C++ binary read char array causes: "free(): double free detected in tcache 2"

Adam Ciesielski 0
Viewed 39

I've browsed topics on this error message: "free(): double free detected in tcache 2" but none covers my case (at least not in the direct way).

The program executes and returns correct values. However, I receive free(): double free detected in tcache 2 after its run. If I remove lines corresponding to char array, marked as //ERROR there's no such issue. Could someone help me to understand what's going on there?

The code:

#include <cstring>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>

#pragma pack(push, r1, 1)
struct Header2 {
    char id[15]; // ERROR
    std::vector<int> length = std::vector<int>(2);
    int count;
};
#pragma pack(pop, r1)

int main() {
  Header2 h;
  strcpy(h.id, "xdddd");  // ERROR
  h.length =  {3, 5};
  h.count = 42;

  std::fstream fh2;
  fh2.open("test.txt", std::fstream::out | std::fstream::binary);
  fh2.write((char*)&h, sizeof(Header2));
  fh2.close();

Header2 h3;

  fh2.open("test.txt", std::fstream::in | std::fstream::binary);
  fh2.read((char *) &h3, sizeof(Header2));
  fh2.close();

   if(!fh2.good()) {
      std::cout << "Error occurred at reading time!" << std::endl;
      return 1;
   }

  std::vector<int> out = h3.length;
  std::cout << h3.id << " "  << std::endl; // ERROR
  std::cout << out[0] << "," <<out[1]  << " " << h3.count << std::endl;
  std::cout << "End of program" << std::endl;
  return 0;
}

In the code, it is important that the sizes of struct variables are fixed, because different programs would read output files.

The problem investigated with TIO Run gives additional message: /srv/wrappers/cpp-gcc: line 5: 15547 Aborted (core dumped) ./.bin.tio "$@" < .input.tio)

binary-data char struct io c++
1 Answers

TL;DR version: Use std::array instead of std::vector.

#include <cstring>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <sstream>
#include <array> // change here

#pragma pack(push, r1, 1)
struct Header2 {
    char id[15];
    std::array<int, 2> length = std::array<int, 2>(); // change here
    int count;
};
#pragma pack(pop, r1)

int main() {
  Header2 h;
  strcpy(h.id, "xdddd");  
  h.length =  {3, 5};
  h.count = 42;

  std::fstream fh2;
  fh2.open("test.txt", std::fstream::out | std::fstream::binary);
  fh2.write((char*)&h, sizeof(Header2));
  fh2.close();

Header2 h3;

  fh2.open("test.txt", std::fstream::in | std::fstream::binary);
  fh2.read((char *) &h3, sizeof(Header2));
  fh2.close();

   if(!fh2.good()) {
      std::cout << "Error occurred at reading time!" << std::endl;
      return 1;
   }

  std::array<int, 2> out = h3.length; // change here
  std::cout << h3.id << " "  << std::endl; // ERROR
  std::cout << out[0] << "," <<out[1]  << " " << h3.count << std::endl;
  std::cout << "End of program" << std::endl;
  return 0;
}

Since the question states In the code, it is important that the sizes of struct variables are fixed, std::vector is not the right tool for the job. Vector is dynamic. std::array is not. It'll always be 2 ints long and is simple enough to dump to a binary file.

std::vector, on the other hand, is far too complex a structure to be written to a file with write. vector is basically a pointer to a data buffer owned (What is ownership of resources or pointers?) by the vector and some book-keeping variables tracking the capacity of the buffer and amount of the buffer currently in use (and are usually also pointers). When you write the vector to the file, the data "in" the vector is not written because it's not in the vector. A pointer to it is. The pointer to the data buffer is written to the file. When the vector is read back out of the file, the "new" vector gets the exact same pointer. Now you have two vectors both owning the same dynamic allocation. Sooner or later both go out of scope and Boom! Double free. Assuming the program holds up that long. Usually when the first vector goes out of scope the second vector is left pointing to invalid memory and becomes a bomb that goes off when the vector's deceased data buffer is accessed.

If you wrote the vector and exited the program, you're still doomed. The data's not in the file. When a future run of the program loads the vector, the pointer refers to the memory allocated in the previous run of the program and almost certainly is no longer valid (and, trust me on this, it's actually worse if the memory IS valid). You need to serialize the vector, basically write the stuff in the vector to the file in a documented and controlled manner (a protocol) and then when reading back, make a new vector that you can read the contents in the file into.

But in this case it appears that vector is overkill and std::array is a better fit.

Also watch out when writing files: Not every system (and sometimes not even the same system if different compilers are used for the programs) agree on the size and byte order of an int. Prefer to use the fixed width integers defined in cstddef and write the values in an agreed-upon endian.

user45813010
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