Some Random C Questions - Topic

lopelurag

Member

Posts: 24,101

Joined: Nov 8 2007

Gold: 5,561.70

Nov 6 2012 07:41am

Trying to understand these:

What situation has occurred after the last assignment of the following code?

Code

int *p = malloc(sizeof(int));
*p = 400;
p = NULL;

-----P points to NULL? Is this right?

Are there any problems with the following code fragment?

Code

double scores[100];
double *p = scores;
scores[0] = 13.2;
free(p);

-----Only problem I could think of was that they never allocated memory for p so there is no need to free it?

Which of the following are valid or invalid.

Code

a. free(NULL); [B]//not sure, I know you can free NULL pointers but NULL?[/B]
b. void *a = malloc(0); [B]//0 mem is allocated[/B]
c. free(b); free(b); [B]//Not sure? Pretty sure this will crash the program[/B]
d. free(NULL); free(NULL); [B]//Again not sure[/B]
e. free(0); [B]//Not sure, but I'd think it's invalid, don't know the reason why?[/B]

-----Not sure if my answers above are right, if someone could explain the ones I had trouble with I'd really appreciate it

carteblanche

Member

Posts: 32,925

Joined: Jul 23 2006

Gold: 3,804.50

Nov 6 2012 11:35am

iirc, NULL is just a typedef for 0, so p = NULL should be fine with respect to compiling. whether it makes sense or does what you intend for it to do is another story.

for the others, just run them and see what happens. not sure why you're asking us if there's a runtime error or not.

animal322

Member

Posts: 287

Joined: Nov 3 2012

Gold: Locked

Trader: Scammer

Nov 6 2012 01:56pm

Quote (carteblanche @ Nov 6 2012 12:35pm)

you're correct

Eep

Member

Posts: 23,862

Joined: Aug 16 2006

Gold: 20.00

Nov 6 2012 03:00pm

nvm. Just looked up how free() works. Lol.

This post was edited by Eep on Nov 6 2012 03:11pm

irimi

Member

Posts: 4,605

Joined: Sep 15 2011

Gold: 9,464.00

Nov 6 2012 03:08pm

Quote (lopelurag @ Nov 6 2012 06:41am)

Are there any problems with the following code fragment?

Code

double scores[100];
double *p = scores;
scores[0] = 13.2;
free(p);

-----Only problem I could think of was that they never allocated memory for p so there is no need to free it?

You shouldn't be calling free because the array is allocated on the stack and not on the heap. So more or less, what you said about "no need to free it because they never allocated memory for p" is correct.

Quote (lopelurag @ Nov 6 2012 06:41am)

Which of the following are valid or invalid.

Code

-----Not sure if my answers above are right, if someone could explain the ones I had trouble with I'd really appreciate it

Depends on what you mean by "valid". There's syntactical validity and there's programmatic validity (what carteblanche said). All of the above are syntactically valid and will compile, but they may lead to unpredictable/unsupported behavior. BTW -- "the behavior is undefined" means that the function can do whatever it damn well pleases.

Read the documentation on malloc and free --
http://www.cplusplus.com/reference/clibrary/cstdlib/malloc/
If size is zero, the return value depends on the particular library implementation (it may or may not be a null pointer), but the returned pointer shall not be used to dereference an object in any case.

http://www.cplusplus.com/reference/clibrary/cstdlib/free/
If ptr does not point to a block of memory allocated with the above functions, the behavior is undefined.

If ptr is a null pointer, the function does nothing.

This post was edited by irimi on Nov 6 2012 03:15pm

Go Back To Programming & Development Topic List