cs50 pset5 在卸载功能时内存泄漏，在释放内存时

Question

我目前正在处理cs50 pset5。

我在尝试释放程序中的所有内存时遇到了内存泄漏。
该错误似乎发生在我的unload 函数中，其中malloc 用于指针cursor。
如果有人可以向我提供有关如何解决此问题的指导，请告诉我。
我还为我的代码提供了 cmets 以突出显示它在做什么。

// 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 = 50;

node *table[N] = {NULL};

// Unloads dictionary from memory, returning true if successful else false
bool unload(void)
{
    // allocate memory for struct type `node` pointer 
    node *cursor = (node*)malloc(sizeof(node));                 // MEMORY LEAK happens here !!!! :(
    node *tmp = (node*)malloc(sizeof(node));

    // if memory cannot be allocated successfully, return false
    if (cursor == NULL || tmp == NULL)
    {
        return false;
    }

    // iterate the table for N'times (N = 50, for now)
    for (int i = 0; i < N; i++)
    {
        // if the table has nothing inside the i'th bucket, return false 
        // this means, table has no linked list inside i'th bucket 
        if (table[i] == NULL)
        {
            continue;
        }
        else
        {
            // assign tmp pointer to the head node, and cursor to anything next to tmp 
            tmp = table[i];
            cursor = tmp;
            cursor = cursor->next;
            // free memory for tmp pointer 
            free(tmp);

            // continue traversing the linked list until cursor pointer points to NULL (end of the linked list)
            while (cursor != NULL)
            {
                tmp = cursor;
                cursor = cursor->next;
                free(tmp);
            }
        }
    }

    // free memory for cursor
    free(cursor);
    return true;
}

我得到的错误，运行后，help50 valgrind ./speller dictionaries/small texts/cat.txt。

==14685== Memcheck, a memory error detector
==14685== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==14685== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==14685== Command: ./speller dictionaries/small texts/cat.txt
==14685== 
32
==14685== Invalid write of size 1
==14685==    at 0x40121F: hash (dictionary.c:67)
==14685==    by 0x401354: load (dictionary.c:99)
==14685==    by 0x400A2B: main (speller.c:41)
==14685==  Address 0x55cc88a is 2 bytes after a block of size 8 alloc'd
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4011D5: hash (dictionary.c:61)
==14685==    by 0x401354: load (dictionary.c:99)
==14685==    by 0x400A2B: main (speller.c:41)
==14685== 
==14685== Invalid read of size 1
==14685==    at 0x40122C: hash (dictionary.c:68)
==14685==    by 0x401354: load (dictionary.c:99)
==14685==    by 0x400A2B: main (speller.c:41)
==14685==  Address 0x55cc88a is 2 bytes after a block of size 8 alloc'd
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4011D5: hash (dictionary.c:61)
==14685==    by 0x401354: load (dictionary.c:99)
==14685==    by 0x400A2B: main (speller.c:41)
==14685== 
19

MISSPELLED WORDS

47
A
32
20
is
27
not
47
a
==14685== Invalid write of size 1
==14685==    at 0x40121F: hash (dictionary.c:67)
==14685==    by 0x401124: check (dictionary.c:32)
==14685==    by 0x400D50: main (speller.c:114)
==14685==  Address 0x55cdf9a is 2 bytes after a block of size 8 alloc'd
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4011D5: hash (dictionary.c:61)
==14685==    by 0x401124: check (dictionary.c:32)
==14685==    by 0x400D50: main (speller.c:114)
==14685== 
==14685== Invalid read of size 1
==14685==    at 0x40122C: hash (dictionary.c:68)
==14685==    by 0x401124: check (dictionary.c:32)
==14685==    by 0x400D50: main (speller.c:114)
==14685==  Address 0x55cdf9a is 2 bytes after a block of size 8 alloc'd
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4011D5: hash (dictionary.c:61)
==14685==    by 0x401124: check (dictionary.c:32)
==14685==    by 0x400D50: main (speller.c:114)
==14685== 
19

WORDS MISSPELLED:     4
WORDS IN DICTIONARY:  2
WORDS IN TEXT:        6
TIME IN load:         0.03
TIME IN check:        0.00
TIME IN size:         0.00
TIME IN unload:       0.00
TIME IN TOTAL:        0.03

==14685== 
==14685== HEAP SUMMARY:
==14685==     in use at exit: 1,000 bytes in 9 blocks
==14685==   total heap usage: 23 allocs, 14 frees, 10,944 bytes allocated
==14685== 
==14685== 56 bytes in 1 blocks are definitely lost in loss record 1 of 4
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4013F1: unload (dictionary.c:128)
==14685==    by 0x400ED0: main (speller.c:154)
==14685== 
==14685== 56 bytes in 1 blocks are definitely lost in loss record 2 of 4
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4013FF: unload (dictionary.c:129)
==14685==    by 0x400ED0: main (speller.c:154)
==14685== 
==14685== 336 bytes in 6 blocks are definitely lost in loss record 3 of 4
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x401131: check (dictionary.c:33)
==14685==    by 0x400D50: main (speller.c:114)
==14685== 
==14685== 552 bytes in 1 blocks are still reachable in loss record 4 of 4
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x5258E49: __fopen_internal (iofopen.c:65)
==14685==    by 0x5258E49: fopen@@GLIBC_2.2.5 (iofopen.c:89)
==14685==    by 0x4012DE: load (dictionary.c:83)
==14685==    by 0x400A2B: main (speller.c:41)
==14685== 
==14685== LEAK SUMMARY:
==14685==    definitely lost: 448 bytes in 8 blocks
==14685==    indirectly lost: 0 bytes in 0 blocks
==14685==      possibly lost: 0 bytes in 0 blocks
==14685==    still reachable: 552 bytes in 1 blocks
==14685==         suppressed: 0 bytes in 0 blocks
==14685== 
==14685== For counts of detected and suppressed errors, rerun with: -v
==14685== ERROR SUMMARY: 15 errors from 7 contexts (suppressed: 0 from 0)

Asking for help...

==14685== 56 bytes in 1 blocks are definitely lost in loss record 1 of 4
==14685==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14685==    by 0x4013F1: unload (dictionary.c:128)
==14685==    by 0x400ED0: main (speller.c:154)

Looks like your program leaked 56 bytes of memory. Did you forget to free memory that you allocated via malloc? Take a closer look at line 128 of dictionary.c.

那么为什么会发生内存泄漏呢？
A memory leak happens when a programmer creates a memory from heap but forgets to free it.

但是，在我的代码中，我已经释放了 cursor 和 tmp 指针在使用后。
我不知道为什么会遇到错误，虽然我在函数结束时释放它。

Answer 1

是的，“内存泄漏发生在这里！！！”如你所说。

您分配了两个缓冲区，但后来它们的指针被table[i] 覆盖而没有被释放。这是内存泄漏。

为避免内存泄漏，请停止分配未使用的缓冲区。

部分

    // allocate memory for struct type `node` pointer 
    node *cursor = (node*)malloc(sizeof(node));                 // MEMORY LEAK happens here !!!! :(
    node *tmp = (node*)malloc(sizeof(node));

    // if memory cannot be allocated successfully, return false
    if (cursor == NULL || tmp == NULL)
    {
        return false;
    }

应该只是

    // allocate memory for struct type `node` pointer 
    node *cursor;
    node *tmp;

指针的内存分配在堆栈上。

也是部分

    // free memory for cursor
    free(cursor);

应该被删除，因为列表已经被释放，并且光标的内存将在从函数返回时自动释放（从堆栈中）。