The Big Programming Thread - Page 874

On April 20 2017 05:32 travis wrote:
Question I thought was clever on an old exam:


Implement the function free_array with prototype shown below that frees the dynamically-allocated
memory associated with each array element in the first parameter passed to the function. The function needs to handle
the scenario where two or more array entries are pointing to the same dynamically-allocated memory (only one free
can be applied to a memory location). It is OK to modify the original array as part of your implementation, but you
MAY NOT define a new array in the function and you may not allocate more memory. If you do you will lose a
significant number of points.

void free_array(void *a[], int length) {

kinda a hard question since based on the exam length we are supposed to be able to write it in about 1 minute, lol

curious how many of you could get this in less than a minute

On April 20 2017 05:32 travis wrote:
curious how many of you could get this in less than a minute

p & j are pointers to the same memory.

free(p);
p = NULL;
free(j); // nothing happens here since j is NULL at this point.

ourfunction(*array[])  {
    for (x = 0; x < total elements; x++)  {
        free(array[x]); // no need to check if array[x] is null since free doesn't care
        array[x] = NULL;
    }
}

free(j); // nothing happens here since j is NULL at this point.

On April 20 2017 06:47 travis wrote:
but j is not null at that point, j is still a pointer to the section of memory that was freed.

p points to null, j contains the address it was assigned when we did j = p

also the above example is bad too. you don't want to free a bunch of memory that was never allocated you'll get seg faults or worse

maybe you guys are referencing behavior of free in c++ ?

On April 20 2017 06:56 Shield wrote:
Yes, it's stupid when people at universities decide to test meaningless, corner case which won't be used in real environment.
Either way, my C experience is limited. I work with C++ which is very far away from C raw pointers nowadays.

On April 20 2017 06:55 BluzMan wrote:


The first thing that comes to mind without allocating memory would be casting the input to an array of ints, sorting it and just using continue if a[i] == a[i - 1].

Double free is a dangerous error and should be avoided at all costs. Furthermore, you can't tell if a pointer has been freed just by looking at it, otherwise, C programming would have been really easy. Freeing a nullptr is generally safe, but you have to set it to nullptr first.

On April 20 2017 07:25 travis wrote:


well that's a way cooler solution than the given solution, I guess technically yours is faster than the given solution too

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

int compare(const void *a, const void *b);

int main(void) {
    int i = 0;
    int len = 2;
    int **ary = malloc(sizeof(int) * len);
    int *p = malloc(sizeof(int));;
    int *j = p;

    ary[0] = p;
    ary[1] = j;

    qsort(ary, len, sizeof(int), compare);

    for (i; i < len; i++) {
	if (i > 0 && ary[i] != ary[i - 1]) {
	    free(ary[i]);
	}
    }

    free(ary);
    
    printf("We are free!\n");

    return 0;
}

int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

On April 20 2017 07:40 Nesserev wrote:
So, I would like to check something with you guys. Today I installed an external harddrive for my father for daily automatic back-ups with the goal of keeping certain data safe from cryptolocker and such; however, I'm not sure whether or not I missed something, and I'm not a windows user :/.

The original setup was simple:
- 1 account called 'daddy69' with admin rights and password (not really 'daddy69', but whatever)
- everyone has access rights to external harddrive

After I was done, the setup was as follows:
- 1 account called 'backup' with admin rights and password, access to external harddrive and daddy69's directories
- 1 account called 'daddy69' with a password and no admin rights, can only detect but not access external harddrive
- only admins have access rights to external harddrive

I wrote a batchfile that automates the whole process of backing up certain directories to a fresh directory on the external drive. Then I added an automated task to run that script once every day when he's supposed to be sleeping. It seems to be working fine.

Anything I should change, or any recommendations? Did I miss anything, and did I reach my goals? Is this setup robust enough to survive cryptolocker or ... ?