c - 尽管内存已被释放，Valgrind 仍检测到内存泄漏

Question

我有一个文件“a”，有 2000 个字符，只有字符“a”，没有空格。

然后我有这段代码，它通过循环运行，将其添加到缓冲区，如果达到限制最终重新分配，并在出现错误时释放 strBuffer 变量。

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

int main()
{
    int maxS = 200;
    int numericExpression;
    int strLength;


    char *strBuffer;
    strBuffer = malloc(sizeof(char)*maxS+1);
        if(strBuffer == NULL)
        {
            printf("Failed to allocate requested memory!\n");
            free(strBuffer);
            strLength = sizeof(strBuffer);
            printf("Freed %d bytes of memory!\n", strLength);
            exit(99);
        }
        else
        {
            numericExpression = sizeof(char)*maxS+1;
            printf("Alocated: %d Bytes of memory.\n", numericExpression);
        }

    // while simulation

    int fcv = -1;
    int numEx;


    // file opening simulation
    FILE* fd = fopen("a", "r");


    int c;
    while((c=fgetc(fd) != EOF)) // condition to make sure we realloc only once
    {
        fcv++;
        strBuffer[fcv] = c;
        if(fcv == (maxS))   
        {
            printf("Additional memory space required!\n");      
            int strlensize = strlen(strBuffer); 
            numEx = (sizeof(char)*(2*strlensize));


            strBuffer = realloc(strBuffer, numEx);

            if(strBuffer == NULL)
            {
                printf("Failed to allocate requested memory!\n");
                strLength = sizeof(strBuffer);
                free(strBuffer);
                printf("Freed %d bytes of memory!\n", strLength);
                exit(99);
            }
            else
            {
                maxS = numEx;
                printf("Reallocation successful!\n");
                printf("Alocated: %d Bytes of memory.\n", numEx);
            }

        }

    }
    strLength = sizeof(strBuffer);
    free(strBuffer);
    printf("Freed %d bytes of memory!\n", strLength);
}

问题是它最后告诉我，我只释放了 8 个字节的内存。我想这是因为 sizeof(strBuffer) 没有响应预期的大小。当我改用 strlen(strBuffer) 时，我只释放了 2001 个字节。

我想这可能只是打印出释放的字节数的问题。我可能做错了。所以也许我只是不知道我释放了多少字节。但是后来我尝试了 valgrind，它告诉我，我没有足够的自由，存在内存泄漏。但是在程序的每个分支中，我都可以释放 strBuffer 使用的内存。

当我通过 valgrind ("valgrind ./realloc") 运行它时，它告诉我:

==780== Memcheck, a memory error detector
==780== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==780== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==780== Command: ./realloc
==780== 
Alocated: 201 Bytes of memory.
Additional memory space required!
==780== Invalid read of size 1
==780==    at 0x4C2E0F4: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400846: main (in /home/dan/Desktop/test_ifj/realloc)
==780==  Address 0x51fd109 is 0 bytes after a block of size 201 alloc'd
==780==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x40078C: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Reallocation successful!
Alocated: 402 Bytes of memory.
==780== Invalid write of size 1
==780==    at 0x400823: main (in /home/dan/Desktop/test_ifj/realloc)
==780==  Address 0x51fd562 is 0 bytes after a block of size 402 alloc'd
==780==    at 0x4C2CE8E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400866: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Additional memory space required!
Reallocation successful!
Alocated: 806 Bytes of memory.
Additional memory space required!
==780== Conditional jump or move depends on uninitialised value(s)
==780==    at 0x4C2E0F8: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400846: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Reallocation successful!
Alocated: 804 Bytes of memory.
Freed 8 bytes of memory!
==780== 
==780== HEAP SUMMARY:
==780==     in use at exit: 568 bytes in 1 blocks
==780==   total heap usage: 5 allocs, 4 frees, 2,781 bytes allocated
==780== 
==780== LEAK SUMMARY:
==780==    definitely lost: 0 bytes in 0 blocks
==780==    indirectly lost: 0 bytes in 0 blocks
==780==      possibly lost: 0 bytes in 0 blocks
==780==    still reachable: 568 bytes in 1 blocks
==780==         suppressed: 0 bytes in 0 blocks
==780== Rerun with --leak-check=full to see details of leaked memory
==780== 
==780== For counts of detected and suppressed errors, rerun with: -v
==780== Use --track-origins=yes to see where uninitialised values come from
==780== ERROR SUMMARY: 1200 errors from 3 contexts (suppressed: 0 from 0)

