释放内存以获取指向struct的指针

Question

我有以下一段我无法更改的代码

typedef struct 
{
    char* firstName;
    char* lastName;
    int id;
    float mark;
}* pStudentRecord;

现在，我以以下方式分配内存：

g_ppRecords = (pStudentRecord*) malloc (sizeof(pStudentRecord*) *(g_numRecords));       /*allocate required memory for the array of pointers to instances of pStudentRecord*/

/*populate the data structure with records from the text file*/
while (count<g_numRecords)
{   
    fscanf(g_pf,"%s %s %i %f\n",&fn,&ln,&i,&m);

    /*allocate memory for each student record. the pointer to each of these will be stored in the array g_ppRecords*/
    g_ppRecords[count]=(pStudentRecord)malloc(sizeof(char*)*2+sizeof(int)+sizeof(float));

    /*allocate memory for the firstName and lastName on the heap for each record. use only as much space as is required*/
    g_ppRecords[count]->firstName=(char*)malloc(strlen(fn));
    g_ppRecords[count]->lastName=(char*)malloc(strlen(ln));

    /*assign values stored in local variables to the ones on heap for each record*/
    strcpy(g_ppRecords[count]->firstName,fn);
    strcpy(g_ppRecords[count]->lastName,ln);
    g_ppRecords[count]->id=i;
    g_ppRecords[count]->mark=m;

    ++count; /*onto next record*/
}

但是我在释放它方面遇到了麻烦。 这就是我在做什么：

while (count<g_numRecords)
{
        /*copy variables in the struct instance into local variables*/
    strcpy(fn,g_ppRecords[count]->firstName);
    strcpy(ln,g_ppRecords[count]->lastName);
    id=g_ppRecords[count]->id;
    mark=g_ppRecords[count]->mark;

    free(g_ppRecords[count]->firstName);
    g_ppRecords[count]->firstName=NULL;
    free(g_ppRecords[count]->lastName);
    g_ppRecords[count]->lastName=NULL;
    free(g_ppRecords[count]);

    ++count;

    #ifdef DEBUG
        printf("\n%s %s %d %.2f",fn,ln,id,mark);
    #endif

    /*detect any errors while writing to file*/
    if(fprintf(g_pf,"%s %s %d %.2f\n",fn,ln,id,mark) ==-1)
        perror("");
    else{
        #ifdef DEBUG
            printf("success");
        #endif
    }

    #ifdef DEBUG
        printf("return val=%d",now);
    #endif
}

    free(g_ppRecords);  /*free memory after writing to file is complete*/
    g_ppRecords=NULL;   /*make pointer point to NULL*/
    fclose(g_pf);       /*close file stream*/
}

我知道这可能与以下方式分配内存有关：

g_ppRecords[count]=(pStudentRecord)malloc(sizeof(char*)*2+sizeof(int)+sizeof(float));

但我无法将其分配为

g_ppRecords[count]=(pStudentRecord)malloc(sizeof(pStudentRecord));

请帮忙！

Answer 1

您的代码应如下所示：

typedef struct 
{
    char* firstName;
    char* lastName;
    int id;
    float mark;
} StudentRecord;

/* Allocate memory for each student record. */
g_ppRecords[count] = (StudentRecord*)malloc(sizeof(StudentRecord));

/* Allocate memory for the firstName and lastName on the heap. */
g_ppRecords[count]->firstName = (char*)malloc(strlen(fn)+1);
g_ppRecords[count]->lastName = (char*)malloc(strlen(ln)+1);

您没有在字符串中为训练零分配空间，这会破坏内存。 为记录本身分配空间时，最好使用sizeof(struct) 。 该结构可能在字段之间或整个记录之后有一些填充，这些填充可能未包含在您的尺寸计算中。

如果您不能更改标题，则应输入：

pStudentRecord DummyPtrVar;
g_ppRecords = (pStudentRecord*)malloc(sizeof(*DummyPtrVar)*g_numRecords);   

在sizeof下写入pStudentRecord*是不正确的。 这将创建双指针，而不是访问记录本身。 C / C ++不允许简单地访问指向的类型。 但是，如果定义变量，则可以进行伪取消引用。 这不是很整洁，但我相信可以与99％的编译器一起使用。

Answer 2

我认为您代码的主要问题是分配g_ppRecords，

"g_ppRecords = (pStudentRecord*) malloc (sizeof(pStudentRecord*) *(g_numRecords));"

应该更改为

"g_ppRecords = (pStudentRecord*) malloc (sizeof(pStudentRecord) *(g_numRecords));"

当然，正如@Kirill所说，char数组应保留更多字节以包含'\\ n'。