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c - 为什么我使用以下代码从 valgrind 获取 "invalid read"和 "invalid write"?

转载 作者:太空宇宙 更新时间:2023-11-04 01:17:58 32 4
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我正在编写一个程序,以确保我了解如何在 C 中正确实现单向链表。我目前正在哈佛的 CS50 类(class)中学习,并且使用本教程,因为 CS50 人员不解释链接详细列出数据结构:https://www.youtube.com/watch?v=7Fz7JSvlr9g

代码似乎运行正常,但是当我使用 valgrind 检查它时出现“无效读取”和“无效写入”错误。

这是我的代码:

// creating and using a singly linked list in C

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

// create structure for nodes
typedef struct sllist
{
int val;
struct sllist *next;
}
sllnode;

// function declarations
sllnode *create(int sz);
void display(sllnode *head);

int main(void)
{
// declare head node and set to NULL
sllnode *head = NULL;

// prompt for size of list
printf("how many numbers would you like to store? ");
int sz;
scanf("%i", &sz);

// create linked list (create passes head pointer back to head)
head = create(sz);
// display linked list
display(head);
}

// function for creating a linked list
sllnode *create(int sz)
{
// initialize head pointer to NULL
sllnode *head = NULL;
// initialize temp pointer (for creating new nodes in the upcoming for loop)
sllnode *temp = NULL;
// initialize p for iterating through the list
sllnode *p = NULL;

for (int i = 0; i < sz; i++)
{
// allocate space for individual node
temp = (sllnode *)malloc(sizeof(sllnode));

// check to make sure we haven't run out of memory
if (temp == NULL)
{
printf("Couldn't allocate memory\n");
exit(1);
}

// prompt user for value to store
printf("enter the value #%i: ", i + 1);
scanf("%i", &(temp->val));

// intialize temp's next pointer to NULL
temp->next = NULL;

// if running first time (linked list is empty) temp becomes the head node
if (head == NULL)
{
head = temp;
}
// if the loop has run a few times (list not empty)
else
{
// start at the head
p = head;
// iterate through the list to find the last node
while (p->next != NULL)
{
p = p->next;
}
// set that node's pointer to the new node
p->next = temp;
}
free(temp);
}
// give the head pointer back to main
return head;
}

// function for displaying the linked list
void display(sllnode *head)
{
// initialize pointer p to head (coming from main)
sllnode *p = head;

// iterate through the list, printing a new value each time
while(p != NULL)
{
printf("%i -> ", p->val);
p = p->next;
}
printf("\n");
}

这是 valgrind 的输出:

==4407== Memcheck, a memory error detector
==4407== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==4407== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==4407== Command: ./ll0
==4407==
how many numbers would you like to store? 2
enter the value #1: 1
enter the value #2: 6
==4407== Invalid read of size 8
==4407== at 0x4209DA: create (ll0.c:74)
==4407== by 0x420713: main (ll0.c:29)
==4407== Address 0x6183048 is 8 bytes inside a block of size 16 free'd
==4407== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4407== by 0x420B73: create (ll0.c:81)
==4407== by 0x420713: main (ll0.c:29)
==4407==
==4407== Invalid write of size 8
==4407== at 0x420B65: create (ll0.c:79)
==4407== by 0x420713: main (ll0.c:29)
==4407== Address 0x6183048 is 8 bytes inside a block of size 16 free'd
==4407== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4407== by 0x420B73: create (ll0.c:81)
==4407== by 0x420713: main (ll0.c:29)
==4407==
==4407== Invalid read of size 4
==4407== at 0x420CAA: display (ll0.c:96)
==4407== by 0x420720: main (ll0.c:31)
==4407== Address 0x6183040 is 0 bytes inside a block of size 16 free'd
==4407== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4407== by 0x420B73: create (ll0.c:81)
==4407== by 0x420713: main (ll0.c:29)
==4407==
1 -> ==4407== Invalid read of size 8
==4407== at 0x420D62: display (ll0.c:97)
==4407== by 0x420720: main (ll0.c:31)
==4407== Address 0x6183048 is 8 bytes inside a block of size 16 free'd
==4407== at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4407== by 0x420B73: create (ll0.c:81)
==4407== by 0x420713: main (ll0.c:29)
==4407==
6 ->
==4407==
==4407== HEAP SUMMARY:
==4407== in use at exit: 0 bytes in 0 blocks
==4407== total heap usage: 2 allocs, 2 frees, 32 bytes allocated
==4407==
==4407== All heap blocks were freed -- no leaks are possible
==4407==
==4407== For counts of detected and suppressed errors, rerun with: -v
==4407== ERROR SUMMARY: 6 errors from 4 contexts (suppressed: 0 from 0)

这似乎与我访问临时节点的方式有关,但我不太了解这个问题。

最佳答案

因为在 create 函数中,您释放了刚刚添加到列表中的节点。

只需从函数中删除 free(temp);

变量名 temp 在这里具有误导性。它根本不是一个临时节点。此变量的正确名称应为 newnode

你应该再读一遍你的 C 课本中处理动态内存分配的章节。


注意 1: 顺便说一句:在您提到的视频中,create 函数中没有 free


注意 2: 请注意,此创建列表的算法效率极低。为了找到最后一个元素,整个列表从头部遍历到最后一个元素。为了提高效率,您应该维护指向最后一个元素的指针。

关于c - 为什么我使用以下代码从 valgrind 获取 "invalid read"和 "invalid write"?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52593407/

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