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 applica-

tion?” 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 pro-

gram 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.

(Con’t, page 8)

At LA Fox

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.

Elsewhere

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.

In this issue

Downgrade from VFP?

Inserting/Retrieving Files

Hot Tip

Page 3

Page 3

Page 7

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

Out and About

by Barry R. Lee

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.

n

and much more