c - 尝试释放内存时出错

Question

我在尝试释放内存时遇到了一些错误。下面发布了我的所有代码。我正在使用 ubuntu 并使用 gcc 编译我的代码。但是当我尝试执行代码时，在尝试释放内存时出现错误。我在我的代码上添加了注释来解释我的疑问。我正在使用堆栈结构。如何在不出错的情况下释放内存以将释放内存转换为字符？如果我不释放分配给数据(字符)的内存，而只释放元素(包含字符数据)的内存，那么分配给数据的内存会发生这种情况？免费吗？

错误

{
    *** glibc detected *** ./pilha: free(): invalid next size (fast):      0x08b86018 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x74f82)[0xb7637f82]
./pilha[0x80485ba]
./pilha[0x804864c]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb75dc4d3]
./pilha[0x8048411]
    ======= Memory map: ========
}

<小时/>

代码

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

typedef struct Stack_element{
    char *data;
    struct Stack_element *next;

}Element;

typedef struct Position{
    Element *top;
    int size;
}stack;

void start(stack *aux){
    aux->top = NULL;
    aux->size = 0;
}

int push(stack *aux, char value){

    Element *new_element;
    if ((new_element = (Element*) malloc(sizeof(Element))) == NULL)
        return -1; //an error occur
    if ((new_element->data = (char*) malloc(sizeof(char))) == NULL)
        return -1; //an error occur

    strcpy(new_element->data, &value);

    new_element->next = aux->top;
    aux->top = new_element;
    aux->size++;

}

int empty(stack *aux){

    if ((aux->size) == 0){
        return -1;
    }
    return 0;

}

char pop(stack *aux){

    Element *element;
    char value='0';

    if (empty(aux)){
        return '1';
    }


    element = aux->top;
    aux->top = aux->top->next;

    /*
        To observe the line below. When a element exist in the stack and
        I try remove this element, first I free the data in that node (element)
        so I turn free memory allocated for the element.
        If I didn't free data memory allocated before (in push fuction), I
        don't get any error. But the memory allocated for the data, what happens?
        Does is it continues allocated?
    */

    value = *(element->data);
    free(element->data);//THE ERROR OCCURS HERE, IN THIS LINE
    free(element);//Just after free the data element memory, I also free the element's memory

    aux->size--;
    return value;
}

int main(){

    stack p;
    char value;

    start(&p);

    //no error occurs. there isn't any element at this moment.
    printf("%c\n",pop(&p));

    //valor = 't';
    if (push(&p, 't')){
        printf("Add a char\n");
    }
    pop(&p);//the error occurrs now, after insert an new element in the stack
    printf("The End.");

}

Answer 1

在您的代码中，

strcpy(new_element->data, &value);

不正确。您只为一个 char 分配了内存，而该字符没有空间用于空终止符。相反，你应该使用

*(new_element->data) = value;

否则，如果 strcpy() 使用不当，您会因内存溢出而弄乱分配的内存，从而导致 undefined behaviour .