LD_PRELOAD compile .so using gcc or g++

Question

I have a wrapper for a socket API which I preload before my application using LD_PRELOAD. However when I compile my .so wrapper library using g++ it doesn't work, i.e., glibc version of the socket implementation is picketd instead.

This is how I compile the wrapper .so library (one of these):

gcc -Wall -fPIC -shared socket_hooks.c -o socket_hooks.so
or
g++ -Wall -fPIC -shared socket_hooks.c -o socket_hooks.so

This is how I compile and run my udp client:

g++ -Wall udp_client.cpp  -o udp_client
LD_PRELOAD=./socket_hooks.so ./udp_client

Now when I enable debug (export LD_DEBUG=all) and run with .so compiled using gcc (working version) compiled example I see:

 17278: symbol=socket;  lookup in file=./udp_client [0]
 17278: symbol=socket;  lookup in file=./socket_hooks.so [0]
 17278: binding file ./udp_client [0] to ./socket_hooks.so [0]: normal symbol `socket' [GLIBC_2.2.5]

When I compile .so using g++ I see the following:

 17285: symbol=socket;  lookup in file=./udp_client [0]
 17285: symbol=socket;  lookup in file=./socket_hooks.so [0]
 17285: symbol=socket;  lookup in file=/lib/x86_64-linux-gnu/libc.so.6 [0]
 17285: binding file ./udp_client [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `socket' [GLIBC_2.2.5]

In both cases "socket" symbol is found, but in the latter case it is not picked up, but libc is used instead.

Could you explain what is happening here?

Answer 1

When you compile your module with C++ compiler, your symbols are name mangled to support function overloading, for example socket may be transformed to _Z10socketiii. That's why runtime loader doesn't find it - it's looking for socket, not _Z10socketiii.

In order to disable name mangling, you have to declare your functions with C linkage using extern "C", probably with conjunction with __cplusplus macro, in order to stay compatible with C compilers:

#ifdef __cplusplus
extern "C" {
#endif

int socket(int domain, int type, int protocol);

#ifdef __cplusplus
}
#endif

Alternatively, set linkage on per-symbol basis:

#ifdef __cplusplus
#define DONT_MANGLE_ME extern "C"
#else
#define DONT_MANGLE_ME
#endif

DONT_MANGLE_ME int socket(int domain, int type, int protocol);