Dynamically loading Linux shared libraries? - linux

Question

I want to create a shared library which can be loaded in two different ways into targets:
LD_PRELOAD
Dynamic loading via dlsym
My shared library looks like this:
#include "stdio.h"
void __attribute__ ((constructor)) my_load(void);
void my_load(void) {
printf("asdf");
}
void someFunc(void) {
printf("someFunc called");
}
I am compiling it like so:
all:
gcc -fPIC -g -c -Wall MyLib.c
gcc -shared -W1,-soname,MyLib.so.1 -o MyLib.so.1.0.1 -lc
I do not wish to install it with ldconfig, etc. The target process looks like this:
#include <stdio.h>
#include <dlfcn.h>
void func1() {
printf("%d\n", 1);
}
void func2() {
printf("%d\n", 2);
}
void func3() {
printf("%d\n", 3);
}
int main() {
void* lib_handle = dlopen("/home/mike/Desktop/TargetProcess/MyLib.so.1.0.1",
RTLD_NOW|RTLD_GLOBAL);
if(lib_handle == NULL) {
printf("Failed loading lib\n");
} else {
printf("Loaded lib successfully\n");
void (*some_func)() = dlsym(lib_handle, "someFunc");
printf("%p\n", some_func);
dlclose(lib_handle);
}
func1();
func2();
func3();
return 0;
}
The target is compiled as so:
all:
gcc TestProg.c -ldl -o TestProg
My questions are:
With the dynamic loading with dlopen as above, why does my_load not appear to be called?
With the same method, why does dlsym always return nil even though dlopen returns non-null? Similarly, nm doesn't list either my_load or someFunc as symbols of the .so.
Is it possible to use LD_PRELOAD to load the library? I tried copying the .so into the same directory as the target then invoking LD_PRELOAD="./MyLib.so.1.0.1" ./TestProg but again my_load seems not to be being called.

Answer 1

Your object files was no linked into your library:
gcc -shared -W1,-soname,MyLib.so.1 -o MyLib.so.1.0.1 -lc
Change it to include your object file MyLib.o:
gcc MyLib.o -shared -W1,-soname,MyLib.so.1 -o MyLib.so.1.0.1 -lc
UPDATE: just tryed your command locally (without any MyLib.c or MyLib.o):
$ gcc -shared -W1,-soname,MyLib.so.1 -o MyLib.so.1.0.1 -lc && echo ok
ok
$ nm MyLib.so.1.0.1
xxxxxxxx a _DYNAMIC
xxxxxxxx a _GLOBAL_OFFSET_TABLE_
w _Jv_RegisterClasses
xxxxxxxx A __bss_start
w __cxa_finalize##xxxxxxxxxxx
xxxxxxxx d __dso_handle
w __gmon_start__
xxxxxxxx t __i686.get_pc_thunk.bx
xxxxxxxx A _edata
xxxxxxxx A _end
xxxxxxxx T _fini
xxxxxxxx T _init
It is an empty dynamic library.