rpath interaction with ld.so and dlopen example

build.sh

#!/bin/sh

set -xe

gcc -fPIC -Wall -shared -Wl,-soname,libtest3.so.0 -o libtest3/libtest3.so.0 libtest3/libtest3.c
gcc -fPIC -Wall -shared -Wl,-soname,libtest2.so.0 -o libtest2/libtest2.so.0 libtest2/libtest2.c
gcc -fPIC -Wall -shared -Wl,-soname,libtest1.so.0 -o libtest1/libtest1.so.0 libtest1/libtest1.c libtest2/libtest2.so.0 -ldl
gcc -fPIC -Wall -o prog/prog prog/main.c -Wl,-rpath-link,libtest2 -Wl,-rpath,$PWD/libtest1:$PWD/libtest2:$PWD/libtest3 libtest1/libtest1.so.0

# - Calling prog/prog won't work because library not found
# - add $PWD/libtest2 to LD_LIBRARY_PATH and it will say that dlopen() failed
# - add both $PWD/libtest2 and $PWD/libtest3 to LD_LIBRARY_PATH and it will work
# - Alterantively, set rpath on libtest1 (that requires libtest2 and libtest3),
#   and it will work w/o LD_LIBRARY_PATH
#      gcc -fPIC -Wall -shared -Wl,-soname,libtest1.so.0 -o libtest1/libtest1.so.0 -Wl,-rpath,$PWD/libtest2:$PWD/libtest3 libtest1/libtest1.c libtest2/libtest2.so.0 -ldl

libtest1.c

// place this file in a subdirectory libtest1
#include <dlfcn.h>
#include <stdio.h>

extern int get_v2(void);

typedef int (*int_getter)(void);

int get_sum()
{
    int_getter get_v3 = NULL;
    void* handle = dlopen("libtest3.so.0", RTLD_NOW | RTLD_GLOBAL);
    if (!handle) {
        fprintf(stderr, "Error loading libtest3.so.0: %s\n", dlerror());
        return -1;
    }
    get_v3 = (int_getter) dlsym(handle, "get_v3");
    if (!get_v3) {
        fprintf(stderr, "Error locating get_v3 in libtest3.so.0: %s\n", dlerror());
        return -1;
    }
    return get_v2() + get_v3();
}

libtest2.c

// place this file in a subdirectory libtest2
extern int get_v2()
{
    return 23;
}

libtest3.c

// place this file in a subdirectory libtest3
extern int get_v3()
{
    return 42;
}

main.c

// place this file in a subdirectory prog
#include <stdio.h>

extern int get_sum(void);

int main()
{
    printf("The sum is: %d\n", get_sum());
    return 0;
}