21 Sep
2017
21 Sep
'17
5:46 p.m.
On Tue, Sep 19, 2017, at 10:42, Larry McVoy wrote:
slib.c:1653 (bk-7.3): open failed: permission denied
which is way way way more useful than just permission denied.
Random832 replied:
Well. It's less useful in one way - it doesn't say what file it was
trying to open. You could pass the filename *instead* of "open failed",
but that still omits the issue I had pointed out: what were you trying
to open the file for (at the very least, were you trying to read, write,
or exec it). Ideally the function would have a format and arguments.
====
Exactly.
The string interpretation of errno is just another
item of data that goes in an error message. There is
no fixed place it belongs, and it doesn't always
belong there, because all that is error does not
fail from a syscall (or library routine).
I do often insert a function of the form
void errmsg(char *, ...)
in my C programs. It takes printf-like arguments.
Normally they just get passed to vfprintf(stderr, ...),
though sometimes there is something more esoteric,
and often fprintf(stderr, "%s: ", progname) ends up
in front.
But errmsg never knows anything about errno. Why
should it? It's supposed to send complaints to
a standard place; it's not supposed to invent the
complaints for itself! If an errno is involved,
I write something like
errmsg("%s: cannot open: %s", filename, strerror(errno));
(Oh, yes, errmsg appends a newline too. The idea
is to avoid cluttering code with minutiae of how
errors are reported.)
I don't print the source code filename or line number
except for `this shouldn't have happened' errors.
For routine events like the user gave the wrong
filename or it had the wrong permissions or his
data are malformed, pointers to the source code are
just unhelpful clutter, like the complicated
%JARGON-OBSCURE-ABBREVIATION prefixes that accompanied
every official error message in VMS.
Of course, if the user's data are malformed, he
should be told which file has the problem and
where in the file. But that's different from
telling him that line 193 of some file he doesn't
have and will probably never see contains the system
call that reported that he typed the wrong filename.
Norman Wilson
Toronto ON