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

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.

CodePudding user response：

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.