Question

Background Info: Windows 7, Visual C++ 2010 Express

Problem: CreateProcess() keeps returning with ‘Invalid command line argument’

Explanation: I’m writing a piece of code that calls external programs using the Windows API’s CreateProcess. I’ve gotten the call to work with one external program so far:

if( !CreateProcess( "C:\\Temp\\convert.exe",
    t_str,        // Arguments
    ...
}
//where t_str is " C:\\img1.jpeg C:\\img1.pgm" (ImageMagick if you're wondering). 

This works perfectly even with the amount of data-mangling I did to push everything into the Windows strings and pointers. So I copied all of the mangling for CreateProcess() for another call to a different external program:

 if( !CreateProcess( "C:\\Temp\\sift.exe",
     t_str2,        // Arguments
     ...
 }
 //where t_str2 is ` < C:\\img1.pgm > C:\\img1.key`

Basically, something very similar, but with all the variable names changed (since I have these two calls running serial). And this is where the problem is; this doesn’t run, and, instead, prints out “Invalid command line argument: < C:\img1.pgm”. Of course, this command works fine in the command prompt, but not in my code.

I switched t_str2 to something else a little less complicated(since I know how sift.exe works) and I get the same result. Same thing happens when I run just sift and not convert.

Question: What could be causing this problem? What could I do to further debug this problem? Any suggestions on alternatives to the methods I’m using? Any help is appreciated. I can provide further code, but It’s pretty straight forward, and not much could go wrong with it.

Answer 1

You cannot use command-line redirection operators with CreateProcess() directly. You have to spawn an instance of cmd.exe and pass the operators to it instead, eg:

CreateProcess( "C:\\windows\\system32\\cmd.exe", t_str2, ...))

Where t_str2 is "/C C:\\Temp\\sift.exe < C:\\img1.pgm > C:\\img1.key". The actual path to cmd.exe can be determined by reading the %COMSPEC% environment variable.