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”，包含2000个字符，仅char“ a”，没有空格。

然后，我得到了这段代码，该代码在循环中运行，将其添加到缓冲区中，如果达到限制，最终将重新分配，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);
}

问题是它最终告诉我，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，我仅释放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。 因此，也许我只是无法说出我释放了多少字节。 但是然后我尝试使用valgrind，它告诉我，我的内存不足，导致内存泄漏。 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)

如何正确释放已分配的内存？ 这样不会引起内存泄漏吗？ 最终-我做错了吗，还有更好的方法吗？ 谢谢您的帮助。

问题更新

我遵循了建议，并在1个上下文中将其纠正为3个错误。 这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);
}

使用相同的valgrind调用（“ valgrind ./realloc”），我得到了：

==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)

任何提示可能导致这一问题？

Answer 1

这是你的问题：

strBuffer = realloc(strBuffer, numEx);

如果您对realloc()调用失败，则它将返回一个空指针，但不会释放原始的内存分配。

您需要先检查返回值，如果成功，则将其分配给原始指针：

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

---

您的代码还有其他一些问题，包括：

• 如果strBuffer为NULL，则没有意义将其传递给free（）。

• 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。 我认为您的意思是strBuffer = malloc(sizeof(char)*(maxS+1)); ，但您只需将strBuffer = malloc(maxS+1); 因为sizeof(char)根据定义为1。

Answer 2

好吧，到目前为止，每个人都为您提供了非常重要的建议，您应该考虑使用它们（主要是tempBuffer ）。 但是您的问题是您忘记关闭文件描述符：

fclose(fd);

同样， sizeof是编译时间，因此它不能给您动态分配的内存大小，并且strlen需要一个\\n字符才能工作。 计算分配和释放的内存是一项艰巨的任务，它的解决方案并不是那么简单。

答案更新

我执行了更新的代码， 从1个上下文中仅收到1个错误 ，可以通过更改以下行来解决： tmpBuffer = realloc(strBuffer, numEx + 1);

除此之外， fclose（fd）之后的所有内存都是可用的。 我在使用gcc 4.8.1的Ubuntu 12.04计算机上。

Answer 3

while((c=fgetc(fd) != EOF))应该是while((c=fgetc(fd)) != EOF) 。 这样做的结果是您试图在字符串中存储1而不是读取的字符。

内存问题源于strlen(strBuffer); 。 您在非空终止的字符串上调用strlen ，这会导致未定义的行为。 （这在valgrind报告中显示为“ strlen中大小为1的无效读取）。

要解决此问题，请摆脱strlenSize并执行以下操作：

maxS = 2 * maxS;
strBuffer = realloc(strBuffer, maxS + 1);

请注意，如果要在内存不足的情况下使用“干净”的valgrind，则需要在将其分配给strBuffer之前检查realloc的返回值，如提示ossifrage所指出。

---

注意 您的错误处理代码不正确。 sizeof(strBuffer)查找指针的大小。 您要打印的值为maxS 。 另外， free(NULL)无效； 在调用exit()块之后没有else块是没有意义的。

Answer 4

this compiles, and does the job
it includes error handling
it eliminated many meaningless variables
it eliminates the errors in the logic of the OPs code



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

int main()
{
    int maxS = 200; // current allocation size

    char *strBuffer = NULL;
    if( NULL == (strBuffer = malloc(maxS) ) )
    { // then, malloc failed
        perror( "malloc failed" );
        exit(99);
    }

    // implied else, malloc successful

    printf("Alocated: %d Bytes of memory.\n", )maxS+1));

    // file opening simulation
    FILE* fd = fopen("a", "r");
    if(fd == NULL)
    { // then fopen failed
        perror( "fopen failed for file: a" );
        free(strBuffer);
        exit(99);
    }

    // implied else, fopen successful

    int c;       // receives input char from fgetc()
    int fcv = 0; // index into malloc'd memory

    // tmpBuffer used in realloc()
    // so will not lose pointer to already allocated memory
    // in case realloc() fails
    char *tmpBuffer;

    while((c=fgetc(fd)) != EOF)
    {
        strBuffer[fcv] = c;
        fcv++;

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

            if( NULL == ()tmpBuffer = realloc(strBuffer, 2*maxS) )
            {
                perror( "realloc failed" );
                free(strBuffer);
                fclose(fd);
                exit(99);
            }

            // implied else, realloc successful

            maxS *= 2;  // only update after being sure realloc successful
            strBuffer = tmpBuffer;

            printf("Reallocation successful!\n");
            printf("Allocated: %d Bytes of memory.\n", maxS);
        } // end if
    } // end while

    free(strBuffer);
    fclose(fd);

    printf("Freed %d bytes of memory!\n", maxS);
    return( 0 );
} // end function: main