Measure the test taker's knowledge about how to prevent the program from being started twice.

To ensure that only one instance is created for QT, The Global mutex variables and file lock methods are used for windows and linux respectively.

The Q_ OS _WIN32 macro is used to indicate that the target platform for compiling and running is windows, and Q_ OS _LINUX indicates that the target platform is linux.

# If defined Q_ OS _WIN32 // for win <br/> # include <windows. h> <br/> bool checkOnly () <br/> {<br/> // create a mutex <br/> HANDLE m_hMutex = CreateMutex (NULL, FALSE, L "fortest_abc123"); <br/> // check the error code <br/> if (GetLastError () = ERROR_ALREADY_EXISTS) {<br/> // If a mutex exists, release the handle and reset the mutex. <br/> CloseHandle (m_hMutex); <br/> m_hMutex = NULL; <br/> // exit the Program <br/> return false; <br/>}< br/> else <br/> return true; <br/>}< br/> # endif <br/> # if defined Q_ OS _LINUX // for linux <br/> # include <sys/types. h> <br/> # include <sys/stat. h> <br/> # include <fcntl. h> <br/> # include <unistd. h> <br/> bool checkOnly () <br/> {<br/> const char filename [] = "/tmp/lockfile "; <br/> int fd = open (filename, O_WRONLY | O_CREAT, 0644); <br/> int flock = lockf (fd, F_TLOCK, 0 ); <br/> if (fd =-1) {<br/> perror ("open lockfile/n"); <br/> return false; <br/>}< br/> // lock the file <br/> if (flock =-1) {<br/> perror ("lock file error/n"); <br/> return false; <br/>}< br/> // after the program exits, automatic File unlocking <br/> return true; <br/>}< br/> # endif <br/> int main (int argc, char * argv []) <br/>{< br/> QTextCodec: setCodecForLocale (QTextCodec: codecForName ("GB18030"); <br/> QTextCodec: setCodecForTr (QTextCodec :: codecForName ("GB18030"); <br/> Q_INIT_RESOURCE (wisdpsclient); <br/> QApplication app (argc, argv ); <br/> // check whether the program has been started <br/> if (checkOnly () = false) <br/> return 0; <br/> Test dialog; <br/> dialog. show (); <br/> return app.exe c (); <br/>}< br/>

Reference http://www.linuxsir.org/bbs/thread117349.html