Launch command line window in hidden mode from a C++ code and read output log file

Question

On windows machine, My code runs ShellExecute to launch abc.exe with SW_HIDE as the last argument to hide the launched command line window. This runs fine as desired.

std::string torun = "/c abc.exe >abc.log";
HINSTANCE retVal = ShellExecute(NULL, _T("open"), _T("cmd"), std::wstring(torun.begin(), torun.end()).c_str(), NULL, SW_HIDE);

My issue is that abc.log which captures the output of abc.exe is not accessible even though it is written successfully. Code below returns "file does not exist".

std::string filename = "abc.log";
if (_access(filename.c_str(), 0) == -1)
    std::cout<<"file does not exist";
else
    std::cout<<"file exists";

I need to read the content of this log file after checking that it exists. Code below also returns "file does not exist".

ifstream fin;
fin.open("abc.log");
if(fin)
    std::cout<<"file exists";
else
    std::cout<<"file does not exist";

Is there some permission issue? I have two needs - launch abc.exe on a command line window which is hidden or runs behind all open windows and also be able to read the abc.log. Appreciate the help.

Show a full program, not snippets. Your last example is examining some variable named `ifile` instead of `fin`, but maybe that's a typo. All the more reason to show an actual program that you compiled, ran and reproduced the behavior. Crucially, it's currently unclear _when_ you try to read the output file. Be aware that `ShellExecute` launches a process asynchronously and does _not_ wait for that process to complete. See [How to wait for ShellExecute to run?](https://stackoverflow.com/questions/17638674/how-to-wait-for-shellexecute-to-run) — paddy, Aug 04 '22 at 05:19

score 0 · Accepted Answer · answered Aug 04 '22 at 10:50

You need to wait for the child process to finish:

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <tchar.h>   
#include <shellapi.h>
...

SHELLEXECUTEINFO sei = { sizeof(sei), SEE_MASK_NOCLOSEPROCESS|SEE_MASK_FLAG_NO_UI };
sei.lpFile = TEXT("cmd.exe");
sei.lpParameters = TEXT("/C ping localhost > abc.log"); // Command that takes a while to complete
sei.nShow = SW_HIDE;
if (ShellExecuteEx(&sei))
{
    if (sei.hProcess)
    {
        WaitForSingleObject(sei.hProcess, INFINITE);
        CloseHandle(sei.hProcess);
        FILE*file = fopen("abc.log", "r");
        if (file)
        {
            char buf[1337];
            size_t cb = fread(buf, 1, sizeof(buf), file);
            if (cb)
            {
                buf[min(cb, sizeof(buf)-1)] = '\0';
                MessageBoxA(NULL, buf, "stdout started with", 0);
            }
            fclose(file);
        }
    }
}

This is still a bit lazy, the optimal way to read stdout from a child process is to call CreateProcess with pipes.

Launch command line window in hidden mode from a C++ code and read output log file

1 Answers1