0
votes

I have a pointer to a struct and one of the objects in the struct is a int **. The double pointer is used to dynamically allocate memory for a 2d array. I am having trouble figuring out how to free the memory for this array. Any ideas?

struct time_data {
    int *week;
    int *sec;
    int **date;
};
typedef struct time_data time_data;

time_data *getTime(time_data *timeptr, int rows, int cols) {
    int i = 0;
    time_data time;

    // allocate memory for time.date field
    time.date = (int **)malloc(rows*(sizeof(int *))); // allocate rows
    if(time.date == NULL)
        printf("Out of memory\n");
    for(i=0; i<rows; i++) {
        time.date[i] = (int *)malloc(cols*sizeof(int));
        if(time.date[i] == NULL)
            printf("Out of memory\n");
    }
    timeptr = &time;
    return timeptr;
}

int main(int argc, const char * argv[]) {
    time_data *time = NULL;
    int rows = 43200, cols = 6;
    int i;
    time = getTime(time, rows, cols);

    for(i=0; i<rows; i++)
        free(time->date[i]); // problem here
    free(time->date);

}

Modified Version (in case anyone else has similar issue)

    struct time_data {
    int *week;
    int *sec;
    int **date;
};
typedef struct time_data time_data;

time_data *getTime(int rows, int cols) {
    int i = 0;
    time_data *time = malloc(sizeof(*time));

    // allocate memory for time.date field
    time->date = (int **)malloc(rows*(sizeof(int *))); // allocate rows
    if(time->date == NULL)
        printf("Out of memory\n");

    for(i=0; i<rows; i++) {
        time->date[i] = (int *)malloc(cols*sizeof(int));
        if(time->date[i] == NULL)
            printf("Out of memory\n");
    }
    return time;
}

int main(int argc, const char * argv[]) {
    time_data *time = NULL;
    int rows = 43200, cols = 6;
    int i;
    time = getTime(rows, cols);

    for(i=0; i<rows; i++)
        free(time->date[i]); // problem here
    free(time->date);
return 0;
}
1
What ideas? you are doing it right. Just remove the cast from malloc. And also, printf("Out of memory\n"); is not enough, you shold abort and not dereferene the pointer.Iharob Al Asimi
You're freeing it correctly. But you're not allocating it correctly. getTime is returning the address of a local variable, which is not valid.Barmar
Why do I want to remove the cast from malloc?biononic

1 Answers

2
votes

Your freeing is ok, but you have a severe error

timeptr = &time;
return timeptr;

you are returning the address of a local variable.

The local variable is allocated in the stack frame of the function, and once the function returns, the data will no longer exist.

You should use malloc for that too

timeptr = malloc(sizeof(*timeptr));

and also you must return an int from main()