How to free allocated memory in C++ DLL

Action Heinz Published at Dev

Action Heinz

i've got following code to encrypt a string in a C++ DLL

EXPORT WCHAR* EncryptString(WCHAR* stringToEncrypt) {
    aes_context ctx;

    WCHAR* in = stringToEncrypt;
    WCHAR* out;
    WCHAR* key = L"TestKey";

    BYTE* buffEnc = (BYTE*)malloc(16);
    BYTE* keyBuffEnc = (BYTE*)malloc(32);

    memset(buffEnc, 0, 16);
    memset(keyBuffEnc, 0, 32);

    memcpy(buffEnc, in, wcslen(in) * 2);
    memcpy(keyBuffEnc, key, wcslen(key) * 2);
    aes_set_key(&ctx, keyBuffEnc, 256);

    aes_encrypt(&ctx, buffEnc, buffEnc);
    out = (WCHAR*)buffEnc;

    // free(buffEnc);   
    // free(keyBuffEnc);

    return out;
}

My problem is that i can not free the allocated memory because otherwise the result is broken. I wonder how can i free the used memory without losing the result? Have i to change the type of return value?

Thanks in advance for your help. Greets Heinz

sharptooth

This is indeed a problematic situation - you return a pointer to allocated memory and it's unclear who should free the memory. You have the following options:

tell the caller free the memory using free() - this will only work if they use the same heap which is hard to guarantee. This is very unreliable and not really recommended.
introduce a memory management interface (such as freeEncrypted() function that is implemented in your library) and tell the caller use it - then memory will be returned to the right heap.
use something well known like CoTaskMemAlloc() for allocation and tell the caller to use the matching function such as CoTaskMemFree() for freeing memory. This is similar to point 2, just uses a well known common memory manager.
change the interface such that it accepts pointer to already allocated data and its size so that the caller both allocates and frees the memory.

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