Read lines from a file opened with CreateFile [duplicate] - c++

Question

In Unix, if you have a file descriptor (e.g. from a socket, pipe, or inherited from your parent process), you can open a buffered I/O FILE* stream on it with fdopen(3).
Is there an equivalent on Windows for HANDLEs? If you have a HANDLE that was inherited from your parent process (different from stdin, stdout, or stderr) or a pipe from CreatePipe, is it possible to get a buffered FILE* stream from it? MSDN does document _fdopen, but that works with integer file descriptors returned by _open, not generic HANDLEs.

Answer 1

Unfortunately, HANDLEs are completely different beasts from FILE*s and file descriptors. The CRT ultimately handles files in terms of HANDLEs and associates those HANDLEs to a file descriptor. Those file descriptors in turn backs the structure pointer by FILE*.
Fortunately, there is a section on this MSDN page that describes functions that "provide a way to change the representation of the file between a FILE structure, a file descriptor, and a Win32 file handle":
_fdopen, _wfdopen: Associates a stream with a file that was
previously opened for low-level I/O and returns a pointer to the open
stream.
_fileno: Gets the file descriptor associated with a stream.
_get_osfhandle: Return operating-system file handle associated
with existing C run-time file descriptor
_open_osfhandle: Associates C run-time file descriptor with an
existing operating-system file handle.
Looks like what you need is _open_osfhandle followed by _fdopen to obtain a FILE* from a HANDLE.
Here's an example involving HANDLEs obtained from CreateFile(). When I tested it, it shows the first 255 characters of the file "test.txt" and appends " --- Hello World! --- " at the end of the file:
#include <windows.h>
#include <io.h>
#include <fcntl.h>
#include <cstdio>
int main()
{
HANDLE h = CreateFile("test.txt", GENERIC_READ | GENERIC_WRITE, 0, 0,
OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
if(h != INVALID_HANDLE_VALUE)
{
int fd = _open_osfhandle((intptr_t)h, _O_APPEND | _O_RDONLY);
if(fd != -1)
{
FILE* f = _fdopen(fd, "a+");
if(f != 0)
{
char rbuffer[256];
memset(rbuffer, 0, 256);
fread(rbuffer, 1, 255, f);
printf("read: %s\n", rbuffer);
fseek(f, 0, SEEK_CUR); // Switch from read to write
const char* wbuffer = " --- Hello World! --- \n";
fwrite(wbuffer, 1, strlen(wbuffer), f);
fclose(f); // Also calls _close()
}
else
{
_close(fd); // Also calls CloseHandle()
}
}
else
{
CloseHandle(h);
}
}
}
This should work for pipes as well.

Answer 2

Here is a more elegant way of doing this instead of CreateFile: specify "N" in fopen(). It's a Microsoft-specific extension to fopen, but since this code is platform-specific anyway, it's ok. When called with "N", fopen adds _O_NOINHERIT flag when calling _open internally.
Based on this:
Windows C Run-Time _close(fd) not closing file