Why does valgrind talk about 'Mismatched free()'

Question

I'm trying to build an tree kind structure, which will store tokens for an IMAP command. I'm trying to add strings to them, and free them. But valgrind complains, and I do not know why.

#include <iostream>
#include <algorithm>
#include <vector>

#include <string.h>

typedef enum : uint8_t
{
    TT_STRING,
    TT_INT32,
    TT_INT64,
    TT_CHAR,
    TT_PAIR
} TokenType;

typedef struct
{
    int32_t p_From;
    int32_t p_To;
} Pair;

struct Token
{
    union {
        char *t_String;
        int32_t t_Int32;
        int64_t t_Int64;
        char t_Char;
        Pair t_Pair;
    };
    TokenType t_Type;
    std::vector<Token> t_Children;
};

typedef struct Token Token;

void _token_free(Token &token)
{
    if (token.t_Type == TT_STRING)
    {
        delete token.t_String;
    }

    for_each(token.t_Children.begin(), token.t_Children.end(), [=](Token &t){
        _token_free(t);
    });
}

Token _token_str_new(const std::string &str)
{
    Token token;
    token.t_Type = TT_STRING;
    token.t_String = new char[str.size() + 1];
    memcpy(reinterpret_cast<void *>(token.t_String), str.c_str(), str.size() + 1);
    return token;
}

int main() {
    Token token;

    token.t_Int64 = 123;
    token.t_Type = TT_INT64;
    token.t_Children = {
            _token_str_new("Hello World")
    };

    _token_free(token);

    return 0;
}

Valgrind says:

Mismatched free() / delete / delete []
_token_free(Token&)
_token_free(Token&)::{lambda(Token&)#1}::operator()(Token&) const
_token_free(Token&)::{lambda(Token&)#1} std::for_each<__gnu_cxx::__normal_iterator, _token_free(Token&)::{lambda(Token&)#1}>(__gnu_cxx::__normal_iterator<Token*, std::vector>, _token_free(Token&)::{lambda(Token&)#1}, _token_free(Token&)::{lambda(Token&)#1})
_token_free(Token&)
main
Address 0x4dc0c80 is 0 bytes inside a block of size 12 alloc'd
operator new[](unsigned long)
_token_str_new(std::__cxx11::basic_string<char, std::char_traits, std::allocator> const&)
main

`t_String` was allocated with `new[]`, but deleted with `delete`. You need to use the `delete[]` form. (Or better, use [smart pointers](https://en.cppreference.com/w/cpp/memory/unique_ptr) and/or [standard containers](https://en.cppreference.com/w/cpp/container/vector) so you don't need to manually handle memory management.) — 0x5453, Jul 23 '20 at 19:16
Unrelated: C++ learned a lot from C. One of the things it learned was the `typedef` struct trick to keep from having to type `struct` or `enum` all the time even though everyone knows darn well it's a `struct` or `enum`. C++ baked the trick in. `struct Pair { members... };` is all you need to do to be able to use `Pair`. — user4581301, Jul 23 '20 at 19:48
When SO tells you to do more explaining and less code dumping, instead of giving us _Lorem Ipsum_, follow the advice... — Asteroids With Wings, Jul 23 '20 at 20:38
Also, your `Token`, class has no copy constructor, but `token.t_Children = {` will copy it, potentially leading to double delete. [What is the Rule of 3?](https://stackoverflow.com/questions/11024438/rule-of-three-in-c?rq=1) [What is the Rule of Zero?](https://stackoverflow.com/q/44997955/845092) — Mooing Duck, Jul 23 '20 at 21:01

score 5 · Answer 1 · answered Jul 23 '20 at 19:15

5

When you assign to token.t_String in _token_str_new, you use new[]. _token_free uses delete, which causes the mismatch.

You need to use delete [] token.t_String in _token_free.

answered Jul 23 '20 at 19:15

1201ProgramAlarm

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Thank you ! Lmao that's one hell of a dumb mistake i made. – skywa04885 Jul 23 '20 at 19:41

Why does valgrind talk about 'Mismatched free()'

1 Answers1