A newbie's elementary guide to spawning processes

Introduction

Often, we find questions coming from newbies on how to spawn a new process, how to open a particular document with the program associated with it, how to launch a web site, how to bring up the mail client's compose window, how to wait for a program to finish, how to print a file etc. Quite a few of these can be accomplished using the CreateProcess call. Unfortunately newbies often find that CreateProcess is not exactly the easiest of calls to understand. But the very good news for newbies is that you can use some of the shell functions available through shell32.dll to accomplish the above jobs.

This article is a sort of miniature FAQ that gives you short code snippets that detail how a particular task can be done. You must look up the functions discussed here on MSDN. And I'd be glad if people can send in more questions on similar lines that might fit in here. I also give an ultra brief introduction to CreateProcess at the end of the article. It is highly insufficient an approach, but I do think it will serve a newbie as an easy stepping block.

ShellExecute and what it does for you

This function is declared in Shellapi.h which you will need to include. You will also have to link with Shell32.lib. ShellExecute is one of the API calls that really saves you a lot of effort. We'll go through the usage of the function through the mini FAQ format. Look up the function on MSDN to get an idea of what each of the parameters mean.

Q: How do I start an application under windows?

A: Simple. Just call ShellExecute, passing the full path of the file name you want to execute.

ShellExecute(this->m_hWnd,"open","calc.exe","","", SW_SHOW );

You can even pass parameters to the application as shown below :-

ShellExecute(this->m_hWnd,"open","notepad.exe",
    "c:\\MyLog.log","",SW_SHOW );

As you can see, I haven't passed the full path of the programs. ShellExecute will look into the PATH environment variable if you don't specify the full path.

Q: I want to open a particular document using the program associated with that document type.

A: You can pass the path of a document to ShellExecute and ShellExecute will open the file in it's associated program. Easy, huh?

ShellExecute(this->m_hWnd,"open",
    "c:\\abc.txt","","",SW_SHOW );

Q: I want to launch my home page in a browser window programmatically.

A: Boy! If you only knew how easy this is. Simply pass the URL to ShellExecute. I bet you have a caustic smile on your lips now as if to say, "Damn! I don't believe this."

ShellExecute(this->m_hWnd,"open",
    "http://www.google.com","","", SW_SHOW );

Q: How do I launch the compose window of the default email client with an address I specify as the To: address?

A: This time we pass a mailto link to ShellExecute.

ShellExecute(this->m_hWnd,"open",
    "mailto:nishinapp@yahoo.com","","", SW_SHOW );

Q: My boss asked me to print that license-agreement text file from the program.

A: Have no fear, ShellExecute is here. [I hope I am not over-doing this]

ShellExecute(this->m_hWnd,"print",
    "c:\\abc.txt","","", SW_HIDE);

Q: I want to bring up the Windows Find window on a particular folder.

A: We use the find verb as the operation parameter and we have the Windows Find window open up with the directory we have specified. This can be rather handy if you want to allow users to find some file within some folder. Just ask them for their folder and pop up a Find Window which has their folder as the root folder.

ShellExecute(m_hWnd,"find","d:\\nish",
    NULL,NULL,SW_SHOW);

Big Brother - ShellExecuteEx

ShellExecuteEx is a more flexible call, in that it allows us to retrieve information about the program we just spawned. You'll need to fill up the SHELLEXECUTEINFO structure and pass it's address to ShellExecuteEx. Please lookup both on your copy of MSDN.

Q: How do I start a program, and halt execution of my current program, till that program exits?

A: You start the program using ShellExecuteEx and use WaitForSingleObject on the process handle.

SHELLEXECUTEINFO ShExecInfo = {0};
ShExecInfo.cbSize = sizeof(SHELLEXECUTEINFO);
ShExecInfo.fMask = SEE_MASK_NOCLOSEPROCESS;
ShExecInfo.hwnd = NULL;
ShExecInfo.lpVerb = NULL;
ShExecInfo.lpFile = "c:\\MyProgram.exe";		
ShExecInfo.lpParameters = "";	
ShExecInfo.lpDirectory = NULL;
ShExecInfo.nShow = SW_SHOW;
ShExecInfo.hInstApp = NULL;	
ShellExecuteEx(&ShExecInfo);
WaitForSingleObject(ShExecInfo.hProcess,INFINITE);

Q: I want to show the File or Folder properties window for a file or a folder.

A: This time what we do is to pass properties as the operation verb. We also need to specify SEE_MASK_INVOKEIDLIST as the fmask parameter of the SHELLEXECUTEINFO structure. That's why we have to use ShellExecuteEx here instead of ShellExecute. I owe this tip to David Lowndes, Microsoft MVP because it was David who helped me with that tip about the SEE_MASK_INVOKEIDLIST flag.

SHELLEXECUTEINFO ShExecInfo ={0};
ShExecInfo.cbSize = sizeof(SHELLEXECUTEINFO);
ShExecInfo.fMask = SEE_MASK_INVOKEIDLIST ;
ShExecInfo.hwnd = NULL;
ShExecInfo.lpVerb = "properties";
ShExecInfo.lpFile = "c:\\"; //can be a file as well
ShExecInfo.lpParameters = ""; 
ShExecInfo.lpDirectory = NULL;
ShExecInfo.nShow = SW_SHOW;
ShExecInfo.hInstApp = NULL; 
ShellExecuteEx(&ShExecInfo);

CreateProcess - an ultra brief introduction

The CreateProcess function is part of Kernel32.dll. Windows uses this call to create a new process and a primary thread for the new process. The primary thread then starts executing the specified executable. Normally, if this is a C++  program, execution starts with your WinMain [actually prior to this the CRT library is loaded and initialized]. For a more comprehensive tutorial on the use of CreateProcess, I recommend that you read Joseph M Newcomer's article, An Introduction to Processes: Asynchronous Process Notification.

PROCESS_INFORMATION ProcessInfo; //This is what we get as an [out] parameter

STARTUPINFO StartupInfo; //This is an [in] parameter

ZeroMemory(&StartupInfo, sizeof(StartupInfo));
StartupInfo.cb = sizeof StartupInfo ; //Only compulsory field

if(CreateProcess("c:\\winnt\\notepad.exe", NULL, 
    NULL,NULL,FALSE,0,NULL,
    NULL,&StartupInfo,&ProcessInfo))
{ 
    WaitForSingleObject(ProcessInfo.hProcess,INFINITE);
    CloseHandle(ProcessInfo.hThread);
    CloseHandle(ProcessInfo.hProcess);
}  
else
{
    MessageBox("The process could not be started...");
}

As you can observe, I am using WaitForSingleObject on the process handle. But since CreateProcess creates a thread object in addition to the process object, I might as well have waited on the thread handle as in :-

WaitForSingleObject(ProcessInfo.hThread,INFINITE);

This might cause problems though if, for some reasons, one or more of your secondary threads are still active even after the main thread finishes. MSDN says that a process is fully terminated only when all it's threads have ceased execution. So I recommend that you wait on the process handle rather than on the thread handle.

Last Updates

• Aug 03 2002 - Added tip to bring up the Find window
• Aug 03 2002 - Added tip to bring up the Properties window