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Re: why could not change the value of char*

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On 03/04/2014 02:46 PM, Edward A. Falk wrote:
> In article <ca781f3b-4de1-41ad-ae1c-71cc3cc5092b@googlegroups.com>,
> jack jack  <qzqsmile@gmail.com> wrote:
>> recently i learn the pointer of c.  wrote a programme below
>>
>>  5     char *s = "Hello world!";
>>  7     *s = 'h';
> 
> Any modern compiler/linker will treat the string "Hello world!"  as a
> constant and put it into read-only memory. Attempting to over-write the
> first byte will probably cause the system to throw a wobbly.
> 
> Frankly, I don't even know why the compiler lets you assign this string
> to a non-const variable, except perhaps that too much legacy code would
> break if the compiler forbade it.

A pointer to the string was used to initialize 's'; the only assignment
in that code was of a character, not a character string.

The fact that the compiler allowed the initialization might have to do
with the fact that the it doesn't violate any of C's rules. The
assignment of 'h' to *s violates 6.4.5p7, but that only specifies that
the behavior is undefined, no diagnostic is required.

The reason the initialization doesn't violate any rules is because
"Hello world!" has the type char[13], whereas it should properly have
the type "const char[13]". That decision was indeed made because 'const'
was a relatively late addition to the language, and by the time it was
added, there was indeed too much legacy code that would break. However,
that decision was made by the C committee, not by the implementor of
that particular compiler.

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why could not change the value of char* jack jack <qzqsmile@gmail.com> - 2014-03-01 00:12 -0800
  Re: why could not change the value of char* Ian Collins <ian-news@hotmail.com> - 2014-03-01 21:16 +1300
  Re: why could not change the value of char* Kaz Kylheku <kaz@kylheku.com> - 2014-03-01 08:42 +0000
    Re: why could not change the value of char* jack jack <qzqsmile@gmail.com> - 2014-03-01 01:03 -0800
  Re: why could not change the value of char* falk@rahul.net (Edward A. Falk) - 2014-03-04 19:46 +0000
    Re: why could not change the value of char* Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-04 15:09 -0500
    Re: why could not change the value of char* James Kuyper <jameskuyper@verizon.net> - 2014-03-04 15:18 -0500
  Re: why could not change the value of char* John Bode <jfbode1029@gmail.com> - 2014-03-07 11:14 -0800
    Re: why could not change the value of char* gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-07 19:29 +0000
      Re: why could not change the value of char* Ken Brody <kenbrody@spamcop.net> - 2014-03-07 14:55 -0500
      Re: why could not change the value of char* glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-07 20:46 +0000
        Re: why could not change the value of char* James Kuyper <jameskuyper@verizon.net> - 2014-03-07 16:12 -0500
          Re: why could not change the value of char* glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-07 21:49 +0000
        Re: why could not change the value of char* Kaz Kylheku <kaz@kylheku.com> - 2014-03-07 22:57 +0000
          Re: why could not change the value of char* anish kumar <yesanishhere@gmail.com> - 2014-03-07 15:05 -0800
            Re: why could not change the value of char* Ian Collins <ian-news@hotmail.com> - 2014-03-08 14:22 +1300
    Re: why could not change the value of char* Ken Brody <kenbrody@spamcop.net> - 2014-03-07 14:43 -0500
      Re: why could not change the value of char* gazelle@shell.xmission.com (Kenny McCormack) - 2014-03-07 19:52 +0000

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