Assume the code below demonstrating a binary semaphore example.

In this example, we have a pthread that reads the source.txt and attempts to copy the content to destination.txt while locking it using a binary semaphore.

What happens in the comment section below without the semaphore?

#include <cstdlib>
#include <memory>
#include <filesystem>
#define _TIMESPEC_DEFINED
#include <pthread.h>
#include <semaphore.h>
#include <thread>
#include <valarray>

pthread_t StartFileAccessThread();
void *CopyFile(void *parameters);

int main(int argc, char* argv[])
{
    pthread_t thread = StartFileAccessThread();

    void *returnValue;
    pthread_join(thread, &returnValue);

    getchar();
    return EXIT_SUCCESS;
}


pthread_t StartFileAccessThread()
{
    std::string sourcePath("source.txt");
    std::string destinationPath("dest.txt");
    sem_t semaphore;
    sem_init(&semaphore, 0, 0);
    pthread_t thread;

    void *parameters[3];
    parameters[0] = &semaphore;
    parameters[1] = &sourcePath;
    parameters[2] = &destinationPath;

    pthread_create(&thread, nullptr, &CopyFile, parameters);

    // What happens without the binary semaphore?
    sem_wait(&semaphore);
    sem_destroy(&semaphore);

    printf("Freeing ressources.\n");

    return thread;
}


void *CopyFile(void *rawParameter)
{
    void **parameters = static_cast<void **>(rawParameter);

    sem_t *semaphore = static_cast<sem_t *>(parameters[0]);
    std::string sourcePath(*static_cast<std::string *>(parameters[1]));
    std::string destinationPath(*static_cast<std::string *>(parameters[2]));

    sem_post(semaphore);

    std::this_thread::sleep_for(std::chrono::seconds(2));

    copy_file(sourcePath, destinationPath, std::experimental::filesystem::copy_options::overwrite_existing);

    printf("File copied \n");

    return nullptr;
}

CodePudding user response：

What happens in the comment section below without the semaphore?

Without the semaphore, function startFileAccessThread() may return before the new thread finishes (or starts) copying its parameters from the argument object. That object is local to startFileAccessThread(), so its lifetime ends when that function returns. Undefined behavior results if the copy thread's accesses to it do not happen before that.