How to avoid a memory leak in this code?

Question

I've the next code, at the click event of a button:

         PNIO_DEV_ADDR addr;
         addr.AddrType = PNIO_ADDR_GEO;     //Para IO Device
         addr.IODataType = PNIO_IO_OUT;     //Escritura en PLC
         addr.u.Geo.Slot = (int)(numericUpDownSlot->Value);
         addr.u.Geo.Subslot = (int)(numericUpDownSubslot->Value);

         CP1626::write(&addr);

PNIO_DEV_ADDR is a C struct, and write is a function linked to a DLL callback, which asks for a PNIO_DEV_ADDR* parameter.

Each time I press the button I can see at the task manager how the memory associated to my app increases a few bytes.

I've googled and read a lot about pointers and references but I don't quite understand what I'm doing wrong.

Could you explain me where is the problem, please?

P.S.: I'm using a C library (built on a DLL) and a C++/CLI application.

Thank you in advance.

EDIT:

PNIO_DEV_ADDR is:

typedef struct {
    PNIO_ADDR_TYPE  AddrType;
    PNIO_IO_TYPE    IODataType;
    union {
        PNIO_UINT32 Addr;
        struct {
            PNIO_UINT32 reserved1[2];
            PNIO_UINT32 Slot;
            PNIO_UINT32 Subslot;
            PNIO_UINT32 reserved2;
        } Geo;                   /* geographical address */
    } u;
} ATTR_PACKED PNIO_DEV_ADDR;

When the two first variables are enums.

EDIT2:

This is the entry point of the function write inside the DLL:

PNIO_UINT32 write(PNIO_DEV_ADDR* addr){
    PNIO_UINT32 result;

    result = PNIOD_trigger_data_write_sync(g_devHndl, addr, PNIO_ACCESS_RT_WITH_LOCK);

    return result;
}

PNIO_trigger_data_write_sync requires a PNIO_DEV_ADDR*. Sorry but I can't access inside this function because it is on a different DLL which is third party. Should I've copied the addr pointer?

Where is the allocation taking place? If `addr` is really a struct and not some pointer type, and you have a function `write(PNIO_DEV_ADDR*)`, then no allocation takes place/should take place anywhere. It is of course possible that `write` leaks memory internally. — Lundin, Feb 22 '18 at 12:49
Nobody will be able to reproduce the problem with what's given here. Windows task manager isn't all that reliable. Are you sure there's a memory leak? — Lundin, Feb 22 '18 at 13:51
@Lundin yes, memory is not oscillating as there is in other applications. But at the moment I click the button it increases a few bytes, and never goes back to the initial value. — areify, Feb 22 '18 at 14:23
https://stackoverflow.com/questions/413477/is-there-a-good-valgrind-substitute-for-windows — Lundin, Feb 22 '18 at 14:26
Task Manager has many memory fields it can show (most of which aren't particularly informative about small memory leaks). Which field are you looking at? — Adrian McCarthy, Feb 22 '18 at 18:08

Abhijit Pritam Dutta · Accepted Answer · 2018-02-22T14:14:31.877

1

There are two kinds of memory allocation in C/C++, one is memory allocation during compile time and other one is dynamic memory(memory allocation during run time) allocation and I am sure you know about this. memory allocation during compile is done form stack of an application and dynamic memory allocation is done from heap of an application. We need to concern about memory release which is allocated (dynamically) from heap to avoid memory leak. For memory release from stack compiler automatically manage it.

In your case the allocation is done from stack. Hence once the control go out of the scope of the method where your current codes are, it will automatically release the memory of stack.

Now in your case PNIO_DEV_ADDR addr;, this is compile time memory allocation and is not dynamically allocated memory at all. So it will not leads to memory leak.

Inside the dll method, I am sure it has copied your object to its local object.

edited Feb 22 '18 at 14:14

answered Feb 22 '18 at 13:03

Abhijit Pritam Dutta

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`static` storage duration variables have nothing to do with the stack at all. Fixed-size arrays are sometimes sloppily called "static", since the size is set at compile time. Perhaps this is what you mean. – Lundin Feb 22 '18 at 13:48
I have modified my answer. But don't understood why you have downvoted. – Abhijit Pritam Dutta Feb 22 '18 at 14:03
I haven't. Although I'm not sure how this answers the question. In fact I don't think it is possible to answer the question, as it stands. – Lundin Feb 22 '18 at 14:09
if you see his question he is asking how he can avid the memory leak in this case:- PNIO_DEV_ADDR addr; (but this is compile time memory allocation) . This is not dynamically allocated memory. So it will not leads to memory leak. – Abhijit Pritam Dutta Feb 22 '18 at 14:13
Thank you for your answer @AbhijitPritam. Maybe the problem is inside the DLL. It's not mine but I have the source code, so I'm posting the entry point on my question. Just give me a minute, please. And again, thank you for your answer. – areify Feb 22 '18 at 14:31
@AbhijitPritam sure! Sadly it seems that recently is easier to get a negative vote than to get a positive one... – areify Feb 22 '18 at 14:40
@AbhijitPritam `PNIO_DEV_ADDR addr;` looks like a local variable. It is not "compile time memory allocation", it is run-time automatic allocation: https://en.wikipedia.org/wiki/Automatic_variable – Andriy Berestovskyy Feb 23 '18 at 07:20

How to avoid a memory leak in this code?

1 Answers1