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Re: [Bug-wget] [PATCH] fix warning and possible invalid free

From: Micah Cowan
Subject: Re: [Bug-wget] [PATCH] fix warning and possible invalid free
Date: Fri, 13 Apr 2012 12:46:09 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120302 Thunderbird/11.0

On 04/13/2012 01:44 AM, Tim Ruehsen wrote:
> Am Thursday 12 April 2012 schrieb Micah Cowan:
>> On 04/12/2012 01:23 AM, TeeRasen wrote:
>>> In main.c we have
>>>           opt.progress_type = "dot";
>> In C, a string literal is of type char[] (which automatically transforms
>> to char*), not const char[] or const char* (even though one must still
>> not modify it. You're either compiling with C++ (a bad idea for wget
>> code), or a nonstandard C invocation that makes string literals out to
>> be const. (Though, a C implementation is allowed to "warn" about
>> whatever it wishes, provided it still behaves properly, I suppose.)
> Maybe, my posting was too long to read...
> The main message was that free("dot") crashes, since literals are read-only. 

Right; but free("dot") crashing has nothing to do with literals being
read-only, and everything to do with literals not having been malloc()'d.

> Example (compile with gcc x.c -o x):
> #include <stdlib.h>
> void main(void)
> {
>         char *buf="dot";
>         free(buf);
> }
> free crashes though gcc did not complain !
> To make gcc check for these potential crashes, use -Wwrite-strings.

-Wwrite-strings doesn't do anything at all to indicate the real problem,
which is the free(). If you change "char *buf" to "const char *buf",
-Wwrite-strings is satisfied, and yet the free() is still just as
dangerous as ever.

If opt.progress_type were const char * (which would probably not be a
terrible idea), the bug would still remain, and -Wwrite-strings wouldn't
say anything about it. So I was just trying to point out that the
warning you mentioned never had anything to do with the real bug, and
doesn't actually indicate a problem (there's nothing wrong with
assigning literals to non-const pointers, provided you never actually
try to modify them). Perhaps it's tangential to the main point of your
email, but I thought it was worth pointing out.


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