April1999

Verifying Only One Instance of the Application is Running

By George Sexton, Rocky Mountain Fox User Group

Ever since the advent of Windows, a popular question has been:

“How do I make sure users only run one instance of my application?” While there are a number of :olutions to this problem, using the Win32 API provides the cleanest. The program below uses mutexes. Mutex means “Mutually Exclusive”. When developing Win32 programs that are multi-threaded, mutexes provide an easy way to synchronize access to a resource. For example, you have one thread reading data from a device into an array and another thread that takes the data and performs some processing. You wouldn’t want both threads altering the array at the same time. For this case, each thread calls CreateMutex() to get a handle to the named mutex. The first thread would call WaitForSingIeObject~to gain ownership of the mutex. If the second thread were to try to get the mutex it would block until the first thread called ReleaseMutexO. For our purposes we take advantage of the fact that if you call CreateMutex() and it already exists, the return value of GetLastError() is ERROR_ALREADY_EXISTS.

In a nutshell we can have our program open a named mutex. If it doesn’t exist then the program proceeds normally. If it does exist, we terminate the program. Any mutexes that a program owns are released by Windows when the process terminates. This means that if our program crashes we can restart it without any problems. As you can see from the implementation program using the Win32 API and mutexes we can come up with a very simple solution to an annoying problem.

‘3/9199 George Sexton - MH Software, Inc.

Simple code to keep two copies of an application from executing at me same time.

The code works by creating a mutex. If the mutex doesn’t exist, tne return coae from GetLastError() iS U. IT it does already exist then the return coae is

ERROR_ALREADY_EXISTS

Closing the mutex is not strictly necessary bUt SflOWfl TOr completeness. According to tne Win32 API documentation wnen the owning process terminates the Mutex wiii be released. This means that if your app errors out and terminates the user will be able to restart the application.

April 19, 1999, 6:30 PM, Rod Paddock. Rod Paddock is president and founder of Dash Point Software, Inc. DPSI based in Seattle, WA, specializes in developing applications and training software developers. DPSI develops with Visual lnterdev, VFP, VB and SQL Server. DPSI’s clients include: ADTC, SBT Accounting, The US Coast Guard, The US Navy, Pinnacle Publishing, Intel and Azalea Software. Rod was a featured spen”3r at 1996/1997 9Q7 German FoxPro DevCons the 1995 and 1998 Visual FoxPro Developers Conference, the 1995-1997 FoxTeach conferences, and the 2nd Annual Southern California Visual FoxPro Conference. Rod writes for several database publications including: Data Based Advisor, Foxtalk and Dbase Advisor. Rod’s books include: VB5 for Web Development and VFP 5 Enterprise Development from Prima Publishing. Both of these books have been revised for the Visual Studio 6. DPSI can be found at: http://www.dashpoint.com.

Conferences play an important and useful part in the FoxPro community. Here’s a rundown of upcoming events:

Microsoft Tech-Ed 99, May 21-28,1999, Dallas Convention Center, Dallas, TX. Micosoft Tech-Ed 99 has been expandec to ive you a choice of three days of focusing on IT operations management - the people who plan, evaluate, deploy, maintair and support IT systems - or five of developer courses for those who design, develop, or test software applications. For more info, go to http://events.microsoft.com/events/teched.

Inserting/Retrieving Files

The LA Fox Developer

A Newsletter for FoxPro Application Developers in Southern California Visit our WebSite at: http://www.mesllc.com/lafox.html April 1999

The April meeting of LA Fox will be held on April 19, 1999, at 6:30 PM at our regular meeting place (the Torrance Airport, 3301 Airport Drive, in Torrance). For details on how to get there, see the map on the back page.