C++ pointer errors using pthreads - c++

Question

I'm new at C++ language and I'm having some troubles with pointers/memory access in a program using pthreads.
Here is the deal: I have a class named Media, as below:
class Media
{
public:
int id;
int freeSpace = CAPACITY;
pthread_mutex_t mutex;
};
and a class named File, as below:
class File
{
public:
int id;
string name;
int size;
pthread_mutex_t mutex;
Media *media;
...
};
This doesn't matter much for the problem, but my program does the following: I have a vector with some random files with random sizes, and I want to put them inside the minimum quantity of medias using threads.
The problem is that I'm getting constantly "double free or corruption (fasttop)", "double free or corruption (!prev)" and "Segmentation fault" errors, and can't find out why. I'm not using mallocs nor frees, but I'm probably using pointers in a wrong way or something like that.
Here is a part of my main class, with the method used in the threads:
vector<Media> medias;
vector<File> files;
vector<File> processedFiles;
void* processFiles(void* args)
{
for (int i = 0; i < mediaQuantity; i++)
{
if (pthread_mutex_trylock(&medias[i].mutex) == 0)
{
Media* media = &medias[i];
for (int j = (int)files.size() - 1; j >= 0; j--)
{
if (pthread_mutex_trylock(&files[j].mutex) == 0)
{
if (files[j].size <= media->freeSpace)
{
media->freeSpace -= files[j].size;
files[j].media = media;
processedFiles.push_back(files[j]);
files.erase(files.begin() + j);
}
pthread_mutex_unlock(&files[j].mutex);
}
}
pthread_mutex_unlock(&medias[i].mutex);
}
}
}
int main()
{
...
Media* media;
...
pthread_t threads[threadQuantity];
pthread_barrier_init(&barrier, NULL, threadQuantity);
...
for (int i = 1; i <= mediaQuantity; i++)
{
media = new Media();
media->id = i;
medias.push_back(*media);
}
//Create threads
for(unsigned i = 0; i < threadQuantity; i++)
pthread_create(&threads[i], NULL, processFiles, NULL);
for(unsigned i = 0; i < threadQuantity; i++)
pthread_join(threads[i], NULL);
...
}
Somehow, I'm getting the double free or corruption and segmentation errors I mentioned in running time. Can anybody see anything wrong?
Sorry for the big question and thanks in advance!

Answer 1

Here is a hint after a quick look at your code, those lines:
processedFiles.push_back(files[j]);
files.erase(files.begin() + j);
are modifing collections that can be used by more than one thread simulanuously. You should add lock to prevent two threads from doing that.