strdup(NULL) supposed to create SIGSEGV?

[Mike Meyer]

On Wed, 23 Apr 2008 10:30:39 +0200 Bernard van Gastel <bvgastel at bitpowder.com> wrote:

> Op 23 apr 2008, om 08:50 heeft Mike Meyer het volgende geschreven:
> > On Tue, 22 Apr 2008 22:40:21 -0700
> > "Garrett Cooper" <yanefbsd at gmail.com> wrote:
> >
> >> Hi all,
> >>     I made an oops in a program, which uncovered "feature" in  
> >> strdup(2)
> >> that I wasn't aware of before. So I was wondering, is  
> >> strdup(pointer = NULL)
> >> supposed to segfault should this just return NULL and set errno?
> >
> > Yes, it's supposed to segfault. Check out what, say, strcpy does if
> > you ask it to copy a NULL pointer. And this is an improvement from the
> > bad old days, when they would happily walk through memory starting at
> > 0.....
> 
> I don't like it this way. I would like:
> 
> strdup(NULL) = NULL
> strdup(string) = copy of string
> 
> strcpy(NULL, NULL) = NULL
> strcpy(s1, NULL) = ERROR
> strcpy(NULL, s2) = NULL (with s2 unchanged)
> strcpy(s1, s2) = normal
> 
> But I am not sure of the implications. Maybe in some situation it is  
> bad... Anyone?

I think someone gave the reason I'm about to: trying to copy a NULL
pointer means I have a bug somewhere earlier in my code that will
eventually produce visibly wrong results - a segfault being such. The
sooner that happens after the bug, the less code I have to search to
find it, the better for me.

So quietly propagating the error is bad in general.

Actually, I'd like to reverse the question: under what conditions
would you be trying to copy a string where not having a string isn't
a sign that something is broken?

> > Besides, errno is used to signal errors from system calls. strdup
> > isn't a system call, it's a library function (says so at the top of
> > the man page).
> 
> But strdup uses malloc, which is a system call (from the strdup  
> manual: If insufficient memory is available, NULL is returned and  
> errno is set to ENOMEM.)

As others have pointed out, malloc isn't a system call. However, what
strdup (and malloc) are doing in this case is passing the system error
from what is eventually an internal system call out to their
caller. Basically, errno being set means some system call went wrong,
even if it was made deep in the heart of a library somewhere.

     <mike


-- 
Mike Meyer <mwm at mired.org>		http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.