On Thu, Jul 13, 2000 at 10:51:41AM -0600, William Astle wrote:
> On Thu, 13 Jul 2000, Andi Kleen wrote:
>
> > The original poster passed it a NULL pointer though, not a random pointer.
> > It would be certainly reasonable to check for NULL pointer in fclose,
> > similar to free()
>
> It seems to me that the argument is about
>
> fclose(fp);
> fclose(fp);
No it wasn't. The original example was:
/*--------------------------------------------*/
#include <stdio.h>
FILE *new;
main () {
fclose (new); /* <---- SIGSEGV on linux only */
}
/*--------------------------------------------*/
There is a common and useful[1] programming style where you try to close
all resources in case of an error in a common function. It is useful to
not have to write if (ptr != NULL) closeit(ptr) all the time to handle
the case where you bombed out in the middle.
The alternative is the error prone and ugly:
if (a = alloca()) {
if (b = allocb()) {
if (c = allocc()) {
if (d =allocd()) {
do_real_work();
freed(d);
}
freec(c);
}
freeb(b);
}
freea(a);
}
which is IMHO just asking for obscure bugs in error paths.
/* a b c d are NULL */
a = alloca();
b = allocb();
c = allocc();
d = allocd();
if (a && b && c && d)
do_real_work();
freea(a);
freeb(b);
freec(c);
freed(d);
Which one do you prefer ?
So handling NULL in the low level library free routine makes it much
less likely that you screw up during resource cleanups. Writing
if (ptr != NULL) before free all the time is just redundant and can
be easily forgotten.
About the only argument I see is to make one undefined behaviour in ISO
C crash instead of behave sanely to point out a potential portability
problem to some hosts (e.g. most Unixes seem to get this correct) early.
I prefer a sane behaviour instead though.
-Andi
[1] It simplifies error handling and makes it more likely to be correct.
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