Determine size of dynamically allocated memory in C

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.

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.)

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.