C++ got wrong value from array of struct

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

eerorika · Accepted Answer · 2017-01-16 09:55:01Z

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.

海牙客移库 · Accepted Answer · 2017-01-16 09:55:42Z

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;

}

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