错误而在结构访问数组元素 [英] Error while accessing element of array in struct

问题描述

我试图写一个的ArrayList计划（到Java的 的ArrayList ），将使用的 的realloc ，这样程序员就不必担心数组的存储空间。这是我的code：

I'm trying to write an "ArrayList" program (Similar to the Java ArrayList) that will expand automaticly using realloc, so that the programmer won't have to worry about storage space in arrays. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#define ARR_DEFAULT_SIZE 20
#define INCR 10

#define ARRTYPE char // Files using this #undef this macro and provide their own type

typedef struct {
    ARRTYPE *arr;
    long size;
    long nextincr;
} arrlst;

arrlst *nlst(void);
void add(arrlst *, ARRTYPE);
ARRTYPE elmat(arrlst *, long);

int main(int argc, char **argv) {
    arrlst *lst = nlst();
    add(lst, 'h');
}

arrlst *nlst() {
    arrlst lst = { malloc(ARR_DEFAULT_SIZE), 0, ARR_DEFAULT_SIZE };
    arrlst *lstptr = &lst;
    return lstptr;
}

void add(arrlst *lst, ARRTYPE elm) {
    if (lst->size >= lst->nextincr) {
        ARRTYPE *tmp = lst->arr;
        lst->nextincr += INCR;
        lst->arr = realloc(lst->arr, lst->nextincr);

        for (int i = 0; i < sizeof tmp; i++)
            lst->arr[i] = tmp[i];
    }

    lst->arr[lst->size++] = elm;
}

ARRTYPE elmat(arrlst *lst, long at) {
    if (lst->size < at)
        strerror(14);

    return lst->arr[at];
}

我的问题是，每当我运行此，调用添加（）产生一个段错误，并且由于在大部分code的补充（）就第一时间跳过调用它，错误的行必须是：

My problem is that whenever I run this, calling add() produces a segfault, and since most of the code in add() is skipped on the first time calling it, the line of error must be:

lst->arr[lst->size++] = elm;

我不知道为什么会出现段错误。请帮助！

And I don't know why this would segfault. Please help!

推荐答案

由于在 NLST 返回一个指针指向一个局部变量和局部变量超出范围和死的时候，它们的功能在返回的定义。使用该指针将导致的未定义行为的，这是崩溃一个非常常见的原因。

Because in nlst you return a pointer to a local variable, and local variables goes out of scope and "dies" when the function they are defined in returns. Using that pointer will lead to undefined behavior, which is a very common reason for crashes.

您有两种解决方法：要么 NLST 应该动态分配 arrlst 结构并返回指针。或者你在一个指针传递 arrlst 结构，从而通过模拟参考合格。

You have two solutions: Either nlst should dynamically allocate the arrlst structure and return that pointer. Or you pass in a pointer to an arrlst structure, thereby emulating pass by reference.

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