Date: Tue, 10 Jul 2007 17:05:13 -0700 (PDT) From: youshi10@u.washington.edu To: current@freebsd.org Subject: Re: HEADS UP: getenv() and family API change Message-ID: <Pine.LNX.4.43.0707101705130.4029@hymn07.u.washington.edu> In-Reply-To: <20070710225752.GA3000@owl.midgard.homeip.net>
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On Wed, 11 Jul 2007, Erik Trulsson wrote:
> On Wed, Jul 11, 2007 at 02:46:19AM +0400, Andrey Chernov wrote:
>> On Wed, Jul 11, 2007 at 07:36:02AM +1000, Peter Jeremy wrote:
>>> On 2007-Jul-10 19:41:48 +0400, Andrey Chernov <ache@nagual.pp.ru> wrote:
>>> >To say strictly, copying somewhere is not neccessary since this way works
>>> >too:
>>> >
>>> >static char *s = "PATH=/bin";
>>> >
>>> >putenv(s);
>>>
>>> I thought the C compiler was still free to place the string into RO
>>> memory and/or coalesce it with other strings in that case.
>>>
>>> Wouldn't the following be clearer (s is forced to be writable):
>>>
>>> static char s[] = "PATH=/bin";
>>>
>>> putenv(s);
>>
>> This two are the same, since there is no "const", so compiler can't put
>> static char *s
>> into RO memory.
>
> Not the pointer, but the string it points to can be put into read-only
> memory.
>
> Example:
>
> static char *s = "PATH=/bin";
> static char *t = "PATH=/bin";
>
>
> Here both 's', and 't' can point into read-only memory where the string
> "PATH=/bin" has been placed. Not only that, they may point to the same
> place, i.e. there need only be one copy of the string "PATH=/bin" in
> the program (but there may be two distinct copies if the compiler does not
> coalesce identical string constants.)
>
>
> If on the other hand you use
>
> static char s[] = "PATH=/bin";
> static char t[] = "PATH=/bin";
>
>
> Then 's' and 't' are no longer pointers to a string constant, but arrays
> that are initialized with the string "PATH=/bin". These arrays are
> modifiable and distinct - i.e. there will be (at least) two copies of the
> string "PATH=/bin" in memory.
>
>
>
>
> --
> <Insert your favourite quote here.>
> Erik Trulsson
> ertr1013@student.uu.se
I'm confused what you're referring to as RO memory -- I thought that only const applied in this case:
#include <stdio.h>
int main () {
static char *s = "PATH=/bin";
s = "PATH=/sbin";
printf("%s\n", s);
return 0;
}
filc9175[409]% gcc -o try try.c
filc9175[410]% ./try
PATH=/sbin
Doesn't static (in terms of variables) only state that the memory address and values are not to be released to the heap again after the function scope exits?
-Garrett
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