Re: opening a non-seekable device file
Ulrich Eckhardt wrote:
How should this code behave?
ofstream out1("/dev/null");
ofstream out2("/dev/null", ios_base::app);
Implementation defined. Both fail under Windows, for example.
The first says that it should open the file and truncate it, i.e. discard
its content. The second says it should leave the content alone and seek to
the end to append there. Since the device file (or a pipe/socket) doesn't
have any content both truncating and appending make little sense, but my
gut feeling is that at least one of them should succeed, or maybe even
both.
I suspect (without checking) that 1) only Unix and Unix
look-alikes will support this, and 2) both calls will map
directly to calls to Posix open, O_WRONLY | O_CREAT | O_TRUNC in
the first case, and O_WRONLY | O_CREAT | O_APPEND in the second.
With the results of filebuf::open reflecting those of the Posix
open. Thus, for an exact response, you really should ask in a
Unix group---at first view, however, I think that the second
should work everywhere; as for the first: "[O_TRUNC] shall have
no effect on FIFO special files or terminal device files. Its
effect on other file types is implementation-defined." (If I
were writing the kernel, I'd treat all character special files
the same: no effect. But in fact, I'm not the author of any of
the Unix kernels around.)
Note that the Solaris documentation is just a copy of the Posix
standard, so you don't know any more, and Linux says that it is
unspecified (which I interpret to mean that it might change from
one version to the next). In practice, I suspect that there is
more than a little code out there which uses the first form in
certain cases (both forms work under Solaris and Linux, at least
the versions I have at hand), and that the kernel distributers
will avoid breaking it. I'd say that having both work is
consistent with the intent of the flags as well: the intent of
the first is that you start with an empty file (which will
always be the case for "/dev/null"), and the intent of the
second is that all writes will be to the end of the file (which
is also always the case for "/dev/null").
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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