Re: Initializing a malloc'ed struct whose fields are const with run-time values

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: Initializing a malloc'ed struct whose fields are const with run-time values
Date	2013-01-22 21:19 +0000
Organization	Aioe.org NNTP Server
Message-ID	<kdmvpg$tb6$1@speranza.aioe.org> (permalink)
References	<kdmbl7$4lc$1@dont-email.me> <slrn3vfskftksd.ddo.ike@sverige.freeshell.org> <kdmo4v$vm5$1@dont-email.me> <50fefdb9$0$16499$426a34cc@news.free.fr>

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Noob <root@localhost> wrote:

(snip)
>> This has the same problem with the initializer list, though.
 
>> I would guess that 'malloc' was being used for lifetime considerations, 
>> but obviously only Noob knows.
 
> As I often do, my simplification went a bit too far, so I'll just
> describe the actual use-case.
> (This involves two threads on a POSIX-compliant platform.)

As far as I know, the constraints on const aren't quite as strict
as Java's static final, or Fortran's PARAMETER. 

You can get away with some things that you probably shouldn't be
able to, as the compiler can't check them. 
 
> I have a foo_start function which malloc's space for a "context"
> struct, populates the struct according to the function's parameters,
> then spawns a new thread which is passed this context. (This is why
> dynamic allocation must be used.)
 
> To make matters more complex, I have a flexible array at the end of
> the struct.
 
(snip)

> In my current version, I don't have any const qualifiers in my code.
> I like to look at the assembly code generated by the compiler, and I
> noticed that every time I need some field from ctx, the compiler has
> to reload it, instead of caching the value in a register.

Have you tried with an actual const struct (with actual compile time
constants) to see what the optimizer does? Or passing such a struct
(or pointer to one) and see what it does?
 
> As far as I understand, this is expected: the address of the struct
> could be stored anywhere, and any function in a different translation
> unit could "pull the rug" from under me. Except that *I* wrote the
> code, and I *know* (by design) that most fields in the struct do NOT
> change after init.

Well, as described in another thread, there is the 'volatile' 
attribute to tell the compiler, in some sense which I won't 
describe here, that a variable could change. 

On the other hand, normal aliasing rules apply. Often the compiler
can't assume that two pointers don't point to the same (or nearby)
variables, and so can't do some optimizations that it might otherwise
do. 
 
> So I set out to sprinkle a few "const" qualifiers here and there, to
> see if that would convince the compiler that some optimizations are
> indeed possible (I know this looks like a clear case of premature
> optimization, but I figured I might as well learn something new along
> the way!)

Did it help any? Does it change with optimization (-O) level?
 
> I don't think I can just const-qualify the entire struct, because
> 1) the prototype for a thread's entry point is imposed by POSIX
> 2) const-qualifying a pointer parameter is only a contract between
> the user and the function's implementer, saying "my function won't
> touch your preciousss struct", it doesn't say that the struct won't
> be changed by something else.
 
> Whereas, I was under the impression that a const-qualified field
> means "hear, hear, this field SHALL NEVER change!"
 
> Anyway, I'd be happy to hear the word from the regs, and/or take
> this to comp.unix.programmer at some point (although I do believe
> that the core of my question is a C question, not POSIX).

-- glen

Thread

Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-22 16:36 +0100
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 11:23 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:26 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:41 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:25 -0800
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:17 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:09 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 14:16 +0100
        Re: Initializing a malloc'ed struct whose fields are const with run-time values "BartC" <bc@freeuk.com> - 2013-01-23 13:25 +0000
          Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 15:21 +0100
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:56 +0000
              Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 16:40 +0100
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 18:26 +0000
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 20:39 -0800
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:14 -0500
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Ike Naar <ike@sverige.freeshell.org> - 2013-01-22 17:59 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 14:09 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 21:59 +0100
        Re: Initializing a malloc'ed struct whose fields are const with run-time values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 21:19 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:47 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:05 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:15 -0500
    Re: Initializing a malloc'ed struct whose fields are const with   run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 09:43 +1300
      Re: Initializing a malloc'ed struct whose fields are const with   run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:19 -0800
        Re: Initializing a malloc'ed struct whose fields are const with   run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:35 -0800
          Re: Initializing a malloc'ed struct whose fields are const with   run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 10:45 +1300
            Re: Initializing a malloc'ed struct whose fields are const with   run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 17:07 -0500
            Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:29 -0800
              Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:54 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:08 -0500
                Re: Initializing a malloc'ed struct whose fields are const with    run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 11:29 +1300
                Re: Initializing a malloc'ed struct whose fields are const with    run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 18:26 -0500
                Re: Initializing a malloc'ed struct whose fields are const with      run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 14:03 +1300
                Re: Initializing a malloc'ed struct whose fields are const with      run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 23:58 -0500
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 23:36 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 13:55 +1300
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 02:44 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 21:39 -0800
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-24 00:54 -0500
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 17:56 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-24 13:58 -0800
        Re: Initializing a malloc'ed struct whose fields are const with   run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:37 -0500
          Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:22 -0800
      Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:06 -0800
        Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 19:55 +1300
          Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 00:48 -0800
            Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 23:06 +1300
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:02 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:12 -0500
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:37 +0000
          Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 22:29 -0500
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 04:28 +0000
              Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 00:07 -0500
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:04 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 22:10 +0100
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:13 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:25 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:21 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:32 -0800
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:50 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Nick Bowler <nbowler@draconx.ca> - 2013-01-22 21:43 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:45 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Keith Thompson <kst-u@mib.org> - 2013-01-24 08:23 -0800
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 20:48 -0800

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