Re: When parameters to printf are in memory

From	BartC <bc@freeuk.com>
Newsgroups	comp.lang.c
Subject	Re: When parameters to printf are in memory
Date	2016-10-04 14:03 +0100
Organization	A noiseless patient Spider
Message-ID	<nt09b2$5vc$1@dont-email.me> (permalink)
References	(11 earlier) <87y4285q9a.fsf@bsb.me.uk> <nsvlm7$3r7$1@dont-email.me> <87fuoc33dx.fsf@bsb.me.uk> <nt013r$961$1@dont-email.me> <874m4s2smc.fsf@bsb.me.uk>

Show all headers | View raw

On 04/10/2016 13:42, Ben Bacarisse wrote:
> BartC <bc@freeuk.com> writes:

>> The main problem would be with passing two arguments for "%d %d", then
>> passing two ints like this means they are 8 bytes apart rather than
>> 4. But it might be workable for a single argument at a time.
>
> Actually the main problem is simply that type is wrong.  I should
> probably not have tried to come up with a specific example, because the
> problem is more general than any one potential pitfall.  Once you you've
> lied the compiler, you are its mercy.

This is for a variadic argument, so the type cannot be wrong, as it's 
not specified! (Except inside the printf implementation, but then that's 
not a compiler issue.)

Nonetheless there is another type-related problem with this: some 
machines pass the first few arguments in registers, but floating point 
arguments might use different registers. So on x64, a double might be 
passed in both rcx and xmm0 when it is for a variadic function, rather 
than just rcx.

You might get around that by always pushing a double member of a union 
instead of uint64_t, but it starts looking more dodgy.

Better here to find out what type is needed and pass the correct member 
of the union using conditional code. But that would still be limited to 
a single operand of printf per call.

-- 
Bartc

Thread

When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 16:20 +0100
  Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 16:43 +0100
    Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-29 20:30 +0100
    Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 21:23 +0100
    Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-09-29 22:03 +0100
      Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 22:35 +0100
        Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-09-29 23:11 +0100
          Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-30 19:53 +0100
            Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-30 20:53 +0100
              Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-01 09:08 +0100
                Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-10-01 11:15 +0100
                Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 08:32 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-01 11:15 +0100
                Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 08:28 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-04 09:49 +0100
                Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-10-04 11:43 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-04 13:42 +0100
                Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-10-04 14:03 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-04 14:16 +0100
                Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 20:01 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-04 20:24 +0100
                Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 20:40 +0100
                Re: When parameters to printf are in memory Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-04 22:51 +0100
            Re: When parameters to printf are in memory "James R. Kuyper" <jameskuyper@verizon.net> - 2016-10-04 14:25 -0400
              Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 20:31 +0100
  Re: When parameters to printf are in memory BartC <bc@freeuk.com> - 2016-09-29 17:03 +0100
    Re: When parameters to printf are in memory gazelle@shell.xmission.com (Kenny McCormack) - 2016-09-30 12:20 +0000
      Re: When parameters to printf are in memory Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-30 05:55 -0700
      Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-30 20:06 +0100
        Re: When parameters to printf are in memory gazelle@shell.xmission.com (Kenny McCormack) - 2016-09-30 22:01 +0000
          Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-01 09:15 +0100
        Re: When parameters to printf are in memory Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-30 23:18 -0700
          Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-01 09:32 +0100
  Re: When parameters to printf are in memory jacobnavia <jacob@jacob.remcomp.fr> - 2016-09-29 18:35 +0200
    Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 19:55 +0100
      Re: When parameters to printf are in memory jacobnavia <jacob@jacob.remcomp.fr> - 2016-09-29 21:22 +0200
  Re: When parameters to printf are in memory Barry Schwarz <schwarzb@dqel.com> - 2016-09-29 11:20 -0700
    Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-09-29 20:08 +0100
  Re: When parameters to printf are in memory Lőrinczy Zsigmond <nospam@for.me> - 2016-10-02 20:15 +0200
    Re: When parameters to printf are in memory Lőrinczy Zsigmond <nospam@for.me> - 2016-10-02 20:19 +0200
      Re: When parameters to printf are in memory gazelle@shell.xmission.com (Kenny McCormack) - 2016-10-02 18:29 +0000
        Re: When parameters to printf are in memory Lőrinczy Zsigmond <nospam@for.me> - 2016-10-03 07:04 +0200
          Re: When parameters to printf are in memory James Harris <james.harris.1@gmail.com> - 2016-10-04 08:36 +0100
    Re: When parameters to printf are in memory jacobnavia <jacob@jacob.remcomp.fr> - 2016-10-02 21:27 +0200

csiph-web