CODEWITHSUNDEEP
c++memory-leaks
kangHan
kangHan

Reputation: 13

Strange Memory leak by new/delete in C++

Below is my question and code:

  1. When code run to line 26, the memory obtained by this process does not return to OS?
  2. But, if I delete line 16, the memory will be released correctly?

I know this is not the regular way to use so many small memory blocks, but I was very curious to know the reason.

I have run this program with MALLOC_MMAP_MAX_=1000000 MALLOC_MMAP_THRESHOLD_=1024, but nothing changed.

int i = 0;
std::cout << "waitting for input, you can check current memory" << std::endl;
std::cin >> i;
char** ptr = new char *[1000000];
std::map<int, char *> tMap;
for (unsigned long i = 0; i < 1000000; i ++)
{
    ptr[i] = new char[3000];
    tMap.insert(make_pair(i, ptr[i]));    //line 16
}
std::cout << "waitting for input, you can check current memory" << std::endl;
std::cin >> i;
for (unsigned long i = 0; i < 1000000; i ++)
{
    delete []ptr[i];
}
delete []ptr;
std::cout << "waitting for input, you can check current memory" << std::endl;
std::cin >> i;   //line 26
return 0;

here are more materials, And I have checked memory of tMap, less than 100M.

1、allocated memory and stop, check memory res:

holds 2.9G memory

2、deallocate memory and stop, check memory res:

holds 2.9G memory

Upvotes: 0

Views: 375

Answers (2)

Tony Delroy
Tony Delroy

Reputation: 106126

1、when code run to line: 26, the memory obtained by this process does not return to OS ?

There's no guarantee any memory will be released by a C++ program to the Operating System just because it's deleted properly by the program. In many C++ runtimes, dynamically allocated memory that's deleted will still be reserved for future use by the same program, and not released to the OS. GCC/Linux is an example of an compiler/runtime-environment where larger allocations are usually done in shared memory segments that can be released to the Operating System before the program terminates, such that the OS or other programs can use them.

2、But, if I delete line: 16, the memory wile be released correctly ?

Line 16 doesn't make any difference to the later deletion/deallocation of the memory at line 22 (which may return it to the pool of dynamic memory that the application may later re-allocate, or actually release it to the OS as mentioned above). It does involve more dynamic allocations for the std::map elements itself though.

Note that the tMap destructor does not itself delete or release the memory in any way. To have memory automatically released - either by pointer-like variables or containers there-of - use smart pointers such as std::shared_ptr or std::unique_ptr (you can google them for information).

Upvotes: 2

Bo Persson
Bo Persson

Reputation: 92271

C++ doesn't have garbage collection, so keeping an extra copy of a pointer doesn't stop the memory from being deallocated.

What happens after delete[] ptr[i] is that the map is full of dangling pointers that can no longer be used.

Another thought: What you might see as a memory leak is the fact that the tMap also allocates dynamic memory to store the inserted data. That memory will be released when the map goes out of scope, just after line 27.

Upvotes: 3

Related Questions