如何理解 valgrind 输出的内存泄漏？

Posted 2023-03-29

【中文标题】如何理解 valgrind 输出的内存泄漏？

【英文标题】：How can I understand a memory leak from valgrind output?

【发布时间】：2021-09-29 03:52:56

【问题描述】：

我正在参加 CS50 课程，我正在第 5 周学习 Pset5 Speller。对于不熟悉的人来说，目标是编辑一个特定的 .c 文件以使五个函数正常运行，以便主函数（位于单独的文件中）可以执行以下操作：

LOAD - 将字典加载到哈希表中 HASH - 通过此函数运行一个单词，以帮助将其加载到字典中或稍后搜索该单词 SIZE - 查看字典中有多少单词 检查 - 查看文本中的单词是否在字典中 UNLOAD - 释放字典以免内存泄漏

请注意，该文件是在课堂上提供给我的，我将编辑函数中的空间 - 我唯一可以更改的另一件事是 const unsigned int N = 1000;，我将其设置为 1000，就像一个任意数字一样，但它可以是任何东西。

我只有一件事有问题（我可以说）。我已尽一切努力让它运行，但 Check50（判断我是否正确执行的程序）告诉我我有内存错误：

Results for cs50/problems/2021/x/speller generated by check50 v3.3.0
:) dictionary.c exists
:) speller compiles
:) handles most basic words properly
:) handles min length (1-char) words
:) handles max length (45-char) words
:) handles words with apostrophes properly
:) spell-checking is case-insensitive
:) handles substrings properly
:( program is free of memory errors
    valgrind tests failed; see log for more information.

当我运行 valgrind 时，这就是它给我的：

==347== 
==347== HEAP SUMMARY:
==347==     in use at exit: 472 bytes in 1 blocks
==347==   total heap usage: 143,096 allocs, 143,095 frees, 8,023,256 bytes allocated
==347== 
==347== 472 bytes in 1 blocks are still reachable in loss record 1 of 1
==347==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==347==    by 0x4A29AAD: __fopen_internal (iofopen.c:65)
==347==    by 0x4A29AAD: fopen@@GLIBC_2.2.5 (iofopen.c:86)
==347==    by 0x401B6E: load (dictionary.c:83)
==347==    by 0x4012CE: main (speller.c:40)
==347== 
==347== LEAK SUMMARY:
==347==    definitely lost: 0 bytes in 0 blocks
==347==    indirectly lost: 0 bytes in 0 blocks
==347==      possibly lost: 0 bytes in 0 blocks
==347==    still reachable: 472 bytes in 1 blocks
==347==         suppressed: 0 bytes in 0 blocks
==347== 
==347== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

这对我来说似乎很神秘，我希望有人可以帮助解释并帮助我解决我的问题（Help50 没有任何建议）。

这是我的实际代码（请记住，第二个文档的主要功能实际上利用了所提供的功能，所以可以，例如，功能似乎没有正确的顺序）。

// Implements a dictionary's functionality

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

#include "dictionary.h"

// Represents a node in a hash table
typedef struct node

    char word[LENGTH + 1];
    struct node *next;

node;

// Number of buckets in hash table
const unsigned int N = 1000;

// Hash table
node *table[N];

// Dictionary size
int dictionary_size = 0;

// Returns true if word is in dictionary, else false
bool check(const char *word)

    // TODO #4!
    
    // make lowercase copy of word
    char copy[strlen(word) + 1];
    for (int i = 0; word[i]; i++)
    
        copy[i] = tolower(word[i]);
    
    copy[strlen(word)] = '\0';
    
    // get hash value
    int h = hash(copy);

    // use hash value to see if word is in bucket
    if (table[h] != NULL)
    
        node *temp = table[h];
        
        while (temp != NULL)
        
            if (strcmp(temp->word, copy) == 0)
            
                return true;
            
            
            temp = temp->next;
        
    
    
    return false;


// Hashes word to a number
unsigned int hash(const char *word)

    // TODO #2
    // source: https://www.reddit.com/r/cs50/comments/1x6vc8/pset6_trie_vs_hashtable/cf9189q/
    // I used this source because I had trouble understanding different variations - this one explained everything well.
    // I modified it slightly to fit my needs
    unsigned int h = 0;
    for (int i = 0; i < strlen(word); i++)
    
        h = (h << 2) ^ word[i];
    
    return h % N;


// Loads dictionary into memory, returning true if successful, else false
bool load(const char *dictionary)

    // TODO #1!
    // open dictionary file
    FILE *file = fopen(dictionary, "r");
    if (file == NULL)
    
        return false;
    
    
    // read strings from file one at a time
    char word[LENGTH + 1];
    while (fscanf(file, "%s", word) != EOF)
    
        node *n = malloc(sizeof(node));
        if (n == NULL)
        
            return false;
        
        
        // place word into node
        strcpy(n->word, word);
        
        // use hash function to take string and return an index
        int h = hash(word);

        // make the current node point to the bucket we want
        n->next = table[h];
        
        // make the bucket start now with the current node
        table[h] = n;
        
        //count number of words loaded
        dictionary_size++;
    

    return true;


// Returns number of words in dictionary if loaded, else 0 if not yet loaded
unsigned int size(void)

    // TODO #3!
    return dictionary_size;


// Unloads dictionary from memory, returning true if successful, else false
bool unload(void)

    // TODO #5!
    for (int i = 0; i < N; i++)
    
        while (table[i] != NULL)
        
            node *temp = table[i]->next;
            free(table[i]);
            table[i] = temp;
        
    
    return true;

【问题讨论】：

fclose(file); 什么时候完成？

【参考方案1】：

就像我们必须 free 每个指针 malloc 一样，我们必须 fclose 每个 FILE* fopen。

您的问题源于此行：

FILE *file = fopen(dictionary, "r");

没有对应的fclose(file) 调用。将此添加到您的 loads 函数的末尾，在返回之前。

Valgrind 可以为调试提供非常有用的信息（尤其是当您的代码使用 -g 编译以获取调试信息时），就像您问题的以下摘录：

==347== 472 bytes in 1 blocks are still reachable in loss record 1 of 1
==347==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==347==    by 0x4A29AAD: __fopen_internal (iofopen.c:65)
==347==    by 0x4A29AAD: fopen@@GLIBC_2.2.5 (iofopen.c:86)
==347==    by 0x401B6E: load (dictionary.c:83)
==347==    by 0x4012CE: main (speller.c:40)

Valgrind 为您提供了分配内存最终泄漏的堆栈跟踪 - 您可以看到您自己代码中的最后一行是 dictionary.c:83，这是调用 fopen 的行。

【讨论】：

如果您在 valgrind 输出中解释帮助您找到此泄漏的线索，答案会更好。

@DanielKleinstein 谢谢！那行得通！感谢您的帮助。