为什么 C 不从 strtok 设置的 malloc'd char* 中释放内存？

Question

此函数将带有空格分隔值的文本文件中的值读取到 2d 数组中。当我运行它时，工作正常 - 但是通过Valgrind进行的内存泄漏检查证实了Xcode的怀疑，即"char *splitString"永远不会被释放，它被调用了两次。我不明白这一点，考虑到我的"字符*缓冲区"似乎被释放得很好。任何帮助都非常感谢！

int** readMatrixFile(char* inFileName, int** matrix, int sizeY, int sizeX)
{
    FILE* matrixFP;
    int ii=0, jj=0, fileValid = 1;
    char *buffer, *splitString;
    const char delim[]=" nr";
    matrixFP = fopen(inFileName, "r");
    if(matrixFP != NULL)
    {
        /*Check if file is the same size as specified by the command line
         *assumed valid until the file is checked*/
        splitString = malloc(100*sizeof(char)); <------where allocated
        buffer = malloc(5000*sizeof(char));
        do
        {
            fgets(buffer, 5000, matrixFP);
            jj=0;
            splitString = strtok(buffer, delim);
            while(splitString != NULL)
            {
                jj++;
                splitString = strtok(NULL, delim);
            }
            if(jj!=sizeX)
            {
                fileValid = 0;
            }
            ii++;
        } while(!feof(matrixFP));
        if(ii != sizeY || buffer==NULL)
        {
            fileValid = 0;
        }
        free(splitString); <-----Appears to do nothing?
        free(buffer);
        if(fileValid) /*Files match to specified command line values*/
        {
            ii=0;
            rewind(matrixFP);
            matrix = (int**)malloc(sizeY * sizeof(int *));
            do
            {
                matrix[ii] = (int*)malloc(sizeX * sizeof(int));
                jj=0;
                do
                {
                    fscanf(matrixFP, "%d", &matrix[ii][jj]);
                    jj++;
                } while(jj<sizeX);
                ii++;
            } while(ii<sizeY && !feof(matrixFP));
        }
        else
        {
            printf("Error: File does not match size specified by the command linen");
        }
        fclose(matrixFP);
    }
    else
    {
        perror("Error: File does not exist or is invalid");
        matrix = NULL;
    }
    return matrix;
}

和瓦尔格林德输出：

==14087== Memcheck, a memory error detector
==14087== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14087== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14087== Command: ./pmms a b 10 3 10
==14087== 
/*irrelevent program output*/ 
==14087== 
==14087== HEAP SUMMARY:
==14087==     in use at exit: 200 bytes in 2 blocks
==14087==   total heap usage: 21 allocs, 19 frees, 11,680 bytes allocated
==14087== 
==14087== 100 bytes in 1 blocks are definitely lost in loss record 1 of 2
==14087==    at 0x4A06A2E: malloc (vg_replace_malloc.c:270)
==14087==    by 0x400B55: readMatrixFile (matrix_reader.c:35)
==14087==    by 0x40095E: main (pmms.c:23)
==14087== 
==14087== 100 bytes in 1 blocks are definitely lost in loss record 2 of 2
==14087==    at 0x4A06A2E: malloc (vg_replace_malloc.c:270)
==14087==    by 0x400B55: readMatrixFile (matrix_reader.c:35)
==14087==    by 0x400982: main (pmms.c:24)
==14087== 
==14087== LEAK SUMMARY:
==14087==    definitely lost: 200 bytes in 2 blocks
==14087==    indirectly lost: 0 bytes in 0 blocks
==14087==      possibly lost: 0 bytes in 0 blocks
==14087==    still reachable: 0 bytes in 0 blocks
==14087==         suppressed: 0 bytes in 0 blocks
==14087== 
==14087== For counts of detected and suppressed errors, rerun with: -v
==14087== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)

Answer 1

您可以在此处更改splitString的值：

            splitString = strtok(NULL, delim);

在这里：

            splitString = strtok(buffer, delim);

所以

它不再指向要释放的内存块，所以free()将不执行任何操作，因为当到达此行时，splitString将被NULL，并且free(NULL)被定义为不执行任何操作。

Answer 2

首先，你迭代

    while(splitString != NULL)
    {
          ...
    }

所以在循环完成之后splitString NULL，而free(NULL)什么也不做。

其次，一旦你分配了一个百字节的内存块

    splitString = malloc(100*sizeof(char));   // (*)

对 splitString 变量的下一个引用是赋值：

    splitString = strtok(buffer, delim);

因此，无论变量中存储什么，它都会被覆盖！ splitString现在指向buffer块的某个地方，在(*)处分配的块的地址将永远丢失。顺便说一下，这表明您从不使用该块，并且您实际上不需要分配它。

Answer 3

在while(splitSring != NULL)循环中，您正在使用修改它的strtok(3)。在循环结束时，splitString将被NULL，本质上您正在调用free(NULL)，同时泄漏最初分配的内存。

Answer 4

这不是对您的问题的直接回答，只是一个可能有助于完全避免该问题的建议：

使用 strtok() 和 family 时，无需为用于捕获返回值的对象显式分配内存：

char delim[] = {" nt"};  //or whatever delimitors you need
char *tok = NULL;
tok = strtok(stringToParse, delim);  
while(tok)
{
    //do something with tok
    ...
    //get next token:
    tok = strtok(NULL, delim);
}

而且，如图所示使用时无需释放tok。