CODEWITHSUNDEEP
cpointersstruct
vinnylinux
vinnylinux

Reputation: 7024

Segfault with pointer to char inside struct, ANSI C code

I'm having a strange behavior with the following simple ANSI C code. I have a pointer to a char inside a struct, and somehow, i have bad pointers, nulls and segfaults. Am i doing something stupid?

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>

#define MAX_ENTITIES 10

typedef enum
{
    COMPONENT_NONE = 0,
    COMPONENT_DISPLACEMENT = 1 << 0,
    COMPONENT_VELOCITY = 1 << 1,
    COMPONENT_APPEARANCE = 1 << 2
} component_t;

typedef struct
{
    float x;
    float y;
} Displacement;

typedef struct
{
    float x;
    float y;
} Velocity;

typedef struct
{
    char *name;
} Appearance;

typedef struct
{
    int entities[MAX_ENTITIES];

    Displacement displacement[MAX_ENTITIES];
    Velocity velocity[MAX_ENTITIES];
    Appearance appearance[MAX_ENTITIES];
} scene_t;

typedef struct
{
    int active;
    scene_t *current_scene;
} game_t;

unsigned int entity_create(scene_t *scene)
{
    unsigned int entity;

    for (entity = 0; entity < MAX_ENTITIES; ++entity) {
        if (scene->entities[entity] == COMPONENT_NONE) {
            printf("Entity created: %u\n", entity);
            return entity;
        }
    }

    printf("Error! No more entities left!\n");
    return MAX_ENTITIES;
}

unsigned int scene_add_box(scene_t *scene, float x, float y, float vx, float vy)
{
    unsigned int entity = entity_create(scene);

    scene->entities[entity] = COMPONENT_DISPLACEMENT | COMPONENT_VELOCITY | COMPONENT_APPEARANCE;

    scene->displacement[entity].x = x;
    scene->displacement[entity].y = y;

    scene->velocity[entity].x = vx;
    scene->velocity[entity].y = vy;

    scene->appearance[entity].name = "Box";

    return entity;
}

unsigned int scene_add_tree(scene_t *scene, float x, float y)
{
    unsigned int entity = entity_create(scene);

    scene->entities[entity] = COMPONENT_DISPLACEMENT | COMPONENT_APPEARANCE;

    scene->displacement[entity].x = x;
    scene->displacement[entity].y = y;

    scene->appearance[entity].name = "Tree";

    return entity;
}

unsigned int scene_add_ghost(scene_t *scene, float x, float y, float vx, float vy)
{
    unsigned int entity = entity_create(scene);

    scene->entities[entity] = COMPONENT_DISPLACEMENT | COMPONENT_VELOCITY;

    scene->displacement[entity].x = x;
    scene->displacement[entity].y = y;

    scene->velocity[entity].x = vx;
    scene->velocity[entity].y = vy;

    return entity;
}

void update_render(scene_t *scene)
{
    const int mask = (COMPONENT_DISPLACEMENT | COMPONENT_APPEARANCE);
    unsigned int entity;
    Displacement *d;
    Appearance *a;

    for (entity = 0; entity < MAX_ENTITIES; ++entity) {
        if ((scene->entities[entity] & mask) == mask) {
            d = &(scene->displacement[entity]);
            a = &(scene->appearance[entity]);

            printf("%s at (%f, %f)\n", a->name, d->x, d->y);
        }
    }
}

void game_init(game_t *game)
{
    scene_t scene;

    memset(&scene, 0, sizeof(scene));
    game->current_scene = &scene;
    game->active = 0;

    scene_add_tree(game->current_scene, 5.0f, -3.2f);
    scene_add_box(game->current_scene, 0.0f, 0.0f, 0.0f, 0.0f);
    scene_add_ghost(game->current_scene, 10.0f, 4.0f, 0.0f, 0.0f);
}

void game_update(game_t *game)
{
    update_render(game->current_scene);
}


int main(int argc, char **argv)
{
    game_t game;

    memset(&game, 0, sizeof(game));
    game_init(&game);

    while (game.active == 0) {
        game_update(&game);
    }

    return 0;
}

Upvotes: 0

Views: 181

Answers (4)

Jackson
Jackson

Reputation: 5657

You also don't assign a value to scene->appearance when you are creating a new ghost. When you then try and print the appearence later you'll get whatever the pointer happens to point to printed ou.

Upvotes: 1

Michael Madsen
Michael Madsen

Reputation: 55009

In game_init, you are declaring a local variable of type scene_t. Once game_init ends, this variable no longer exists - you cannot use it correctly outside that function, but that's what you try to do when you access the scene inside your game_t variable.

If you want to create a scene_t which can be used outside of the function, you need to allocate the memory for it manually by using 

scene_t* scene = malloc(sizeof(scene_t));

...and then working with it as a pointer instead.

Upvotes: 2

6502
6502

Reputation: 114521

The problem is that you're not allocating a scene:

scene_t scene;

memset(&scene, 0, sizeof(scene));
game->current_scene = &scene;

the scene_t object here is on the stack and that memory will be reclaimed once you exit from the function. You should instead use

scene_t *pscene = malloc(sizeof(scene_t));

memset(pscene, 0, sizeof(scene));
game->current_scene = pscene;

this way the memory will be allocated from the heap, surviving the exit from the function.

You will need later to free this memory once done with the object.

Upvotes: 1

AndersK
AndersK

Reputation: 36082

You are saving a local address, when function goes out of scope "scene" will be destroyed.

scene_t scene;

memset(&scene, 0, sizeof(scene));
game->current_scene = &scene;

You should instead allocate scene

scene_t* scene = malloc(sizeof(scene_t));

Upvotes: 1

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