mfc, HOW TO redirect stderr to a file??? (freopen NOT supported)

microsoft.public.vc.mfc - Microsoft Visual C


mfc >> HOW TO redirect stderr to a file??? (freopen NOT supported)

by Fabio » Fri, 21 Nov 2003 18:09:28 GMT

Hi i'm doing a porting from an application console for win32
to pocketpc 2002, this applicancion have a lot of call to function
similar to those:

fprintf(stderr, "%s: Unknown error\n", msg);
................
................
fprintf(stderr, "dropping packet #%u\n", seqno);
...............

i want to preserve those functions and redirect stderr to a file but
pocketpc sdk and EVC30 doe not support the function "freopen"
anyone can soggest me a way to do that redirection?

Thanks



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mfc >> HOW TO redirect stderr to a file??? (freopen NOT supported)

by Joseph M. Newcomer » Sat, 22 Nov 2003 07:47:00 GMT


fclose(stderr);
FILE * f = fopen("whatever.txt", "w");
stderr = f;

At least that's how I used to do it decades ago on Unix, and I have no reason to expect
that Windows will run any differently.
joe




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mfc >> HOW TO redirect stderr to a file??? (freopen NOT supported)

by Doug Harrison [MVP] » Sun, 23 Nov 2003 03:27:00 GMT





That's obviously wrong for at least a couple of reasons:

1. There's no guarantee that stderr is an lvalue, and in fact it isn't in
VC.

2. You might be able to get away with *stderr = *f, but it's undefined, and
thus it's a gamble. In particular, you wouldn't want to fclose(stderr)
before overwriting *stderr, so if that's what you meant, well... However,
you would want to fflush(stderr) before overwriting *stderr and orphaning
the file handle, buffer (if any), etc. Note that it's really kind of a
library glitch that it's even possible to copy FILE objects, because the
standard doesn't define what makes up the FILE struct, and it doesn't define
copying them. To the user, a FILE* is in all respects an opaque pointer.
Quiz time: Why then is struct FILE completely defined by <stdio.h>? Why
doesn't <stdio.h> simply declare FILE like this:

struct FILE;

Every library API is defined in terms of FILE*, so why isn't this
sufficient?

Anyway, the proper way to do this on Unix and other OSes whose compilers
implement a Unix-like I/O library layer has always been to use dup/dup2, or
as MS calls them, _dup/_dup2, e.g.

// NB: From distant memory, error-checking omitted.

// Flush any pending output.
fflush(stderr);

// Open new file.
int fd = _open(...);

// dup a copy of stderr's file handle, because dup2 will close it.
int orig = _dup(_fileno(stderr));

// Associate fd with stderr.
_dup2(fd, _fileno(stderr));

// We no longer need the handle fd, so close it.
_close(fd);

// Now output through stderr goes to the file opened above.

// When done writing, flush the stream.
fflush(stderr);

// Restore the original stderr file handle.
_dup2(orig, _fileno(stderr));

// And finally close the copy we made.
_close(orig);

// Now stderr is once again associated with its original file handle.

--
Doug Harrison
Microsoft MVP - Visual C++

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HOW TO redirect stderr to a file??? (freopen NOT supported)

by Joseph M. Newcomer » Sun, 23 Nov 2003 18:00:19 GMT

OK, this is something I haven't done in a large number of years. I stand corrected.
joe





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