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Re: string literals vs char*

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Barry Schwarz <schwarzb@dqel.com> writes:
> On 27 Dec 2014 05:37:56 GMT, arnuld <sunrise@invalid.address> wrote:
>>C90 requires that string literls are constant, then why I am not getting 
>>warning here. Is this a compiler problem ?  (if it is a compiler problem 
>
> There is a difference between constant and const.  String literals
> have always been considered constant (modifying one causes undefined
> behavior).  However, for historical reasons (not breaking really old
> code) they are not const.  Hence no diagnostic.  You are however free
> to define the pointer properly as 
>      const char *p1;
> which would have no effect on the operation of your program.

All this applies to C90, C99, and C11.

Yes, there's a difference between constant and const, but you haven't
stated it correctly.

A *constant expression* is one that can be (and in some cases must be)
evaluated at compile time.  (And a "constant", used as a noun, is a
token in source code, referred to in some languages as a "literal".)

The "const" keyword, on the other hand, means read-only.

String literals are read-only, but as you say they're not made "const"
for historical reasons.  Attempting to modifying a string literal has
undefined beahvior, but you can't count on the compiler to diagnose the
attempt.

Some pre-ANSI C compilers, IIRC, actually allowed you to modify string
literals, so that this:

    char *s = "hello";
    s[0] = 'H';
    puts(s);

would actually print "Hello".  (It might also affect other uses of the
same string literal.)  And in fact a conforming C11 compiler could do
the same thing; the behavior is undefined, so it can do anything it
likes.

>>then why g++ catches the error at compile time when gcc does not):

Because g++ is a C++ compiler, and in C++ string literals are const.

> I don't use g++ but based on the text of the diagnostic, it appears
> that g++ treats the literals as const due to the specified option
> being in effect.  If you compiled the program without that option, I
> would expect the diagnostic to disappear.
>
>>[arnuld@arch64 c $] cat test.c
>>#include<stdio.h>
>>
>>int main(void)
>>{
>>    char *p1 = "India", *p2;
>>    p2=p1;
>>    p1 = "BIX";
>>    printf("%s %s\n", p1, p2);
>>    return 0;
>>}
>>
>>================== OUTPUT =======================
>>[arnuld@arch64 c $] gcc -ansi -pedantic -Wall -Wextra test.c
>>[arnuld@arch64 c $] ./a.out 
>>BIX India
>>
>>[arnuld@arch64 c $] g++ -ansi -pedantic -Wall -Wextra test.c
>>test.c: In function ‘int main()’:
>>test.c:5:16: warning: deprecated conversion from string constant to 
>>‘char*’ [-Wwrite-strings]
>>     char *p1 = "India", *p2;
>>                ^
>>test.c:7:8: warning: deprecated conversion from string constant to 
>>‘char*’ [-Wwrite-strings]
>>     p1 = "BIX";
>>        ^
>>[arnuld@arch64 c $] 

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

string literals vs char* arnuld <sunrise@invalid.address> - 2014-12-27 05:37 +0000
  Re: string literals vs char* Kaz Kylheku <kaz@kylheku.com> - 2014-12-27 06:36 +0000
  Re: string literals vs char* Barry Schwarz <schwarzb@dqel.com> - 2014-12-27 00:58 -0800
    Re: string literals vs char* Ian Collins <ian-news@hotmail.com> - 2014-12-27 22:03 +1300
    Re: string literals vs char* Keith Thompson <kst-u@mib.org> - 2014-12-27 11:55 -0800
      Re: string literals vs char* glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-28 07:55 +0000
        Re: string literals vs char* Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-28 11:39 +0000
          Re: string literals vs char* Tim Rentsch <txr@alumni.caltech.edu> - 2015-03-24 08:55 -0700
            Re: string literals vs char* Keith Thompson <kst-u@mib.org> - 2015-03-24 12:17 -0700
  Re: string literals vs char* James Kuyper <jameskuyper@verizon.net> - 2014-12-27 11:36 -0500
    Re: string literals vs char* Vincenzo Mercuri <nomail@yahoo.it> - 2014-12-27 18:20 +0100
      Re: string literals vs char* Vincenzo Mercuri <nomail@yahoo.it> - 2014-12-27 18:55 +0100
    Re: string literals vs char* Kaz Kylheku <kaz@kylheku.com> - 2014-12-27 17:25 +0000
    Re: string literals vs char* Keith Thompson <kst-u@mib.org> - 2014-12-27 12:08 -0800
    Re: string literals vs char* James Kuyper <jameskuyper@verizon.net> - 2014-12-28 07:01 -0500
  Re: string literals vs char* John Bode <jfbode1029@gmail.com> - 2014-12-30 10:52 -0800

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