高内存范围内的堆分配地址

Question

I am facing this strange issue in which the addresses malloc 'ed between two calls to malloc differ a lot in terms of the address allocated, for example:

iovec = 0x6201f0

and:

iovec = 0x7ffff00008e0

and the size of the iovec structure is 40 bytes. Is there a possible reason to explain this behavior ?

Answer 1

Whatever the address, it should not be an issue. One potential reason is that there are two different routes for the dynamic memory system to get memory from the system. One is brk() and the other anonymous (ie MAP_ANON ) mmap() . I believe glibc uses brk() for small amounts of memory but mmap() for large. If you have an intervening call to allocate a large amount of memory, the subsequent malloc() for a 40 byte iovec may sit in the end of the page(s) grabbed using mmap() . If you really want to know what's happening, I suggest you use strace and find out the memory addresses returned.

Also note that if you allocated one on the stack rather than the heap, then it will (of course) have a different looking memory address.