那些遇到过的问题

有没有办法验证我的程序没有内存泄漏？

Jak*_*son 6 c memory debugging memory-leaks memory-management

我想确定以下程序（查找最大子数组的实现）是否泄漏内存。有没有通用的方法来确定这一点？例如使用调试器的某些功能？什么是一般策略？

struct Interval {
   int max_left;
   int max_right;
   int sum;
};

struct Interval * max_crossing_subarray(int A[], int low, int mid, int high) {
    struct Interval * crossing = malloc(sizeof(struct Interval));

    int left_sum = INT_MIN;
    int sum = 0;

    for(int i = mid; i >= low; --i) {
        sum = sum + A[i];
        if(sum > left_sum) {
            left_sum = sum;
            crossing->max_left = i;
        }
    }

    int right_sum = INT_MIN;
    sum = 0;

    for(int j = mid+1; j <= high; ++j) {
        sum = sum + A[j];
        if(sum > right_sum) {
            right_sum = sum;
            crossing->max_right = j;
        }
    }

    crossing->sum = left_sum + right_sum;

    return crossing;
}

struct Interval * max_subarray(int A[], int low, int high) {
    if(high == low) {
        struct Interval * base = malloc(sizeof(struct Interval));
        *base = (struct Interval) { low, high, A[low] };
        return base;
    } else {
        int mid = floor((low+high)/2);
        struct Interval * left = malloc(sizeof(struct Interval));
        struct Interval * right = malloc(sizeof(struct Interval));
        left = max_subarray(A, low, mid);
        right = max_subarray(A, mid+1, high);
        struct Interval * cross = max_crossing_subarray(A, low, mid, high);
        if(left->sum >= right->sum & right->sum >= cross->sum) {
            free(right);
            free(cross);
            return left;
        } else if(right->sum >= left->sum & right->sum >= cross-> sum) {
            free(left);
            free(cross);
            return right;
        } else {
            free(left);
            free(right);
            return cross;
        }
    }
}

int main()
{
    int A[] = {-10, 7, -5, -3, 40, 4, -1, 8, -3, -1, -5, 20, 7};
    struct Interval * result = max_subarray(A, 0, 12);

    printf("left: %i, right: %i, sum: %i\n", result->max_left, result->max_right, result->sum);

    return 0;
}
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由于程序的递归性质，它很难遵循（至少对我而言）。我想我已经把所有东西都塞进去了，但我想找到一种方法来确定。

编辑：所选答案中建议的软件允许我找到所有泄漏，正如评论中指出的那样，没有理由分配左右，下面是无内存泄漏的代码。

struct Interval {
   int max_left;
   int max_right;
   int sum;
};

struct Interval * max_crossing_subarray(int A[], int low, int mid, int high) {
    struct Interval * crossing = malloc(sizeof(struct Interval));

    int left_sum = INT_MIN;
    int sum = 0;

    for(int i = mid; i >= low; --i) {
        sum = sum + A[i];
        if(sum > left_sum) {
            left_sum = sum;
            crossing->max_left = i;
        }
    }

    int right_sum = INT_MIN;
    sum = 0;

    for(int j = mid+1; j <= high; ++j) {
        sum = sum + A[j];
        if(sum > right_sum) {
            right_sum = sum;
            crossing->max_right = j;
        }
    }

    crossing->sum = left_sum + right_sum;

    return crossing;
}

struct Interval * max_subarray(int A[], int low, int high) {
    if(high == low) {
        struct Interval * base = malloc(sizeof(struct Interval));
        *base = (struct Interval) { low, high, A[low] };
        return base;
    } else {
        int mid = floor((low+high)/2);
        struct Interval * left = max_subarray(A, low, mid);
        struct Interval * right = max_subarray(A, mid+1, high);
        struct Interval * cross = max_crossing_subarray(A, low, mid, high);
        if(left->sum >= right->sum & right->sum >= cross->sum) {
            free(right);
            free(cross);
            return left;
        } else if(right->sum >= left->sum & right->sum >= cross-> sum) {
            free(left);
            free(cross);
            return right;
        } else {
            free(left);
            free(right);
            return cross;
        }
    }
}

int main()
{
    int A[] = {-10, 7, -5, -3, 40, 4, -1, 8, -3, -1, -5, 20, 7};
    struct Interval * result = max_subarray(A, 0, 13-1);

    printf("left: %i, right: %i, sum: %i\n", result->max_left, result->max_right, result->sum);

