_msize does not have equivalent function in Nucleus

Hi all,

A standard realization of realloc function in Microsoft VC as below:
void* realloc(void* buf, size_t len)
{
    void* newBuf ;
     // Get rid of degenerate cases
    if (buf == 0) return malloc(len);
    if (len == 0)
    {
        free(buf);
        return 0;
    }
    // Need to move memory, acquire new chunk
    newBuf = malloc(len);
    if (!newBuf ) return 0;

    // Move data to the new location
    memcpy(newBuf , buf, min(_msize(buf), len));
    free(buf);
    return newBuf ;
}
I need to port this function to Nucleus(RTOS) but there is no function equivalent to _msize which return the size of a buffer. Any advice?
Does anyone know how Microsoft return the size of a buffer and why RTOS can not?
Thanks
Hai