即使在释放 C 语言中的 malloc 空间后，Valgrind 也能检测到可访问的块（cs50 问题集 4 恢复）

问：

我已经完成了 cs50 的问题集 4 称为 recover。您将在下面看到我已将 malloc 用于名为 sfile 的 char*。在文件的末尾，我记得释放分配的空间（又名 sfile）并关闭我打开的 2 个文档。

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

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./recover IMAGE\n");
        return 1;
    }
    printf("%s", argv[1]);

    FILE *file = fopen(argv[1], "r");

    if (file == NULL)
    {
        printf(" can't be opened for reading\n");
        return 2;
    }

    unsigned char arr[512];

    int s = 0;

    FILE *output_file = NULL;

    char *sfile = malloc(sizeof(char) * 8);

    while (fread(arr, sizeof(char), 512, file) != 0)
    {

        if (arr[0] == 0xff && arr[1] == 0xd8 && arr[2] == 0xff && (arr[3] & 0xf0) == 0xe0)
        {
            sprintf(sfile, "%03i.jpg", s);

            output_file = fopen(sfile, "w");

            s++;
        }

        if (output_file != NULL)
        {
            fwrite(arr, sizeof(char), 512, output_file);
        }

    }

    free(sfile);

    fclose(output_file);
    fclose(file);


    return 0;
}

但是，当运行 check50 时，这是一个检查您是否完成了练习的所有要点的工具，我只收到一条红色消息，表明 valgrind 检测到内存错误。所以我的下一步是运行：

$ valgrind ./recover card.raw

这是返回的内容：

==53590== HEAP SUMMARY:
==53590==     in use at exit: 23,128 bytes in 49 blocks
==53590==   total heap usage: 1,091 allocs, 1,042 frees, 240,880 bytes allocated
==53590== 
==53590== 23,128 bytes in 49 blocks are still reachable in loss record 1 of 1
==53590==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==53590==    by 0x4A076CD: __fopen_internal (iofopen.c:65)
==53590==    by 0x4A076CD: fopen@@GLIBC_2.2.5 (iofopen.c:86)
==53590==    by 0x109304: main (recover.c:37)
==53590== 
==53590== LEAK SUMMARY:
==53590==    definitely lost: 0 bytes in 0 blocks
==53590==    indirectly lost: 0 bytes in 0 blocks
==53590==      possibly lost: 0 bytes in 0 blocks
==53590==    still reachable: 23,128 bytes in 49 blocks
==53590==         suppressed: 0 bytes in 0 blocks
==53590== 
==53590== For lists of detected and suppressed errors, rerun with: -s
==53590== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

该程序运行良好，这是剩下的唯一问题。

我看过 cs50 恢复门槛，但没有 valgrind 错误，但我无法真正理解它，因为他的代码在我眼中是意大利面条代码（没有冒犯！！

我还是 C 的新手，拥有 Python 编程背景使我很难理解与内存有关的任何事情，因为 python 更直接。

任何帮助都不胜感激:)