So here is what I have:
struct __attribute__((packed)) HackedHandlePasser {
using HANDLE32 = std::int32_t;
DWORD NumberOfHandles = 3; // 4 Byte
BYTE FlagsPerHandle[3]; // 3 * 1 Byte
HANDLE32 Handles[3]; // 3 * 4 Byte
};
static_assert(sizeof(HackedHandlePasser) == 19);
HackedHandlePasser handles;
#ifndef FOPEN
#define FOPEN 0x01
#endif
#ifndef FDEV
#define FDEV 0x40
#endif
handles.FlagsPerHandle[0] = 0;
std::memset(&handles.FlagsPerHandle[1], FOPEN | FDEV, 2);
const HANDLE invalid = INVALID_HANDLE_VALUE;
std::memcpy(&handles.Handles[0], &invalid, 4);
std::memcpy(&handles.Handles[1], &pipe.Write, 4);
std::memcpy(&handles.Handles[2], &pipe.Write, 4);
startInf.cbReserved2 = sizeof(HackedHandlePasser);
startInf.lpReserved2 = reinterpret_cast<LPBYTE>(&handles);
Some explanations:
* I have the HANDLE32 because I start a 32 bit application from within a 64 bit,
as far as I understood the structure needs the right HANDLE size, thus for a 64
bit application it should be std:int64_t, but since I don't need that I never
tested it.
* In cbReserved2 there is the size of the data in lpReserved2.
* The data in lpReserved2 starts with 2 bytes which indicate how many handles
are passed to the spawned process (they may be INVALID_HANDLE_VALUE). Followed
by one byte per handle with the opened flags - and here comes the trick: If it
has the flag FDEV the ms runtime printf assumes it writes to a console and thus
flushes. And then followed by the handles.
* My use case needs only stdout (and I also open stderr, in my case to the same
handle), these are the handles number 1 and 2. Number 0 would be stdin.
So when using it with QProcess I think one should copy the handles from the
STARTUPINFOW structure into this struct, then I think the normal QIODevice
interface should keep working. If one should clear the STARTF_USESTDHANDLES flag
I also don't know. I stopped my experiments when I achieved success for my
"simple" use case.
Let me know what works for you.
And last but not least some sources:
https://stackoverflow.com/questions/40965496/windows-how-to-stop-buffering-of-redirected-stdout-using-createprocess
which apparently now also has a C/C++ answer, when I needed it there was only
the python code.
http://www.catch22.net/tuts/undocumented-createprocess does use the fields for
some other stuff, but explains a bit of background.
https://github.com/cansou/msvcrt/blob/master/src/isatty.c One if the involved
files to handle the input.
Kind Regards,
Björn.
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