c - 学习C，segfailt混淆

Question

我一直在尝试在春假期间自学 C，这比我预期的更具挑战性!我已经取得了一些良好的进步，但我似乎遇到了障碍。我一直在编写自己的双向链表实现。经过大约八个小时的努力，我得到了一个看起来相当连贯的程序。编译器让我一切都清楚，并且我已经多次成功构建了该项目。现在，我开始用java编程；因此，我认为上述保证是成功的铁定保证。然而，C 似乎对事物的看法有所不同(请原谅双关语)!当我尝试运行我的程序时，出现“段错误”运行时错误。我花了一些时间阅读这个错误，并且(我确信我即将被告知)这是内存分配错误的问题。我尝试过调试，但无论我在哪里设置断点，我似乎都无法抢占错误。我有点左右为难，我真的很感激你们为我提供的任何见解!我对我刚刚潦草地写下的冗长的文字表示歉意……我会尽量保持帖子脚本简短。这是我的设置的粗略轮廓:

• 具有 Cygwin 环境的 NetBeans 6.8 IDE
• 当前运行 Windows 7 x64

我将在下面发布我的代码，但当然没有义务对其进行筛选。我真正希望的是关于如何处理一般段错误的一些建议。我感觉我会经常见到他们，我想制定一个策略来解决这个特定问题。

提前致谢!没有你们，我就会陷入困境!

main.c

#include <stdlib.h>
#include <stdbool.h>
#include"dll.h"

int main(int argc, char** argv) {

    VECT coord1;
    coord1->x = 0.0012345;
    coord1->y = 0.012345;
    coord1->z = 0.12345;

    LIST coords = init_list();
    list_add(coords, coord1);

    return (EXIT_SUCCESS);
}

dll.c(双向链表源文件)

#include "dll.h"
#include <stdlib.h>
#include <string.h>
#include<stdbool.h>
#include"dll.h"

LIST init_list() {
    LIST list = (LIST) malloc(sizeof (struct list));
    list->length = 0;
    list->first = NULL;
    list->last = NULL;
    list->destructor = free;
    return list;
}

LIST_ITR list_iterator(LIST list, bool from_front) {
    LIST_ITR iter = (LIST_ITR) malloc(sizeof (struct list_itr));
    if (from_front) {
        iter->current = list->first;
    } else if (!from_front) {
        iter->current = list->last;
    } else return NULL;
    iter->started = 0;
    return iter;
}

void list_add(LIST list, VECT coords) {
    NODE node = (NODE) malloc(sizeof (struct node));
    node->coord_vector = coords;

    if (list->first == NULL) {
        node->prev = NULL;
        node->next = NULL;
        list->first = node;
        list->last = node;
    } else {
        list->last->next = node;
        node->prev = list->last;
        node->next = NULL;
        list->last = node;
    }
    list->length++;
}

VECT list_itr_current(LIST_ITR iter) {
    if (iter->started && iter->current != NULL)
        return iter->current->coord_vector;
    else {
        return NULL;
    }
}

VECT list_itr_next(LIST_ITR iter) {
    if (!iter->started && iter->current != NULL) {
        iter->started = 1;
        return iter->current->coord_vector;
    }
    if (iter->current != NULL) {
        iter->current = iter->current->next;
        return list_itr_current(iter);
    }
    return NULL;
}

VECT list_prev(LIST_ITR iter) {
    if (!iter->started && iter->current != NULL) {
        iter->started = 1;
        return iter->current->coord_vector;
    }
    if (iter->current != NULL) {
        iter->current = iter->current->prev;
        return list_itr_current(iter);
    }
    return NULL;
}

VECT list_get_first(LIST list) {
    return list->first->coord_vector;
}

VECT list_get_last(LIST list) {
    return list->last->coord_vector;
}

VECT list_pop(LIST list) {
    NODE last = list->last;
    if (last == NULL) return NULL;

    if (list->first == list->last) {
        list->first = NULL;
        list->last = NULL;
    } else {
        list->last = last->prev;
        last->prev->next = NULL;
    }

    VECT data = last->coord_vector;
    free(last);
    list->length--;
    return data;
}

