我目前正在进行 cs50 pset5.
当我试图释放程序中的所有内存时，我遇到了一个内存泄漏。该错误似乎发生在我的 unload 函数，其中 malloc 对于指针 cursor. 如果有人能给我提供如何解决这个问题的指导，请告诉我。我也为我的代码提供了注释，以强调它在做什么。
// 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;
}
运行后，我得到的错误是 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？ 当程序员从堆中创建了一个内存，但忘记释放它时，就会发生内存泄漏。
然而，在我的代码中，我已经释放了两个 cursor 和 tmp 指针使用后。我不知道为什么会遇到这个错误，虽然我在函数结束时释放了它。
是的，"MEMORY LEAK发生在这里！！！"正如你所说。
你分配了两个缓冲区，但后来它们的指针被覆盖了。table[i] 而不被释放。这就是内存泄漏。
为了避免内存泄漏，请停止分配未使用的缓冲区。
该部分
// 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;
}
应是
// allocate memory for struct type `node` pointer
node *cursor;
node *tmp;
指针的内存是在栈上分配的。
另外，部分
// free memory for cursor
free(cursor);
应该被删除，因为列表已经被释放，并且游标的内存将在从函数返回时自动释放（从堆栈）。