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

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 
#define MSIZE(ptr) malloc_usable_size((void*)ptr)
#elif defined __APPLE__
#include 
#define MSIZE(ptr) malloc_size(const void *ptr)
#elif defined _WIN32
#include 
#define MSIZE(ptr) _msize(ptr)
#else
#error "unknown system"
#endif