opposed to the gcc compiler) it mangles the symbols in the object file so you can’t load them later. Here’s a “Hello World!” dynamic library in C++ example for your enjoyment.
First, create the library file, called dynamic.cpp:
#include <cstdio>
extern "C" void _init();
extern "C" void hello(void);
extern "C" void _fini();
void _init() {
printf("Initialization Phase");
}
void hello(void) {
printf("Hello, Dynamic World!");
}
void _fini() {
printf("Deconstruction Phase");
}
The _init() and _fini() functions will be called automagically when you load or unload the dynamic module.
You can compile this on the command line (we’ll create a Makefile in a minute):
g++ -fPIC -c dynamic.cpp
ld -shared -o dynamic.so dynamic.o
We first compile the function with g++ first because if we just do the whole thing together it automatically adds tons of other symbols you just don’t need, not to mention not allowing you to put your own _init() and _fini() functions in. If you’re curious, go ahead and try to compile it with the command g++ -fPIC -shared -o dynamic.so dynamic.cpp—it probably won’t work!
Next, we need a program to test it with, and in a fit of creative genius I called it main.cpp:
#include <cstdio>
#include <cstdlib>
#include <dlfcn.h>
int main() {
void *handle;
void (*hello)(void);
char *error;
handle = dlopen("/path/to/my/dynamic.so", RTLD_LAZY);
if(handle == NULL) {
fprintf(stderr, "Error: open/load error of dynamic.so failed: %s\n", dlerror());
exit(1);
}
hello = (void(*)(void)) dlsym(handle, "hello");
if((error = dlerror()) != NULL) {
fprintf(stderr, "Error: symbol lookup in dynamic.so failed: %s\n", dlerror());
exit(2);
}
hello();
dlclose(handle);
return 0;
}
Now all we need is a good Makefile:
# Makefile for dynamic module loading APP = test CC = g++ LD = ld all: $(APP) dynamic.o: dynamic.cpp $(CC) -fPIC -c dynamic.cpp dynamic.so: dynamic.o $(LD) -shared -o dynamic.so dynamic.o $(APP): dynamic.so main.cpp $(CC) -o $(APP) main.cpp -ldl clean: @rm -f *.o *.so $(APP)
I realize I don’t explain a lot of this code, but it’s not too complicated, so it should be pretty easy to understand if you are familiar with C or C++.
One Response to “Dynamic Libraries in C/C++ and Linux”
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Quoth Ram’s Blog » The Story of the .so files
November 23rd, 2005 at 5:04 am [...] ilename.c or the same way with .cpp. For a basic tutorial on how so work , I found it over here. First create a file named dynamic.cppas include extern “C [...] |