VECT list_poll(LIST list) {
    NODE first = list->first;

    if (first == NULL)
        return NULL;

    if (list->first == list->last) {
        list->first = NULL;
        list->last = NULL;
    } else {
        list->first = first->next;
        first->next->prev = NULL;
    }

    VECT data = first->coord_vector;
    free(first);
    list->length--;
    return data;
}

void list_remove(LIST list, bool from_front) {
    VECT data;
    if (from_front)
        data = list_poll(list);
    else if (!from_front)
        data = list_pop(list);
    else return;
    list->destructor(data);
}

void destroy_list(LIST list) {
    NODE current = list->first;
    NODE next;
    while (current != NULL) {
        next = current->next;
        list->destructor(current->coord_vector);
        free(current);
        current = next;
    }
    free(list);
}

dll.h(双向链表头文件)

#include<stdbool.h>

#ifndef _DLL_H
#define _DLL_H

#ifdef  __cplusplus
extern "C" {
#endif

    /* A C implementation of a doubly-linked list. Contains void pointer values.
   Can be used as a LIFO stack of FIFO queue. */

#define FRONT 0
#define BACK 1

struct vector{
    double x;
    double y;
    double z;
};

typedef struct vector* VECT;

struct node{
        VECT    coord_vector;
    struct node* next;
    struct node* prev;
};

typedef struct node* NODE;

struct list{
    int length;
    NODE first;
    NODE last;
    void (*destructor)(void*);
};

typedef struct list * LIST;

struct list_itr{
    NODE current;
    char started;
};

typedef struct list_itr * LIST_ITR;

//Initializes the list
LIST init_list();

//initializes the list iterator
LIST_ITR list_iterator(LIST list, bool from_front);

//append element to end
void list_add(LIST list, VECT coords);

//Gets the data stored in the first item of the list or NULL if the list is empty
VECT list_get_first(LIST list);

//Gets the data stored in the last item of the list or NULL if the list is empty
VECT list_get_last(LIST list);

//LIFO pop: remove element and return data
VECT list_pop(LIST list);

//FIFO poll: remove element and return data
VECT list_poll(LIST list);

//Deletes element and frees memory
void list_remove(LIST list, bool from_front);

//Delete list and free all memory
void destroy_list(LIST list);

//returns the data of the element pointed to by current
VECT list_itr_current(LIST_ITR list_itr);

//Increments the index of current by 1 and returns the data stored there
VECT list_itr_next(LIST_ITR list_itr);

//Decrements the index of current by 1 and returns the data stored there
VECT list_prev(LIST_ITR list_itr);

#ifdef  __cplusplus
}
#endif

#endif  /* _DLL_H */

Answer 1

您应该使用 -Wall 标志编译器构建代码。在编译时它将打印:

main.c:9:15:警告:在此函数中使用“coord1”时未初始化 [-Wuninitialized]

这向您指出了问题所在。coord1 是一个指针类型，您可以分配给它，但 coord1 在初始化之前没有内存支持它。在下面的代码片段中，coord1 通过分配内存来存储其组件来初始化。这消除了段错误。

VECT coord1 = NULL;

coord1 = (VECT)malloc(sizeof(struct vector));
if (NULL == coord1)
{
    fprintf(stderr, "Out of memory!\n");
    exit(1);
}

coord1->x = 0.0012345;
coord1->y = 0.012345;
coord1->z = 0.12345;

通常，当程序访问操作系统尚未分配给它的内存时，就会发生段错误。未初始化的指针通常指向地址零，该地址未分配给任何程序。编译时始终使用 gcc -Wall，这会多次指出这些潜在问题。立刻帮我找到了。

此外，您还可以将 VECT 类型声明为 typedef 结构 vector (非指针类型)。

VECT coord1;
VECT* v_coord1 = &coord1;
v_coord1->x = 0.0012345;
v_coord1->y = 0.012345;
v_coord1->z = 0.12345;`

此外，变量命名约定也可以提供帮助。

struct vector{
    double x;
    double y;
    double z;
 };

typedef struct vector VECT;
typedef struct vector* pVECT;