为什么我会丢失“3,744,768 字节”的内存？

Question

我是编程新手，我正在编写一个程序来通过比较 4 个连续字节来恢复“card.raw”中存在的 JPEG 文件。如果他们划分了 JPEG，程序必须将 512 字节的块复制到保存为 xxx.jpg（000.jpg、001.jpg 等）的新文件中。如果复制完该块后，找到新JPEG的开头，则关闭当前文件并打开下一个文件复制下一个JPG。否则下一个块将被复制到同一个文件中。

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    int counter = 0;
    if(argc != 2)
    {
        printf("Usage: ./recover image\n");
        return 1;
    }
    typedef uint8_t BYTE;
    FILE *recover = fopen(argv[1], "r");
    if(recover == NULL)
    {
        printf("ERRNO 1 IS %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }
    fseek(recover, 0L, SEEK_END);
    unsigned long size = ftell(recover);
    fseek(recover, 0L, SEEK_SET);
    BYTE *CHUNK = (BYTE*)malloc(size * sizeof(BYTE));
    fread(CHUNK, sizeof(BYTE), size, recover);      //Break recover into bytes
    int j = 0;
    char file[8] = "xxx.jpg";
    FILE *f = NULL;
    int s = 0;
    while(j  < size)
    {
        if(CHUNK[j] == 0xff && CHUNK[j + 1] == 0xd8 && CHUNK[j + 2] == 0xff)                           //Check if byte is JPEG format 1st byte
        {
                            if(s == 0)
                            {
                                if(f != NULL)
                                {
                                    f = NULL;
                                }
                                sprintf(file ,"%03d.jpg",counter);  //Create custom file of format xxx.jpg
                                f = fopen(file,"w");
                                if(f == NULL)
                                {
                                    printf("ERRNO 2 is %s\n", strerror(errno));
                                    exit(EXIT_FAILURE);
                                }
                                fwrite(&CHUNK[j], 512, sizeof(BYTE), f);  //Copy 512 bytes from start of JPEG file as 512 bytes form one 'block`
                                j += 512; //Increment to check initial bytes of next 'block'
                                s++;
                            }
                            else
                            {
                                fclose(f);
                                counter++;
                                s = 0;
                            }
        }
        else if(s > 0)   //Else continue searching
        {
            fwrite(&CHUNK[j], 512, sizeof(BYTE), f);
            j += 512;
        }
        else j += 512;
    }
    fclose(f);
    fclose(recover);
    free(&CHUNK);
    return 0;
}

除了 Valgrind 给我以下报告外，该程序运行完美：

==1560== Memcheck, a memory error detector
==1560== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==1560== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==1560== Command: ./recover card.raw
==1560== 
==1560== Invalid free() / delete / delete[] / realloc()
==1560==    at 0x4C32D3B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1560==    by 0x400BF4: main (recover.c:69)
==1560==  Address 0x1fff000510 is on thread 1's stack
==1560==  in frame #1, created by main (recover.c:9)
==1560== 
==1560== 
==1560== HEAP SUMMARY:
==1560==     in use at exit: 3,744,768 bytes in 1 blocks
==1560==   total heap usage: 103 allocs, 103 frees, 3,981,816 bytes allocated
==1560== 
==1560== 3,744,768 bytes in 1 blocks are definitely lost in loss record 1 of 1
==1560==    at 0x4C31B0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1560==    by 0x4009FA: main (recover.c:26)
==1560== 
==1560== LEAK SUMMARY:
==1560==    definitely lost: 3,744,768 bytes in 1 blocks
==1560==    indirectly lost: 0 bytes in 0 blocks
==1560==      possibly lost: 0 bytes in 0 blocks
==1560==    still reachable: 0 bytes in 0 blocks
==1560==         suppressed: 0 bytes in 0 blocks
==1560== 
==1560== For counts of detected and suppressed errors, rerun with: -v
==1560== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

Valgrind 似乎说问题出在 line 26:

中的内存分配

BYTE *CHUNK = (BYTE*)malloc(size * sizeof(BYTE));

但是我在最后用free(&CHUNK);释放了分配的内存。我想知道为什么 Valgrind 报告说 3,744,768 bytes in 1 blocks 已经 definitely lost。

Answer 1

free(&CHUNK) 要求 free space 用于变量 CHUNK，即（例如 4 或 8 ) 指针本身占用的字节；除了不释放你正在考虑的内存之外，这是未定义的行为，因为你可以 free 只有 malloc 和朋友分配的内存，我很惊讶你没有因为在分配器内部进行一些完整性检查。

你实际需要做的是释放指向 CHUNK的内存，所以你必须做free(CHUNK).

为什么我会丢失“3,744,768 字节”的内存？

Why am I losing `3,744,768 bytes` of memory?

c

pointers

malloc

valgrind

free