Thread: Issues with calling free() on a malloced double pointer

Hey everyone,

I was messing around with pointers trying to get a better understanding of them, and I came across something that I haven't been able to solve in regards to freeing a double pointer I malloced.

Code:

int main(int argc, char** argv) {

    
    char** temp = (char**)malloc(argc * sizeof(char*));
    //free(temp);                  <-------------------------------------------- free's fine here
    char** temp2;
    int i;
    for (i = 0; *argv != NULL; i++)
        *(temp + i) = *argv++;

    *(temp + argc) = NULL;

    temp2 = temp;
    while (*temp != NULL)
        printf("%s", *temp++);
   
    free(temp2);                 <--------------------------------------------- doesn't free fine here (SIGABRT)

    return (EXIT_SUCCESS);
}

I don't understand why it free's fine right after I malloc and then when I add strings to the double pointer, print them, and attempt to assign the char** temp to char** temp2 (equivalently storing the value of temp, which was malloced, into temp2 because I increment temp in the while) I run into an error when I free what I malloced...

This might be because I'm ignorant on the operation of malloc, but if someone could set me straight, I'd appreciate it. Thanks.

Free the row pointers (in a loop), first.

Then free the array pointer.

Whatever you assign to a malloc'ed array, is not somehow removed when you free the pointer with that address. That value or values, will remain as they are, until the OS (perhaps through your program), see's fit to assign some other data, to that memory.

The pointer address will not automatically be returned to NULL or something like that. This is like dating - just because you broke up with your ex- that doesn't mean you have a date for Saturday night. It just means you're "available". Same with the RAM.

Originally Posted by grumpy

The line *(temp + argc) = NULL falls off the end of the array you allocated. The result is undefined behaviour.

It's got nothing to do with a "double pointer". Undefined behaviour is undefined behaviour.

One common symptom of such undefined behaviour is overwriting some area of memory used later by something else. The internal workings of malloc() and free() are an example of such a "something else".

Well, what you said didn't really help me out at all, but when you mentioned that my pointer fell off the end of the array, it reminded me that I forgot to add room for a NULL character in my malloc call. I went back and did that and it ended up working.

Next time maybe you can try to be a little less demeaning in your answer. I'm very aware of the consequences involved with undefined behavior, I'm just not use to the mechanics of C.

Originally Posted by Adak

Free the row pointers (in a loop), first.

Then free the array pointer.

Whatever you assign to a malloc'ed array, is not somehow removed when you free the pointer with that address. That value or values, will remain as they are, until the OS (perhaps through your program), see's fit to assign some other data, to that memory.

The pointer address will not automatically be returned to NULL or something like that. This is like dating - just because you broke up with your ex- that doesn't mean you have a date for Saturday night. It just means you're "available". Same with the RAM.

Thanks for the suggestion Adak, however you only free what you malloc and since I only malloced one time, I only free one time. If I attempted to free the individual strings, it would have thrown a SIGABRT because I didn't allocate memory for them since they're arguments to main.

I just compiled it by attempting to free the strings like you said and it did exactly as I described above.

Of course. I misunderstood your original post.

>> Next time maybe you can try to be a little less demeaning in your answer.
Most of the posts on this board are written in a window of like ten minutes. If anyone comes off as demeaning I think you could find it in your heart to let it go without comment. If we took a long time people would complain about that too, so we just can't win, ever.