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 a way to find out the size of memory associated with p?

Answer 1

There is no standard way to find this information. However, some implementations provide functions like msize to do this. For example:

• _msize on Windows
• malloc_size on MacOS
• malloc_usable_size on systems with glibc

Keep in mind though, that malloc will allocate a minimum of the size requested, so you should check if msize variant for your implementation actually returns the size of the object or the memory actually allocated on the heap.

Answer 2

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.)

Answer 3

The C mentality is to provide the programmer with tools to help him with his job, not to provide abstractions which change the nature of his job. C also tries to avoid making things easier/safer if this happens at the expense of the performance limit.

Certain things you might like to do with a region of memory only require the location of the start of the region. Such things include working with null-terminated strings, manipulating the first n bytes of the region (if the region is known to be at least this large), and so forth.

Basically, keeping track of the length of a region is extra work, and if C did it automatically, it would sometimes be doing it unnecessarily.

Many library functions (for instance fread()) require a pointer to the start of a region, and also the size of this region. If you need the size of a region, you must keep track of it.

Yes, malloc() implementations usually keep track of a region's size, but they may do this indirectly, or round it up to some value, or not keep it at all. Even if they support it, finding the size this way might be slow compared with keeping track of it yourself.

If you need a data structure that knows how big each region is, C can do that for you. Just use a struct that keeps track of how large the region is as well as a pointer to the region.

Answer 4

Here's the best way I've seen to create a tagged pointer to store the size with the address. All pointer functions would still work as expected:

Stolen from: https://stackoverflow.com/a/35326444/638848

You could also implement a wrapper for malloc and free to add tags (like allocated size and other meta information) before the pointer returned by malloc. This is in fact the method that a c++ compiler tags objects with references to virtual classes. Here is one working example:

#include <stdlib.h>
#include <stdio.h>

void * my_malloc(size_t s) 
{
  size_t * ret = malloc(sizeof(size_t) + s);
  *ret = s;
  return &ret[1];
}

void my_free(void * ptr) 
{
  free( (size_t*)ptr - 1);
}

size_t allocated_size(void * ptr) 
{
  return ((size_t*)ptr)[-1];
}

int main(int argc, const char ** argv) {
  int * array = my_malloc(sizeof(int) * 3);
  printf("%u\n", allocated_size(array));
  my_free(array);
  return 0;
}

The advantage of this method over a structure with size and pointer

 struct pointer
 {
   size_t size;
   void *p;
 };

is that you only need to replace the malloc and free calls. All other pointer operations require no refactoring.

Answer 5

No, the C runtime library does not provide such a function.

Some libraries may provide platform- or compiler-specific functions that can get this information, but generally the way to keep track of this information is in another integer variable.

Answer 6

Everyone telling you it's impossible is technically correct (the best kind of correct).

For engineering reasons, it is a bad idea to rely on the malloc subsystem to tell you the size of an allocated block accurately. To convince yourself of this, imagine that you were writing a large application, with several different memory allocators — maybe you use raw libc malloc in one part, but C++ operator new in another part, and then some specific Windows API in yet another part. So you've got all kinds of void* flying around. Writing a function that can work on any of these void*s impossible, unless you can somehow tell from the pointer's value which of your heaps it came from.

So you might want to wrap up each pointer in your program with some convention that indicates where the pointer came from (and where it needs to be returned to). For example, in C++ we call that std::unique_ptr<void> (for pointers that need to be operator delete'd) or std::unique_ptr<void, D> (for pointers that need to be returned via some other mechanism D). You could do the same kind of thing in C if you wanted to. And once you're wrapping up pointers in bigger safer objects anyway, it's just a small step to struct SizedPtr { void *ptr; size_t size; } and then you never need to worry about the size of an allocation again.

However.

