Determine size of dynamically allocated memory in C

Question

Is there a way in C to find out the size of dynamically allocated memory?

For example, after

char* p = malloc (100);

Is there

Answer 1

Well now I know this is not answering your specific question, however thinking outside of the box as it were... It occurs to me you probably do not need to know. Ok, ok, no I don't mean your have a bad or un-orthodox implementation... I mean is that you probably (without looking at your code I am only guessing) you prbably only want to know if your data can fit in the allocated memory, if that is the case then this solution might be better. It should not offer too much overhead and will solve your "fitting" problem if that is indeed what you are handling:

if ( p != (tmp = realloc(p, required_size)) ) p = tmp;

or if you need to maintain the old contents:

if ( p != (tmp = realloc(p, required_size)) ) memcpy(tmp, p = tmp, required_size);

of course you could just use:

p = realloc(p, required_size);

and be done with it.

Answer 2

This code will probably work on most Windows installations:

template <class T>
int get_allocated_bytes(T* ptr)
{
 return *((int*)ptr-4);
}

template <class T>
int get_allocated_elements(T* ptr)
{
 return get_allocated_bytes(ptr)/sizeof(T);
}

Answer 3

I was struggling recently with visualizing the memory that was available to write to (i.e using strcat or strcpy type functions immediately after malloc).

This is not meant to be a very technical answer, but it could help you while debugging, as much as it helped me.

You can use the size you mallocd in a memset, set an arbitrary value for the second parameter (so you can recognize it) and use the pointer that you obtained from malloc.

Like so:

char* my_string = (char*) malloc(custom_size * sizeof(char));
if(my_string) { memset(my_string, 1, custom_size); }

You can then visualize in the debugger how your allocated memory looks like:

Answer 4

If you use malloc then you can not get the size.

In the other hand, if you use OS API to dynamically allocate memory, like Windows heap functions, then it's possible to do that.

Answer 5

Like everyone else already said: No there isn't.

Also, I would always avoid all the vendor-specific functions here, because when you find that you really need to use them, that's generally a sign that you're doing it wrong. You should either store the size separately, or not have to know it at all. Using vendor functions is the quickest way to lose one of the main benefits of writing in C, portability.

Answer 6

comp.lang.c FAQ list · Question 7.27 -

Q. So can I query the malloc package to find out how big an allocated block is?

A. Unfortunately, there is no standard or portable way. (Some compilers provide nonstandard extensions.) If you need to know, you'll have to keep track of it yourself. (See also question 7.28.)