Format String C++ in CreateProcess - c++

Question

ello all I hope someone can help me with this issue I am using the following example from msdn for createprocess function.
#include <windows.h>
#include <stdio.h>
#include <tchar.h>
void _tmain( int argc, TCHAR *argv[] )
{
STARTUPINFO si;
PROCESS_INFORMATION pi;
ZeroMemory( &si, sizeof(si) );
si.cb = sizeof(si);
ZeroMemory( &pi, sizeof(pi) );
if( argc != 2 )
{
printf("Usage: %s [cmdline]\n", argv[0]);
return;
}
// Start the child process.
if( !CreateProcess( NULL, // No module name (use command line)
argv[1], // Command line
NULL, // Process handle not inheritable
NULL, // Thread handle not inheritable
FALSE, // Set handle inheritance to FALSE
0, // No creation flags
NULL, // Use parent's environment block
NULL, // Use parent's starting directory
&si, // Pointer to STARTUPINFO structure
&pi ) // Pointer to PROCESS_INFORMATION structure
)
{
printf( "CreateProcess failed (%d).\n", GetLastError() );
return;
}
// Wait until child process exits.
WaitForSingleObject( pi.hProcess, INFINITE );
// Close process and thread handles.
CloseHandle( pi.hProcess );
CloseHandle( pi.hThread );
}
This is accessed via dos and works perfectly using this command in command prompt. When I type this
this.exe "my.exe test" > result.txt
The my.exe is another console application that takes the input test and the > result.txt is for my output log. I am trying to remove the need for the command line so I am trying to feed the path into the createprocess function call. This is where I get stuck here is what I am trying
if( !CreateProcess( NULL, // No module name (use command line)
"\"my.exe test \" > result.txt", // this.exe "my.exe test" > result.txt
NULL, // Process handle not inheritable
NULL, // Thread handle not inheritable
still doesnt work I thought \" would give me the result I needed but it doesnt seem to work, will parse the my.exe and test part but not the > result.txt output. However it works from the command prompt fine if i pass it to argv[1].
Any help much appreciated.
So in summary
In console I can parse
this.exe "my.exe test" > result.txt // Works fine via cmd.exe
To app I tried
my.exe test > result.txt // Not work but missing ""
and
\"my.exe test \" > result.txt // work for first section

Answer 1

CreateProcess only does basic command line parsing by breaking up the words into individual arguments -- it doesn't understand file redirection or anything. If you pass it "> result.txt", it tries to parse that as two arguments named literally ">" and "result.txt".
If you want to redirect the output of the command, you have two options:
Do the redirection yourself. To do that, you'd first open the file with CreateFile (passing in security attributes which make the handle inheritable), and then assign the resulting handle to the hStdOut member of the STARTUPINFO structure you pass in. Then, remember to close the file after CreateProcess returns, since otherwise you'd leak a handle.
Use another program to do the redirection. When you type in commands on the command line, it's cmd.exe which parses your command line and does the file redirection. So instead of creating a my.exe process, you could instead create a cmd.exe process with a command line something like this:
cmd.exe "my.exe test > result.txt"

Answer 2

CreateProcess only expects an executable name and some arguments. The redirection is actually not a program argument. That is interpreted by the shell (cmd.exe).
What is happening when you invoke your own program is as follows...
cmd> this.exe "my.exe test" > result.txt
argv[0] = "this.exe"
argv[1] = "my.exe test"
Output is sent to result.txt by the shell
Now, your one that doesn't work:
cmd> this.exe my.exe test > result.txt
argv[0] = "this.exe"
argv[1] = "my.exe"
argv[2] = "test"
Output is sent to result.txt by the shell
You'll see that because you only send argv[1] to CreateProcess, the behaviour is not as you expected.
Now, as I mentioned, CreateProcess does not actually redirect output. To do that, you should use the system call which invokes cmd.exe or whatever command interpreter the system uses:
system( "my.exe test > result.txt" );
See: http://msdn.microsoft.com/en-us/library/277bwbdz(v=vs.80).aspx