如何正确释放分配的内存？所以不会造成内存泄漏？最终-我做错了吗，有更好的方法吗？感谢您的帮助。

问题更新

我听从了建议，在 1 个上下文中出现了 3 个错误。这就是我的代码现在的样子:

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

int main()
{
    int maxS = 200;
    int numericExpression;


    char *strBuffer;
    strBuffer = malloc((maxS+1));
        if(strBuffer == NULL)
        {
            printf("Failed to allocate requested memory!\n");
            printf("Freed %d bytes of memory!\n", maxS);
            exit(99);
        }
        else
        {
            numericExpression = sizeof(char)*maxS+1;
            printf("Alocated: %d Bytes of memory.\n", numericExpression);
        }

    // while simulation

    int fcv = -1;
    int numEx;


    // file opening simulation
    FILE* fd = fopen("a", "r");

    if(fd == NULL)
    {
        printf("Error opening a file!\n");

        if(strBuffer != NULL)
        {free(strBuffer);}


        exit(99);
    }


    int c;

    char *tmpBuffer;
    while((c=fgetc(fd)) != EOF) // condition to make sure we realloc only once
    {
        fcv++;
        strBuffer[fcv] = c;

        if(fcv == (maxS))   
        {
            printf("Additional memory space required!\n");      

            numEx = ((2*fcv));


            tmpBuffer = realloc(strBuffer, numEx);
            if(!tmpBuffer)
            {
                free(strBuffer);
                printf("Realloc() failed!\n");
                exit(99);
            }       
            else
            {
                strBuffer = tmpBuffer;
            }   


            if(strBuffer == NULL)
            {
                printf("Failed to allocate requested memory!\n");
                printf("Freed %d bytes of memory!\n", maxS); // well this is questionable, I think
                exit(99);
            }
            else
            {
                maxS = numEx;
                printf("Reallocation successful!\n");
                printf("Alocated: %d Bytes of memory.\n", maxS);
            }

        }

    }

    free(strBuffer);fclose(fd); // ADDED, still errors occur

    printf("Freed %d bytes of memory!\n", maxS);
}

通过相同的 valgrind 调用(“valgrind ./realloc”)我得到了这个:

==1213== Invalid write of size 1
==1213==    at 0x4007FD: main (in /home/dan/Desktop/test_ifj/realloc)
==1213==  Address 0x51fd560 is 0 bytes after a block of size 400 alloc'd
==1213==    at 0x4C2CE8E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1213==    by 0x400831: main (in /home/dan/Desktop/test_ifj/realloc)
==1213== 
Additional memory space required!
Reallocation successful!
Alocated: 800 Bytes of memory.
Additional memory space required!
Reallocation successful!
Alocated: 1600 Bytes of memory.
Additional memory space required!
Reallocation successful!
Alocated: 3200 Bytes of memory.
Freed 3200 bytes of memory!
==1213== 
==1213== HEAP SUMMARY:
==1213==     in use at exit: 568 bytes in 1 blocks
==1213==   total heap usage: 6 allocs, 5 frees, 6,769 bytes allocated
==1213== 
==1213== LEAK SUMMARY:
==1213==    definitely lost: 0 bytes in 0 blocks
==1213==    indirectly lost: 0 bytes in 0 blocks
==1213==      possibly lost: 0 bytes in 0 blocks
==1213==    still reachable: 568 bytes in 1 blocks
==1213==         suppressed: 0 bytes in 0 blocks
==1213== Rerun with --leak-check=full to see details of leaked memory
==1213== 
==1213== For counts of detected and suppressed errors, rerun with: -v
==1213== ERROR SUMMARY: 3 errors from 1 contexts (suppressed: 0 from 0)

有什么提示可以导致这个吗？

Answer 1

这是你的问题:

strBuffer = realloc(strBuffer, numEx);

如果您对 realloc() 的调用失败，则它会返回一个空指针，但不会释放原始内存分配。

需要先检查返回值，如果成功则赋值给原指针:

char *tmpBuffer = realloc(strBuffer, numEx);
if (!tmpBuffer) {
  free(strBuffer);
  puts("realloc() failed");
  exit(1);
}
else {
  strBuffer = tmpBuffer;
}

---

您的代码还有一些其他问题，包括:

• 如果 strBuffer 为 NULL，则将其传递给 free() 没有意义。

• sizeof() 不是运行时函数，因此它不知道分配了多少内存。

• strBuffer = malloc(sizeof(char)*maxS+1); 有点草率。我想你的意思是 strBuffer = malloc(sizeof(char)*(maxS+1));，但你可以只输入 strBuffer = malloc(maxS+1);因为根据定义，sizeof(char) 为 1。