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Re: double strtod( const char *nPtr, char **endPrt);

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Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
> On Thursday, November 12, 2015 at 6:55:52 AM UTC, G G wrote:
>> double strtod( const char *nPtr, char **endPrt );
>> (and functions like it)
>> 
>> nPtr is a pointer to a character constant
>> 
>> endPtr is a pointer to a pointer to a character or
>> endPtr is a pointer to a pointer of type character?
>>  
>> 
>> nPtr is not changeable. i mean what nPtr points
>> to is not changeable.
>> 
>> endPtr is a pointer which points to another to a character 
>> that is allowed to change?
>> 
> Consider this.
>
>
> char line[1024]; /* modifiable */
> const char * end;
> double x;
>
> fgets(line, 1024, stdin);
> x = strtod(line, &end);
> /* we want to null terminate the numerical part of line */
> if(end != line)
>   *end = 0; 
> /* we can't do that if end is a const char * */
>
> It illustrates the problems with const. You could define strtod_const
> and strtod_variable, but the language ends up talking to itself.

You shouldn't have the "const" on the declaration of "end".

It's true that strtod can give you a pointer to non-const that points
into a read-only string.  Another illustration of this:

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    const char *line = "1.23xyz";
    char *end;
    double x = strtod(line, &end);
    // x == 1.23, end points to the 'x'
    *end = '\0'; // kaboom
}

Several other functions, including strstr() and strchr(), have the same
problem.

(C++ has addressed this, at least in some cases, by providing
const-consistent overloaded versions.)

IMHO the best solution would have been to provide two different versions
of each of these functions, one for const data and one for non-const
data.

-- 
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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double strtod( const char *nPtr, char **endPrt); G G <gdotone@gmail.com> - 2015-11-11 22:55 -0800
  Re: double strtod( const char *nPtr, char **endPrt); Richard Heathfield <rjh@cpax.org.uk> - 2015-11-12 07:24 +0000
    Re: double strtod( const char *nPtr, char **endPrt); G G <gdotone@gmail.com> - 2015-11-11 23:42 -0800
  Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 08:31 -0800
    Re: double strtod( const char *nPtr, char **endPrt); supercat@casperkitty.com - 2015-11-12 08:52 -0800
    Re: double strtod( const char *nPtr, char **endPrt); G G <gdotone@gmail.com> - 2015-11-12 09:43 -0800
      Re: double strtod( const char *nPtr, char **endPrt); Richard Heathfield <rjh@cpax.org.uk> - 2015-11-12 17:59 +0000
      Re: double strtod( const char *nPtr, char **endPrt); Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-12 13:10 -0500
      Re: double strtod( const char *nPtr, char **endPrt); James Kuyper <jameskuyper@verizon.net> - 2015-11-12 13:19 -0500
      Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 11:32 -0800
        Re: double strtod( const char *nPtr, char **endPrt); supercat@casperkitty.com - 2015-11-12 12:17 -0800
          Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 12:48 -0800
            Re: double strtod( const char *nPtr, char **endPrt); supercat@casperkitty.com - 2015-11-12 13:39 -0800
              Re: double strtod( const char *nPtr, char **endPrt); glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-12 23:24 +0000
              Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 17:14 -0800
                Re: double strtod( const char *nPtr, char **endPrt); supercat@casperkitty.com - 2015-11-12 17:42 -0800
                Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 18:20 -0800
            Re: double strtod( const char *nPtr, char **endPrt); glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-12 23:15 +0000
              Re: double strtod( const char *nPtr, char **endPrt); Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-12 18:30 -0500
                Re: double strtod( const char *nPtr, char **endPrt); supercat@casperkitty.com - 2015-11-12 16:47 -0800
                Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 18:07 -0800
                Re: double strtod( const char *nPtr, char **endPrt); glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-13 02:41 +0000
                Re: double strtod( const char *nPtr, char **endPrt); glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-13 02:53 +0000
        Re: double strtod( const char *nPtr, char **endPrt); G G <gdotone@gmail.com> - 2015-11-12 12:17 -0800
  Re: double strtod( const char *nPtr, char **endPrt); Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-12 12:34 -0800
    Re: double strtod( const char *nPtr, char **endPrt); Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-12 23:54 +0000
      Re: double strtod( const char *nPtr, char **endPrt); Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-12 16:23 -0800
        Re: double strtod( const char *nPtr, char **endPrt); Ian Collins <ian-news@hotmail.com> - 2015-11-13 14:09 +1300
          Re: double strtod( const char *nPtr, char **endPrt); Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-12 17:31 -0800
            Re: double strtod( const char *nPtr, char **endPrt); Ian Collins <ian-news@hotmail.com> - 2015-11-13 14:46 +1300
        Re: double strtod( const char *nPtr, char **endPrt); Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-13 02:07 +0000
        Re: double strtod( const char *nPtr, char **endPrt); David Brown <david.brown@hesbynett.no> - 2015-11-13 08:48 +0100
    Re: double strtod( const char *nPtr, char **endPrt); Keith Thompson <kst-u@mib.org> - 2015-11-12 16:57 -0800

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