Why dynamically allocated memory return different size that i actually tried to allocate? [duplicate]

Question

I want to add a string after the struct in memory. How to check that i dynamically allocated right amount of bytes?

Example:

 const wchar_t* add_str = L"test string"; struct test_{ wchar_t* name; size_t namelen; } test; void* ptest_void = malloc(sizeof(test) + wcslen(add_str)*sizeof(wchar_t)); // i cant dereference void*, hence, cant check sizeof(*ptest_void) // then i try to get sizeof of a ptr which was cast to (test_*): test_* ptest = (test_*)ptest_void; size_t ptest_sz = sizeof(*ptest); // ptest_sz has the size of _test struct, but without size of add_str... free(ptest_void);

sizeof(pointer) returns size of pointer, not the content it points to. — Yksisarvinen
– Yksisarvinen, Commented Aug 18, 2022 at 12:05
it was a typo, i edited the question the now there is sizeof(*ptest) — hgrev
– hgrev, Commented Aug 18, 2022 at 12:08
also this code supose to be C or C++? There is a difference how it should look like. — Marek R
– Marek R, Commented Aug 18, 2022 at 12:08
Don't use malloc/free in C++. Use new/new[]/delete/delete[] if you must, but in general try to avoid manual memory management entirely. Use containers first, smart pointers if you have to, raw manual memory management almost never. — Jesper Juhl
– Jesper Juhl, Commented Aug 18, 2022 at 12:09
As @JesperJuhl said: Use STL containers (std::vector, std::string) first. Then only if you really have to write your own datastructure prefer using std::make_unique/std_unique_ptr (or std::make_shared/std::shared_ptr if you have good enough design reasons to do so). Check it here (I don't think I needed new/delete anymore ever since C++11 came out): isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines — Pepijn Kramer
– Pepijn Kramer, Commented Aug 18, 2022 at 12:19

dbush · Accepted Answer · 2022-08-18 12:06:36Z

sizeof(ptest) doesn't tell you how much allocated memory ptest points to. It tells you the size in bytes of the variable ptest, most likely either 4 or 8 depending on your system.

There's also no standard way to check "how big" an allocated block of memory it. You're expected to keep track of that yourself.

If a call to malloc does not return NULL, that means the call was successful, and you can access as many bytes as you told it to allocate.

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