Initializing nested structures

Question

I have a problem when trying to initialize my nested structures.

On the compilation, there is no errors, but on the execution, I'm getting a segfault. In valgrind, I get

> Invalid read of size 8  at 0x402175: new_animal  
  > Address 0x38 is not stack'd, malloc'd or (recently) free'd

Here is the code :

    void new_animal(int i, int j, int species){

   struct animal * a;

    if (array[i][j]==NULL) 
    {
        a = malloc(sizeof(struct animal));
        assert (a);
        a->espece=espece;
        if(array[i][j]->player==NULL)
        {
        a->player->id_fisherman=0;
        strcpy(a->player->Name,"N"); //I want it set to NULL.
        }
    }
   grille[i][j] = a;
  }

And here is the two structures :

struct fisherman {
    int id_fisherman;
    char Name[10];
};

struct animal {
    int species;
    struct fisherman* player;
};

It worked fine until I added the fisherman. I don't know if it is due to the memory allocation, or when I'm initializing.

Answer 1

a->player = malloc(sizeof(struct fisherman));

You need to allocate memory to struct Fisherman before writing something to it

Answer 2

One of the problems is here:

if (array[i][j]==NULL) {
    // ... then ...
    if(array[i][j]->player==NULL)
}

If array[i][j] is NULL, then you can't dereference it. You must first assign it to a valid pointer.

Answer 3

Let's break the explanation into parts, following the code.

When you do a = malloc(sizeof(struct animal));, you have a structure with 0 on the species attribute, and memory garbage on the player attribute, for it's is an uninitialized pointer.

As @Gopi said, you have to initialize this pointer before using it:

a->player = malloc(sizeof(struct fisherman));

Now, the following statment is valid:

a->player->id_fisherman=0;

Before using a pointer, as a->player, is better to assert it. The C compiler is not very reliable on possible run-time error detection. In production code, you can set the compiler to don't assert.

A tip to avoid this type of error is to create an initialization function along with the definition of the structure. You could have written:

struct fisherman {
        int id_fisherman;
        char Name[10];
};

*fisherman malloc_fisherman(){
        return malloc(sizeof(struct fisherman));
}

*fisherman malloc_fisherman(int id){
        struct fisherman *a = malloc(sizeof(struct fisherman));
        a->id_fisherman = id;
        return a;
}

Here, you are creating two functions, one for default creation and other to create a struct with a given argument. The struct animal can rely on those functions to make its own creation functions.

Philip Guo has written a great article on basic C programming good practices. Check it out, specially the section Use assert statements to document and enforce function pre-conditions and other assumptions.

OBS: Isn't the following statement wrong? Isn't the attribute named species and not espece?

a->espece=espece;