Scoping question in C

[Henry Spencer]

On Sun, 12 Jun 2005 pking123-rieW9WUcm8FFJ04o6PK0Fg at public.gmane.org wrote:
> The following code segfaults under Cygwin:
> ...
> char *x;
> int func ()
> {
>    *x = "something";

Yep, sure does.  But the original poster didn't put that `*' in the
assignment.  That one small change makes your code completely different
from his. 

He set `x' to point to the literal string.  Your version, well, is more
complicated.  It tries to set the character that `x' points to.  There
are two problems with this:

+ `x' has never been assigned a value, so where does it point?  In fact,
since it's a global variable, it gets implicitly initialized to a zero
of the appropriate type, in this case a null pointer.  But a null pointer
doesn't point anywhere!

+ The value being assigned to that character should be a character value
(that is, a single character).  What your code tries to assign is a
string, that is, a pointer to an initialized *array* of characters.  When
I give this code to GCC, it produces a warning message about that.  The
pointer is being converted to an integer, and the result is being treated
as a character for purposes of the assignment. 

C doesn't do array assignments, e.g. copying whole strings, with `='.
For that, you have to call strcpy().

Incidentally, the code as you showed it wouldn't even compile, since you
wrote "#indlude" instead of "#include" once.  This presumably means you
retyped it rather than doing cut-and-paste or reading it from a file into
your mail message.  That's a bad idea, because typos can make major
differences in how a program behaves; when asking about a programming
problem, it's very important to show *exactly* the code you tried to run. 

> When I compiled your source code with a slight modification: single 
> quotes around the %s...
> I got
> 'something
> '
> Note the \n came *before* the closing quote in the output but not in 
> the printf().

That's peculiar, but I suspect that another typo might be at fault.  I'm
not going to try to diagnose this one without seeing the exact code.

                                                          Henry Spencer
                                                       henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org



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