Subj : Re: How to deal with errno in a multipthread program
To   : comp.programming.threads
From : Eric Sosman
Date : Fri Apr 15 2005 02:40 pm



icoming wrote:
> So I even do not need to save errno in a local variable?

    (Context: Is `int e = errno;  f();  errno = e;'
required in threaded programs?)

    You need to save errno if you want to preserve its
value while doing something that might change it.  If
you don't need to preserve the value, you don't need
to save it.

	FILE *old, *new;
	old = fopen("old.txt", "r");
	if (old == NULL)
	    return FAILURE;
	new = fopen("new.txt", "w");
	if (new == NULL) {
	    fclose (old);
	    return FAILURE;
	}

If the FAILURE return is all you care about, this is
good enough.  If you want to return FAILURE *and* let
the caller inspect errno to get more information about
the reason for the failure, this may not work because
fclose(old) may change errno and thus obliterate any
information from the failed fopen().  If you want to
preserve that information, write

	if (new == NULL) {
	    int e = errno;
	    fclose (old);
	    errno = e;
	    return FAILURE;
	}

    The decision to preserve errno or not has nothing to
do with whether the program is single- or multi-threaded;
it's a matter of the "contract" between the function
and its callers.

-- 
Eric.Sosman@sun.com

.