Ben Pfaff <blp@cs.stanford.edu> writes:
> CBFalconer <cbfalconer@yahoo.com> writes:
>> int fcopy(char const *dst, char const *src) {
>> FILE *sf, *df;
>> int ch
>>
>> if (sf = fopen(src, "r") { /* text files */
>> if (df = fopen(dst, "w") {
>> while (EOF != (ch = getc(sf))) putc(ch, df);
>> fclose(df);
>> }
>> fclose(sf);
>> }
>> return sf && df; /* non-zero for success */
>
> I think that this is a bad idea: it is a lot like checking the
> value of a pointer after it has been freed (often fclose will
> actually call free on the stream, in fact). I'm sure it works in
> practice on almost every implementation though.
>
>> }

If the first fopen() fails, no value is assigned to df, and accessing
it invokes undefined behavior.

In addition, I think you're write; though fclose() cannot change the
value of of its argument, it can cause it to become indeterminate.
(In practice, it will probably appear to be be null or non-null after
fclose() if it was, respectively, null or non-null before fclose().)

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"