Re: does char *str="abcd"; alloc addressable memory?

From	Barry Schwarz <schwarzb@delq.com>
Newsgroups	comp.lang.c
Subject	Re: does char *str="abcd"; alloc addressable memory?
Date	2020-07-06 09:58 -0700
Organization	A noiseless patient Spider
Message-ID	<uhk6gfd8o7g7cmg3ftcknb877odfik5e6m@4ax.com> (permalink)
References	<rdv1l5$clo$1@reader1.panix.com>

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On Mon, 6 Jul 2020 11:24:22 +0000 (UTC), John Forkosh
<forkosh@panix.com> wrote:

The answer to the question in the title is "yes".  But addressable can
mean readable, writable, or both.  

The standard guarantees that if argv[1] exists you can modify it.  

The standard also says that any attempt to modify a string literal
results in undefined behavior.  Your compiler implements this
restriction by placing the string literal in non-writable memory.

>Or, to put it another way, why does the following code
>segfault when run without a command-line arg, but runs
>fine when one is supplied...
>
>#include <stdio.h>
>#include <stdlib.h>
>#include <string.h>
>int main ( int argc, char *argv[] ) {
>  /* --- command-line arg --- */
>  char *str = ( argc>1? argv[1] : "01234&56789" );

The fact that this statement and ...

>  /* --- other variables --- */
>  char *str1=str, *amp=strchr(str,'&'), *str2=amp+1;

... this one both execute correctly demonstrates that the string
literal is addressable for reading.

>  /* --- display str's --- */
>  if ( amp != NULL ) *amp = '\000';

The fact that this statement fails is a result of the compiler
protecting you from yourself.  The compiler can neither detect nor
prevent most examples of undefined behavior.  It is a value judgement
whether its approach in this case is good or bad.  Hobbyists and
commercial developers have different priorities.

>    else str2 = str1;
>  printf("str1=\"%s\", str2=\"%s\"\n",str1,str2);
>  exit(0); }
>
>And then...
>   bash-5.0$ ./tester
>   Segmentation fault
>but...
>   bash-5.0$ ./tester "abc&def"
>   str1="abc", str2="def"
>
>Seems like *amp = '\000'; segfaults when
>the allocation is char *str = "01234&56789";
>So, those 12 bytes "01234&56789" have to be
>somewhere. But they're in non-addressable
>memory? Or what???

If they were in non-addressable memory, the call to strchr or perhaps
the initialization of str would have failed first.  They are in
non-writable (read-only) addressable memory.

