Trying LD_PRELOAD with Julia

base.c

/* build with gcc -shared -fPIC -o base.so base.c */

#include <stdio.h>

void doTheThing() {
    printf("BASE\n");
}

inject.c

/* build with gcc -shared -fPIC -ldl -o inject.so inject.c */
/* See http://www.goldsborough.me/c/low-level/kernel/2016/08/29/16-48-53-the_-ld_preload-_trick/ */

#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>

typedef void (*real_doTheThing_t)();

/*  Call the real "doTheThing" in the regular library (base.so, in this case) */
void real_doTheThing() {
    ((real_doTheThing_t)dlsym(RTLD_NEXT, "doTheThing"))();
}


void doTheThing() {
    printf("INJECT ");
    real_doTheThing();
}

try.jl

#= Run with
LD_PRELOAD=./inject.so julia try.jl
=#

using Libdl

# We must use LD_PRELOAD to make the wrapping in inject.c work (we can't just dlopen("./inject.so") )

# Load the base library
base = "./base.so"
b = dlopen(base, RTLD_GLOBAL | RTLD_DEEPBIND | RTLD_LAZY)  # Need RTLD_GLOBAL so ccall(:fcn, ...)
                                                           # can find :fcn without specifying the library
                                                           # RTLD_DEEPBIND doesn't seem to matter

# This will honor LD_PRELOAD
ccall(:doTheThing, Cvoid, ())

# This will not honor LD_PRELOAD since we are specifying the library to use
ccall((:doTheThing, "./base.so"), Cvoid, ())