    return 0;
}
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dbu*_*ush 8

您可以使用valgrind。它是 Linux 和其他类 UNIX 系统的内存调试工具，可以发现内存泄漏以及无效的内存访问。

当我通过 valgrind 运行此代码时，它输出以下内容：

[dbush@db-centos7 ~]$ valgrind ./x1
==3406== Memcheck, a memory error detector
==3406== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3406== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==3406== Command: ./x1
==3406== 
left: 4, right: 12, sum: 69
==3406== 
==3406== HEAP SUMMARY:
==3406==     in use at exit: 300 bytes in 25 blocks
==3406==   total heap usage: 49 allocs, 24 frees, 588 bytes allocated
==3406== 
==3406== LEAK SUMMARY:
==3406==    definitely lost: 300 bytes in 25 blocks
==3406==    indirectly lost: 0 bytes in 0 blocks
==3406==      possibly lost: 0 bytes in 0 blocks
==3406==    still reachable: 0 bytes in 0 blocks
==3406==         suppressed: 0 bytes in 0 blocks
==3406== Rerun with --leak-check=full to see details of leaked memory
==3406== 
==3406== For counts of detected and suppressed errors, rerun with: -v
==3406== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
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所以你有一些泄漏。现在让我们传递--leak-check=full选项以查看这些泄漏的确切位置：

==11531== Memcheck, a memory error detector
==11531== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==11531== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==11531== Command: ./x1
==11531== 
left: 4, right: 12, sum: 69
==11531== 
==11531== HEAP SUMMARY:
==11531==     in use at exit: 300 bytes in 25 blocks
==11531==   total heap usage: 49 allocs, 24 frees, 588 bytes allocated
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 1 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 2 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 3 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 4 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 5 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 6 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 7 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 8 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 9 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 10 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 11 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 12 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 13 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 14 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 15 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 16 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 17 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 18 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 19 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 20 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 21 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 22 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 23 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007A8: max_subarray (x1.c:49)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 24 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x4007B6: max_subarray (x1.c:50)
==11531==    by 0x4007CE: max_subarray (x1.c:51)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x4007E9: max_subarray (x1.c:52)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== 12 bytes in 1 blocks are definitely lost in loss record 25 of 25
==11531==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==11531==    by 0x40065B: max_crossing_subarray (x1.c:13)
==11531==    by 0x400802: max_subarray (x1.c:53)
==11531==    by 0x400931: main (x1.c:73)
==11531== 
==11531== LEAK SUMMARY:
==11531==    definitely lost: 300 bytes in 25 blocks
==11531==    indirectly lost: 0 bytes in 0 blocks
==11531==      possibly lost: 0 bytes in 0 blocks
==11531==    still reachable: 0 bytes in 0 blocks
==11531==         suppressed: 0 bytes in 0 blocks
==11531== 
==11531== For counts of detected and suppressed errors, rerun with: -v
==11531== ERROR SUMMARY: 25 errors from 25 contexts (suppressed: 0 from 0)
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大多数泄漏都来自以下两条线：

    struct Interval * left = malloc(sizeof(struct Interval));
    struct Interval * right = malloc(sizeof(struct Interval));
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如果我们看看接下来的两行，很明显为什么：

    left = max_subarray(A, low, mid);
    right = max_subarray(A, mid+1, high);
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因此，在将分配的内存地址分配给这些指针后，您立即用其他值覆盖这些地址，从而导致泄漏。这可以通过摆脱malloc调用并使用函数调用的结果进行初始化来解决：

    struct Interval * left = max_subarray(A, low, mid);
    struct Interval * right = max_subarray(A, mid+1, high);
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最后一个在 max_crossing_subarray

struct Interval * crossing = malloc(sizeof(struct Interval));
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这个指针是从函数返回的，所以我们需要看看缺失free的地方。环顾四周后，我们看到它被调用 from max_subarray，最终将其返回main为result：

struct Interval * result = max_subarray(A, 0, 13-1);

printf("left: %i, right: %i, sum: %i\n", result->max_left, result->max_right, result->sum);

return 0;
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但是正如你所看到的，这里没有调用free，所以让我们添加它：

struct Interval * result = max_subarray(A, 0, 13-1);

printf("left: %i, right: %i, sum: %i\n", result->max_left, result->max_right, result->sum);

free(result);
return 0;
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现在，在完成这些修复之后，我们将再次运行 valgrind：

    struct Interval * left = malloc(sizeof(struct Interval));
    struct Interval * right = malloc(sizeof(struct Interval));
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并且泄漏消失了。

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