C++ got wrong value from array of struct - c++

Question

The problem is as described above. When I try to read values from loaded *.so file (using libdl), whih are in struct I am getting wrong values
Code of application:
#include <dlfcn.h>
#include <iostream>
/* For face data type reproduction */
#define GET_FACE_XYZ_SIZE 1
/* For face_array reproduction */
#define GET_FACE_ARRAY_SIZE 2
#define GET_OBJECT_DATA 3
typedef struct face {
float x[1000];
float y[1000];
float z[1000];
int vertices;
} face;
int main()
{
void *hook;
int (*fn)(int request_type, void *ptr);
hook = dlopen("/root/osms/dlopen-test/lib.so", RTLD_LAZY);
if(!hook)
{
std::cout << "Couldn't find lib.so" << std::endl;
}
fn = dlsym(hook, "object_info");
int face_array_size = fn(GET_FACE_ARRAY_SIZE, NULL);
std::cout << "FACE_ARRAY_SIZE: " << face_array_size << std::endl;
face pointer[face_array_size];
fn(NULL, pointer);
dlclose(hook);
std::cout << "pointer[0].z[1]: " << pointer[0].z[1] << std::endl;
return 0;
}
and code of lib.so:
/* For face data type reproduction */
#define GET_FACE_XYZ_SIZE 1
/* For face array reproduction */
#define GET_FACE_ARRAY_SIZE 2
#define GET_OBJECT_DATA 3
typedef struct face {
float x[1000];
float y[1000];
float z[1000];
int vertices;
} face;
extern "C" int object_info(int request, void *ptr)
{
face face_array[2];
face_array[0].x[0] = 1.1;
face_array[0].y[0] = 0.5;
face_array[0].z[0] = 1.2;
face_array[0].x[1] = 1.6;
face_array[0].y[1] = -0.11;
face_array[0].z[1] = -12;
face_array[0].x[2] = -0.12;
face_array[0].y[2] = 0.24;
face_array[0].z[2] = -0.12;
face_array[0].vertices = 3;
face_array[1].x[0] = -1.1;
face_array[1].y[0] = 0.15;
face_array[1].z[0] = -1.2;
face_array[1].x[1] = -1.6;
face_array[1].y[1] = 0.11;
face_array[1].z[1] = 1.2;
face_array[1].x[2] = 0.12;
face_array[1].y[2] = -0.24;
face_array[1].z[2] = 0.12;
face_array[1].vertices = 3;
if(request == GET_FACE_ARRAY_SIZE)
{
return 2;
}
else
{
ptr = face_array;
}
}
The expected output is pointer[0].z[1]: -12 but I am getting pointer[0].z[1]: -0.12. What's wrong in my code ?
Thanks in advance

Answer 1

Accessing
pointer[0].z[1]
Has undefined behaviour, because it has an indeterminate value.
object_info never modifies the array pointed by ptr. It simply modifies a local array, and assigns the local ptr to point to that local array.
A solution: Don't declare a local array, and instead modify the array pointed by the argument. In other words, repace face face_array[2]; with:
face* face_array = (face*)ptr;
And get rid of the ptr = face_array; that does nothing meaningful.
object_info is declared to return int, but not all code paths return a value. When the function reaches the end of object_info without a return statement, the behaviour is undefined.
A solution: Always return a value if the function is not void.
face_array_size is not a compile time constant value, so face pointer[face_array_size]; will declare a variable length array. VLA are not allowed in C++.
Either use C (VLA are supported since C99, but only optionally supported since C11) instead or use a dynamic array: std::vector<face> or make peace with the fact that your program is not standard compliant.

Answer 2

The variable "face_array" in function object_info and the variable "pointer" in main are not the same variable.
The statement "ptr = face_array" does not change the content of "pointer".
extern "C" int object_info(int request, face *face_array)
{
if(request == GET_FACE_ARRAY_SIZE)
return 2;
face_array[0].x[0] = 1.1;
face_array[0].y[0] = 0.5;
face_array[0].z[0] = 1.2;
face_array[0].x[1] = 1.6;
face_array[0].y[1] = -0.11;
face_array[0].z[1] = -12;
face_array[0].x[2] = -0.12;
face_array[0].y[2] = 0.24;
face_array[0].z[2] = -0.12;
face_array[0].vertices = 3;
face_array[1].x[0] = -1.1;
face_array[1].y[0] = 0.15;
face_array[1].z[0] = -1.2;
face_array[1].x[1] = -1.6;
face_array[1].y[1] = 0.11;
face_array[1].z[1] = 1.2;
face_array[1].x[2] = 0.12;
face_array[1].y[2] = -0.24;
face_array[1].z[2] = 0.12;
face_array[1].vertices = 3;
}