Thread: Malloc and Free.....

Hi Guys, got a question here about malloc and free calls...

Say I want to allocate a dynamic array of struct, I go something like

tydef struct somestruct mystruct;
mystruct *pt = (mystruct*)malloc(num*sizeof(mystruct));


So I can do pointer arthmetic right? For example:

mystruct *temp_pt = pt;
for(i = 0; i < num; i++)
// do some work with temp_pt

Now, my question is....if I want to free up the allocated memory, do I have to do it individually:

mystruct *temp_pt = pt;
for(i = 0; i < num; i++)
{
free(temp_pt);
temp_pt++;
}

or: I only have to free from the base of the allocated memory:

free(pt);



Thanks in advance!

You only have to free from the base of the allocated memory:

free(pt);

Originally posted by heljy
Say I want to allocate a dynamic array of struct, I go something like

tydef struct somestruct mystruct;
mystruct *pt = (mystruct*)malloc(num*sizeof(mystruct));

Just something to note: in C the cast isn't necessary. An assignment to/from a void pointer to a pointer of another type doesn't require a cast. This excludes function pointers IIRC.

Casting malloc (and realloc etc) can hide a bug... that of not including stdlib.h. On many platforms you 'get away' with it at link time, but as far as C is concerned it's undefined behaviour. It could do anything from toast your hard drive through to making demons (or daemons) fly out of your nose.

So I can do pointer arthmetic right? For example:

mystruct *temp_pt = pt;
for(i = 0; i < num; i++)
// do some work with temp_pt

Yep, but ITYM

temp_pt+i (or *(temp_pt+i) which is equivalent to temp_pt[i])

or

temp_pt++ at some point in the loop

and so on and so forth.

Now, my question is....if I want to free up the allocated memory, do I have to do it individually:

mystruct *temp_pt = pt;
for(i = 0; i < num; i++)
{
free(temp_pt);
temp_pt++;
}

Nope!

or: I only have to free from the base of the allocated memory:

free(pt);

Yep! The only valid things you can pass to free are the values which have been returned from malloc, realloc and calloc. Passing anything else (except NULL) is a bug, and a nasty one at that. Usually programs will carry on merilly after this, and then crash spontaneously next time... or the 5000th time you allocate or free some more memory. Again, it's undefined behaviour.

Ian Woods

Thats because I am not very active here I guess =P

Guess I should be!! *Motivated now *