There are also good reasons why you might legitimately want to know the actual underlying size of an allocation. For example, maybe you're writing a profiling tool for your app that will report the actual amount of memory used by each subsystem, not just the amount of memory that the programmer thought he was using. If each of your 10-byte allocations is secretly using 16 bytes under the hood, that's good to know! (Of course there will be other overhead as well, which you're not measuring this way. But there are yet other tools for that job.) Or maybe you're just investigating the behavior of realloc on your platform. Or maybe you'd like to "round up" the capacity of a growing allocation to avoid premature reallocations in the future. Example:

SizedPtr round_up(void *p) {
    size_t sz = portable_ish_malloced_size(p);
    void *q = realloc(p, sz);  // for sanitizer-cleanliness
    assert(q != NULL && portable_ish_malloced_size(q) == sz);
    return (SizedPtr){q, sz};
}
bool reserve(VectorOfChar *v, size_t newcap) {
    if (v->sizedptr.size >= newcap) return true;
    char *newdata = realloc(v->sizedptr.ptr, newcap);
    if (newdata == NULL) return false;
    v->sizedptr = round_up(newdata);
    return true;
}

To get the size of the allocation behind a non-null pointer which has been returned directly from libc malloc — not from a custom heap, and not pointing into the middle of an object — you can use the following OS-specific APIs, which I have bundled up into a "portable-ish" wrapper function for convenience. If you find a common system where this code doesn't work, please leave a comment and I'll try to fix it!

#if defined(__linux__)
// https://linux.die.net/man/3/malloc_usable_size
#include <malloc.h>
size_t portable_ish_malloced_size(const void *p) {
    return malloc_usable_size((void*)p);
}
#elif defined(__APPLE__)
// https://www.unix.com/man-page/osx/3/malloc_size/
#include <malloc/malloc.h>
size_t portable_ish_malloced_size(const void *p) {
    return malloc_size(p);
}
#elif defined(_WIN32)
// https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/msize
#include <malloc.h>
size_t portable_ish_malloced_size(const void *p) {
    return _msize((void *)p);
}
#else
#error "oops, I don't know this system"
#endif

#include <stdio.h>
#include <stdlib.h>  // for malloc itself

int main() {
    void *p = malloc(42);
    size_t true_length = portable_ish_malloced_size(p);
    printf("%zu\n", true_length);
}

Tested on:

• Visual Studio, Win64 — _msize
• GCC/Clang, glibc, Linux — malloc_usable_size
• Clang, libc, Mac OS X — malloc_size
• Clang, jemalloc, Mac OS X — works in practice but I wouldn't trust it (silently mixes jemalloc's malloc and the native libc's malloc_size)
• Should work fine with jemalloc on Linux
• Should work fine with dlmalloc on Linux if compiled without USE_DL_PREFIX
• Should work fine with tcmalloc everywhere

Answer 7

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 8

I would expect this to be implementation dependent.
If you got the header data structure, you could cast it back on the pointer and get the size.

Answer 9

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 10

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 11

Quuxplusone wrote: "Writing a function that can work on any of these void*s impossible, unless you can somehow tell from the pointer's value which of your heaps it came from." Determine size of dynamically allocated memory in C"

Actually in Windows _msize gives you the allocated memory size from the value of the pointer. If there is no allocated memory at the address an error is thrown.

int main()
{
    char* ptr1 = NULL, * ptr2 = NULL;
    size_t bsz;    
    ptr1 = (char*)malloc(10);
    ptr2 = ptr1;
    bsz = _msize(ptr2);
    ptr1++;
    //bsz = _msize(ptr1);   /* error */
    free(ptr2);

    return 0;
}

Thanks for the #define collection. Here is the macro version.

#define MALLOC(bsz) malloc(bsz)
#define FREE(ptr) do { free(ptr); ptr = NULL; } while(0)
#ifdef __linux__
#include <malloc.h>
#define MSIZE(ptr) malloc_usable_size((void*)ptr)
#elif defined __APPLE__
#include <malloc/malloc.h>
#define MSIZE(ptr) malloc_size(const void *ptr)
#elif defined _WIN32
#include <malloc.h>
#define MSIZE(ptr) _msize(ptr)
#else
#error "unknown system"
#endif

Answer 12

Note: using _msize only works for memory allocated with calloc, malloc, etc. As stated on the Microsoft Documentation

The _msize function returns the size, in bytes, of the memory block allocated by a call to calloc, malloc, or realloc.

And will throw an exception otherwise.

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/msize?view=vs-2019