Question

I have a problem with semaphores in Windows between two application. An application waits for release signal(Qt) and the other application sends release signal(MSVC2008). But it dose not work.

I tested Qt-Qt and MSVC2008-MSVC2008 modes and they was succeed. But when I try Qt-MSVC2008 mode it fails.

// MSVC2008:
#include <windows.h>
#include <stdio.h>
int main()
{
    const WCHAR semName[] = L"TestSem";

    PHANDLE sem = (HANDLE *) CreateSemaphore (NULL, 0, 1, semName);

    if (sem == NULL)
    {
         sem = (HANDLE *)OpenSemaphore (SEMAPHORE_ALL_ACCESS, 0, semName);
    }

    if (sem == NULL)
    {
        printf("OPEN/CREATE ERROR\n");
        return 0;
    }

    BOOL r = ReleaseSemaphore(sem, 1 ,NULL);
    if (r)
        printf("OK\n");
    else
        printf("RELEASE ERROR\n");
    CloseHandle (sem);

    return 0;
}

and

// Qt 4.8.0 :
#include <QSystemSemaphore>
#include <QCoreApplication>
#include <iostream>

int main()
{
    QSystemSemaphore *sem_read = new QSystemSemaphore("TestSem");

    std::cout << "Wait for signal: " << std::endl;

    while (1)
    {
        sem_read->acquire();
        std::cout << "Hi" << std::endl;
    }

    return 0;
}

I expect when Qt-app is running, after executing MSVC2008-app, it prints one “Hi” in the screen. But it dose not. What is the problem?!

Note: I’m using Windows 7 and MinGW compiler for Qt

Answer 1

Looking at the Qt sources (4.7.3) I see at corelib/kernel/qsystemsemaphore_p.h:79 that the semaphore name generated by Qt is prefixed by qipc_systemsem_.

[UPDATE] The sha1 hash of “TestSem” is also appended, so the resulting Qt semaphore name is qipc_systemsem_TestSem3ec37c26f212774998f34a4e6722cac152ad17fa

Confirmed working.

To generate the semaphore name:

QString prefix = "qipc_systemsem_";
QString key = "TestSem";

QString result = prefix;

QString part1 = key;
part1.replace(QRegExp(QLatin1String("[^A-Za-z]")), QString());
result.append(part1);

QByteArray hex = QCryptographicHash::hash(key.toUtf8(), QCryptographicHash::Sha1).toHex();
result.append(QLatin1String(hex));

qDebug() << result;