Bob Moore's Coding Tips

46. I can't make CreateProcess launch my process or handle its parameters. Why?

There are two main things that can go wrong when using CreateProcess. 

A. Unqualified App Names

The first problem is a failure to realise how CreateProcess behaves when you give an application name. Let's say I call CreateProcess like this :

BOOL bRet ;
STARTUPINFO sui ;
PROCESS_INFORMATION pi ;

sui.cb = sizeof (STARTUPINFO);
GetStartupInfo (&sui);
sui.dwFlags = STARTF_USESHOWWINDOW ;
sui.wShowWindow = SW_SHOW ;

bRet = CreateProcess ("myapp.exe",
                      NULL,
                      NULL, NULL, FALSE,
                      0, NULL, NULL,
                      &sui,
                      &pi);

Chances are that will fail. Why ? Because the application name is not fully qualified. If you simply use the EXE name, in the first parameter, CreateProcess requires that the EXE be fully qualified, i.e. it contain the full path to the EXE file. If, on the other hand, you called it like this :

bRet = CreateProcess (NULL,
                      "myapp.exe",
                      NULL, NULL, FALSE,
                      0, NULL, NULL,
                      &sui,
                      &pi);

That would probably work, because if the EXE name is given as part of the command line Windows will apply the same searching rules as if you had simply typed the name at the command prompt, i.e. it will search in the following places :

• The directory from which the application loaded.
• The current directory for the parent process.
• Windows 95/98: The Windows system directory.
  Windows NT/2000: The 32-bit Windows system directory (usual name System32).
• Windows NT/2000: The 16-bit Windows system directory (usual name System).
• The Windows directory. 
• The directories that are listed in the PATH environment variable

If you give a command line which contains spaces, you'll need to use quotes to make the actual command line unambiguous, because Windows takes the first space-delimited token on the command line to be the EXE name.

B. Faulty Command Line

The second thing that commonly goes wrong is when dealing with parameters. People forget that the second parameter to CreateProcess is the full command line, not just the parameters to the EXE. Lets take two examples :

CreateProcess ("c:\\notepad.exe",
               "c:\\notepad.exe c:\\wibble.txt",
               ...);

will work fine (if there is a copy of notepad.exe and a file called wibble.txt in the root of C:), whereas

CreateProcess ("c:\\notepad.exe",
               "c:\\wibble.txt",
               ...);

will launch the EXE but fail to open the file. What this means is that when the help systems calls the second parameter the command line, it ain't lying - it wants the whole command line. The annoying thing is that the help says

"Note that it is a common practice to repeat the module name as the first token in the command line".

I didn't notice a "required" anywhere in there....