-- 
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does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-06 11:24 +0000
  Re: does char *str="abcd"; alloc addressable memory? jacobnavia <jacob@jacob.remcomp.fr> - 2020-07-06 13:29 +0200
    Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) gazelle@shell.xmission.com (Kenny McCormack) - 2020-07-06 12:48 +0000
      Re: Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) BGB <cr88192@gmail.com> - 2020-07-06 11:55 -0500
        Re: Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) Vir Campestris <vir.campestris@invalid.invalid> - 2020-07-06 21:21 +0100
          Re: Mark Twain had something to say about this... John Forkosh <forkosh@panix.com> - 2020-07-07 03:21 +0000
          Re: Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) mark.bluemel@gmail.com - 2020-07-09 06:30 -0700
      Re: Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) Richard Damon <Richard@Damon-Family.org> - 2020-07-06 22:16 -0400
        Re: Mark Twain had something to say about this... Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-06 23:22 -0700
      Re: Mark Twain had something to say about this... John Forkosh <forkosh@panix.com> - 2020-07-07 02:46 +0000
      Re: Mark Twain had something to say about this... (Was: does char *str="abcd"; alloc addressable memory?) jacobnavia <jacob@jacob.remcomp.fr> - 2020-07-07 09:29 +0200
    Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-06 11:25 -0700
      Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 03:05 +0000
        Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-06 23:25 -0700
          Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 06:47 +0000
            Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 07:00 +0000
              Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-07 01:24 -0700
              Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-07 20:47 +0100
                Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-08 03:43 +0000
        Re: does char *str="abcd"; alloc addressable memory? David Brown <david.brown@hesbynett.no> - 2020-07-07 09:35 +0200
          Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 07:54 +0000
            Re: does char *str="abcd"; alloc addressable memory? David Brown <david.brown@hesbynett.no> - 2020-07-07 11:27 +0200
          Re: does char *str="abcd"; alloc addressable memory? Richard Damon <Richard@Damon-Family.org> - 2020-07-09 06:53 -0400
            Re: does char *str="abcd"; alloc addressable memory? David Brown <david.brown@hesbynett.no> - 2020-07-09 14:48 +0200
              Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-09 17:01 +0100
                Re: does char *str="abcd"; alloc addressable memory? David Brown <david.brown@hesbynett.no> - 2020-07-09 22:10 +0200
                Re: does char *str="abcd"; alloc addressable memory? scott@slp53.sl.home (Scott Lurndal) - 2020-07-09 20:23 +0000
                Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-09 23:56 +0100
                Re: does char *str="abcd"; alloc addressable memory? antispam@math.uni.wroc.pl - 2020-07-10 03:25 +0000
                Re: does char *str="abcd"; alloc addressable memory? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-07-10 11:56 -0400
                Re: does char *str="abcd"; alloc addressable memory? richard@cogsci.ed.ac.uk (Richard Tobin) - 2020-07-10 12:31 +0000
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-09 13:35 -0700
                Re: does char *str="abcd"; alloc addressable memory? scott@slp53.sl.home (Scott Lurndal) - 2020-07-09 22:17 +0000
                Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-09 23:50 +0100
            Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-09 16:52 +0100
            Re: does char *str="abcd"; alloc addressable memory? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-07-09 10:13 -0700
              Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-09 11:24 -0700
                Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-09 22:59 +0000
                Re: does char *str="abcd"; alloc addressable memory? Manfred <noname@add.invalid> - 2020-07-10 16:36 +0200
                Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-10 15:54 +0100
                Re: does char *str="abcd"; alloc addressable memory? Manfred <noname@add.invalid> - 2020-07-10 18:33 +0200
                Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-10 22:50 +0100
                Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-11 16:21 +0100
                Re: does char *str="abcd"; alloc addressable memory? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-07-11 08:55 -0700
                Re: does char *str="abcd"; alloc addressable memory? Bart <bc@freeuk.com> - 2020-07-11 17:09 +0100
                Re: does char *str="abcd"; alloc addressable memory? Manfred <noname@add.invalid> - 2020-07-11 18:34 +0200
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-10 13:17 -0700
                Re: does char *str="abcd"; alloc addressable memory? Manfred <noname@add.invalid> - 2020-07-11 02:00 +0200
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-10 17:46 -0700
                Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-11 03:09 +0000
                Re: does char *str="abcd"; alloc addressable memory? Manfred <noname@add.invalid> - 2020-07-11 18:43 +0200
                Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-11 15:09 -0400
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-11 13:39 -0700
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-11 15:50 -0700
                Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-12 01:06 +0000
                Re: does char *str="abcd"; alloc addressable memory? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-07-10 08:54 -0700
                Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-11 03:26 +0000
                Re: does char *str="abcd"; alloc addressable memory? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-07-11 03:56 -0700
                Re: does char *str="abcd"; alloc addressable memory? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-07-10 08:46 -0700
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-10 13:38 -0700
                Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-10 13:54 -0700
        Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-07 08:52 -0400
    Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 02:56 +0000
      Re: does char *str="abcd"; alloc addressable memory? antispam@math.uni.wroc.pl - 2020-07-07 19:08 +0000
        Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-08 04:07 +0000
          Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-07 21:54 -0700
  Re: does char *str="abcd"; alloc addressable memory? Ike Naar <ike@sdf.org> - 2020-07-06 11:56 +0000
  Re: does char *str="abcd"; alloc addressable memory? Bonita Montero <Bonita.Montero@gmail.com> - 2020-07-06 13:57 +0200
    Re: does char *str="abcd"; alloc addressable memory? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2020-07-06 05:37 -0700
      Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-06 19:52 +0100
        Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-06 12:20 -0700
      Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-07 03:32 +0000
        Re: does char *str="abcd"; alloc addressable memory? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-07-07 06:26 -0700
          Re: does char *str="abcd"; alloc addressable memory? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-07-07 10:03 -0700
            Re: does char *str="abcd"; alloc addressable memory? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-07-09 10:07 -0700
          Re: does char *str="abcd"; alloc addressable memory? John Forkosh <forkosh@panix.com> - 2020-07-08 03:57 +0000
            Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-08 11:02 -0400
    Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-06 09:43 -0700
      Re: does char *str="abcd"; alloc addressable memory? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-07-06 19:35 +0100
    Re: does char *str="abcd"; alloc addressable memory? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2020-07-06 18:50 -0700
  Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-06 08:35 -0400
    Re: does char *str="abcd"; alloc addressable memory? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-07-06 11:21 -0400
  Re: does char *str="abcd"; alloc addressable memory? Barry Schwarz <schwarzb@delq.com> - 2020-07-06 09:58 -0700
    Re: does char *str="abcd"; alloc addressable memory? scott@slp53.sl.home (Scott Lurndal) - 2020-07-06 23:07 +0000
      Re: does char *str="abcd"; alloc addressable memory? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-07-06 20:03 -0400
        Re: does char *str="abcd"; alloc addressable memory? scott@slp53.sl.home (Scott Lurndal) - 2020-07-07 14:36 +0000
          Re: does char *str="abcd"; alloc addressable memory? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2020-07-07 11:58 -0400

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