Question

我对以下简单的ANSI C代码有一种奇怪的行为。我有一个指向结构中的字符的指针，不知何故，我有坏指针，空值和段错误。我做了些蠢事吗？

#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;
}

Answer 1

在game_init中，您声明了scene_t类型的局部变量。 game_init结束后，此变量不再存在 - 您无法在该函数之外正确使用它，但这是您在访问game_t变量中的场景时尝试执行的操作

如果你想创建一个可以在函数外部使用的scene_t，你需要使用

手动为它分配内存

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

...然后将其作为指针使用。

Answer 2

您正在保存本地地址，当功能超出范围时，“场景”将被销毁。

scene_t scene;

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

您应该改为分配场景

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

Answer 3

问题在于您没有分配scene：

scene_t scene;

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

此处的scene_t对象位于堆栈中，一旦退出该函数，该内存将被回收。你应该改为使用

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

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

这样，内存将从堆中分配，从函数退出中幸存下来。

一旦完成对象，您将需要稍后free这个记忆。

Answer 4

创建新鬼时，您也不会为scene->appearance指定值。然后，当您尝试打印外观后，您将获得指向打印的任何指针。

带有指向struct里面char的指针的Segfault，ANSI C代码

4 个答案: