在结构中设置数组元素时出现总线错误

Question

I have the following struct:

struct State {
    int num_walls;
    Wall walls[];

    int num_persons;
    Person persons[];
};

I want to have persons contain two Persons and walls contain one Wall:

int num_walls = 1;
int num_preson = 2;

State simState = *(State*)malloc( sizeof(simState) + num_person*sizeof(Person) + num_walls*sizeof(Wall));
simState.num_walls = num_walls;
simState.num_persons = num_person;
simState.walls[0] = w;
simState.persons[0] = p1;
simState.persons[1] = p2;

When I do that I get Bus error (core dumped). When I only set the persons, everything works fine. Ie this works:

int num_walls = 0;
int num_preson = 2;

State simState = *(State*)malloc( sizeof(simState) + num_person*sizeof(Person) + num_walls*sizeof(Wall));
simState.num_walls = num_walls;
simState.num_persons = num_person;
// simState.walls[0] = w;
simState.persons[0] = p1;
simState.persons[1] = p2;

The code is c++11 and I'm using gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609

Why does that happen?

Answer 1

So this is legal C++, it may be what you are trying to achieve

struct State {
    int num_walls;
    Wall* walls;
    int num_persons;
    Person* persons;
};

State simState;
simState.num_walls = num_walls;
simState.walls = new Wall[num_walls];
simState.num_persons = num_person;
simState.persons = new Person[num_person];
simState.walls[0] = w;
simState.persons[0] = p1;
simState.persons[1] = p2;

It's not good C++, but at least it's legal.

The important points (in either language) is that you do not have to allocate simState , it gets it's memory just by being declared, but you do have to allocate your two variable length arrays simState.persons and simState.walls

Answer 2

It's not allowed to define dimensionless arrays in C++, you should be using pointer with allocation (new or even malloc) as stated by others.

Dimensionless arrays are valid construct in C99, it's not called VLA but flexible array member . This feature of language can be used only as last member of structure with several other restrictions on definition and usage. Definitely not regularly used feature.

Reason for "Bus error" on first example is illegal definition of wall it's not last member of struct. Since person is correctly defined you are accessing memory past structure and overwriting something else which is really not what you want to do. GCC is reason why your program compiles even when it's not valid since lot of extensions is enabled by default. Please use -pedantic flag together with version of C++ standard (-std=c++11) to get warning about that.