LD_PRELOAD help

Question

I'm trying to use LD_PRELOAD.

original.cpp

void myPuts() {  
    puts ("Hello myPuts");  
}  
int main() {  
    myPuts();  
    return 0;  
}

hacked.cpp

void myPuts() {  
    std::cout >> "Hello hacked myPuts";  
}

I compile original.cpp:

g++ original.cpp

And hacked.cpp:

g++ -shared -fPIC hacked.cpp

I try:

LD_PRELOAD=./hacked.so ./original.out

The string "Hello hacked myPuts" should be seen, by "Hello myPuts" appears. (If I try to "overwrite" the puts function, it works correctly)

What am I missing?

Answer 1

From man ld.so

LD_PRELOAD

A whitespace-separated list of additional, user-specified, ELF shared libraries to be loaded before all others. This can be used to selectively override functions in other shared libraries .

If myPuts was in shared library linked to main application it would work, but not when myPuts exists in the application and does not resolved in an external library.

Answer 2

You should have:

main.cpp

int main() {  
    myPuts();  
    return 0;  
}

original.cpp

void myPuts() {  
    puts ("Hello myPuts");  
}  

hacked.cpp

void myPuts() {  
    std::cout << "Hello hacked myPuts";  
}

Compiling all:

g++ -shared -fPIC original.cpp -o liboriginal.so
g++ -shared -fPIC hacked.cpp -o libhacked.so
g++ main.cpp -loriginal -o main.out

And using:

LD_PRELOAD=./libhacked.so ./main.out