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

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 2

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

Answer 3

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.