Date: Tue, 10 Jul 2007 18:21:33 -0700 (PDT) From: youshi10@u.washington.edu To: current@freebsd.org Subject: Re: HEADS UP: getenv() and family API change Message-ID: <Pine.LNX.4.43.0707101821330.4029@hymn07.u.washington.edu> In-Reply-To: <94B3BCF3-D490-48A5-8488-2E931E30C07B@mac.com>
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On Tue, 10 Jul 2007, Chuck Swiger wrote:
> On Jul 10, 2007, at 5:05 PM, youshi10@u.washington.edu wrote:
>> On Wed, 11 Jul 2007, Erik Trulsson wrote:
>>> 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:
>
> It means that the string can be placed in a section of the executable file
> which is loaded with read-only memory protection set so that attempts to write
> to it fail; ELF calls this .rodata in contrast to the normal .bss and .data
> sections which contain (respectively) zero-filled memory and preinitialized but
> writable data.
Ok, thanks for the explanation :).
>> #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?
>
> static implies that the variables are held in memory address space which is
> made permanently available, either from pages mapped in from the executable
> file (that's the .data segment) or from zero-filled heap pages (.bss), rather
> than going away after the enclosing function has returned. The default
> automatic scope actually typically uses the stack, not the heap...
>
> --
> -Chuck
Ok :).
-Garrett
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