linux-kernel - Re: [PATCH v5 10/12] string: factorize skip_spaces and export it to be generally available

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Date:	Mon, 16 Nov 2009 10:21:48 -0200
From:	André Goddard Rosa <andre.goddard@...il.com>
To:	Anders Larsen <al@...rsen.net>
Cc:	Anonymous <aim0shei@...abit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 10/12] string: factorize skip_spaces and export it to 
	be generally available

Hi, Anders!

On Sun, Nov 15, 2009 at 7:51 PM, Anders Larsen <al@...rsen.net> wrote:
> On 2009-11-15 21:02:15, Anonymous wrote:
>>
>> On Sun, Nov 15, 2009 at 08:33:29PM +0100, Anders Larsen wrote:
>> > On 2009-11-15 08:15:03, André Goddard Rosa wrote:
>> > >+char *skip_spaces(const char *str)
>> > >+{
>> > >+    while (isspace(*str))
>> > >+            ++str;
>> > >+    return (char *)str;
>> > >+}
>> >> Is there a good reason why the parameter 'str' is declared 'const'
>> > when skip_spaces() returns a non-const pointer into str ?
>> >Declaring return value as const won't let us modificate string after
>> skipping spaces. Declaring parameter as non-const won't let us
>> giving (const char *) to this function. So i think it is ok.
>
> skip_spaces() _implicitly_ casts away the 'const' of the parameter,
> which may come as a (nasty) surprise to users of the function.
>
> Consider this (contrieved and buggy) example:
>
>        const char* my_string = " do not modify me! ";
>        char* result = strcat(skip_spaces(my_string, "boom!"));
>
> The proposed implementation of skip_spaces() effectively prevents
> the compiler from catching this obvious bug.
>

I agree with the above comment. It's possible to make it differently
char* skip_spaces(char *),
but it's necessary to change many of its callers at the same time:

CC      lib/argv_split.o
lib/argv_split.c: In function ‘count_argc’:
lib/argv_split.c:24: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’
lib/argv_split.c: In function ‘argv_split’:
lib/argv_split.c:78: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’
...
  CC      lib/vsprintf.o
lib/vsprintf.c: In function ‘vsscanf’:
lib/vsprintf.c:1737: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’
lib/vsprintf.c:1738: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’
lib/vsprintf.c:1808: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’
lib/vsprintf.c:1850: warning: passing argument 1 of ‘skip_spaces’
discards qualifiers from pointer target type
include/linux/string.h:65: note: expected ‘char *’ but argument is of
type ‘const char *’

    There are other functions following the same pattern presently:

        char *strstr(const char *haystack, const char *needle);
        char *strchr(const char *s, int c);
        char *strrchr(const char *s, int c);

    on both glibc and the kernel.

Thanks,
André
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