Pointer to Pointer null C

Question

I have a question about a double pointer in C. For example:

int** point = malloc(sizeof(int**));
int* point2 = malloc(sizeof(int*));
*point2 = 5;
*point = point2;
free(point2);
printf("%d", *point);

My question is, if I have a double pointer pointing to another pointer (point2) and I make a free on (point2), the double pointer (point) is going to point to a memory address that has nothing. So I should do * point = NULL?

yes, that could be a good idea to avoid keeping an invalid pointer ref. Note that you don't need to use malloc for those cases. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Oct 16, 2017 at 18:58
int* point2 = malloc(sizeof(int*)); point2 is an integer pointer; it should point to an int. So, you should allocate sizeof(int), or sizeof *point2 — wildplasser
– wildplasser, Commented Oct 16, 2017 at 18:58
int* point = malloc(sizeof(int)) this is what @wildplasser means, I think — akshayk07
– akshayk07, Commented Oct 16, 2017 at 19:01
Your mallocs are indeed wrong. Do instead int **point = malloc(sizeof *point) and int *point2 = malloc(sizeof *point2) to avoid these errors. — Antti Haapala
– Antti Haapala, Commented Oct 16, 2017 at 19:08
I suggest avoiding the term "double pointer" unless you're talking about the type double*. — Keith Thompson
– Keith Thompson, Commented Oct 16, 2017 at 19:17

Sergey Kalinichenko · Accepted Answer · 2017-10-16 19:32:31Z

2

Strictly speaking, you don't have to assign NULL to *point. However, it is a very good idea to do it anyway, because it helps you distinguish valid pointers from invalid when you debug your program.

the double pointer (point) is going to point to a memory address that has nothing.

Memory always has something in it. However, it's not always under your control, so it may not be legal to read its content.

For instance, when you free point2, the value stored in point is not going to disappear. However, it would become illegal to use that value, as it becomes indeterminate.

edited Oct 16, 2017 at 19:32

answered Oct 16, 2017 at 19:01

Sergey Kalinichenko

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2 Comments

Antti Haapala Over a year ago

... UB to use the value as it becomes indeterminate...

Sergey Kalinichenko Over a year ago

@AnttiHaapala Edited. Thanks!

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