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Re: Pointers and Sequence Point

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On 05/02/2011 07:38 AM, arnuld wrote:
> I have this program which is causing confusion to me. I am told that last 
> printf() of main() will always print "Second", not "First". I think line:
> 
>  x = myAlloc(x);
> 
> is UB. My colleague is disagreeing on this while I was saying that its UB 
> because there is no sequence point here. It is just like using this line:
> 
>  i = i++;

No, it's not. 6.5p2 contains two separate "shalls" that apply between
consecutive sequence points. "i=i++" violates the first one, against
modifying the value of an object twice. Your example modifies the value
of x only once. I presume, therefore, that you're actually worried about
the second one, against reading the value for any purpose other than to
determine what the new value is.

> But he says that sequence point will not affect the pointers and hence 
> this is perfectly valid ANSI C program. I am confused on this:

There is no special exemption of pointers from the rules involving
multiple modifications with no intervening sequence points. "x=x++"
would be just as thoroughly undefined as "i = i++". It's just that
there's a whole lot more relevant sequence points involved in the
evaluation of "x = myAlloc(x)" than you're taking into consideration.

> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> 
> enum { SIZE_LIMIT = 10 };
> 
> char* myAlloc(char* c);
> 
> int main(void)
> {
>   char* x = malloc(SIZE_LIMIT * sizeof *x);
>   if(NULL == x)
>     {
>       printf("Out of Memory\n");
>       return EXIT_FAILURE;
>     }
> 
>   strcpy(x, "First");
>   printf("x = %s\n", x);
>   x = myAlloc(x);

There's a sequence point separating evaluation of 'x' as an argument to
myAlloc() from the call to myAlloc() (6.5.2.2p10).

>   printf("x = %s\n", x);
>       
>   return 0;
> }
> 
> 
> char* myAlloc(char* c)
> {
>   c = malloc(SIZE_LIMIT * sizeof *c);

There's a sequence point separating the multiplication from the call to
malloc(), another just before malloc() returns (7.1.4p3), a third at the
end of the full expression (6.8p4).


>   if(NULL == c)

There's a sequence point at the end of the full expression.

>     {
>       printf("Out of Memory\n");
>       return NULL;
>     }
>   else
>     {
>       strcpy(c, "Second");

There's a sequence point separating the evaluation of the arguments from
the call to strcpy(), a second just before strcpy() returns, and a third
at the end of the full expression.

>     }
> 
>   return c;

There's a sequence point at the end of the full expression.

> }

I count a total of 9 sequence points separating the reading of the value
of x from its modification. One is all that's needed to avoid violating
6.5p2.

-- 
James Kuyper

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Pointers and Sequence Point arnuld <sunrise@invalid.address> - 2011-05-02 11:38 +0000
  Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-02 08:23 -0400
  Re: Pointers and Sequence Point Martin Ambuhl <mambuhl@earthlink.net> - 2011-05-02 09:58 -0400
    Re: Pointers and Sequence Point Chad <cdalten@gmail.com> - 2011-05-02 09:36 -0700
      Re: Pointers and Sequence Point Angel <angel+news@spamcop.net> - 2011-05-02 17:21 +0000
      Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-02 23:20 -0400
        Re: Pointers and Sequence Point Keith Thompson <kst-u@mib.org> - 2011-05-02 23:26 -0700
          Re: Pointers and Sequence Point James Kuyper <jameskuyper@verizon.net> - 2011-05-03 05:33 -0400
    Re: Pointers and Sequence Point Kenneth Brody <kenbrody@spamcop.net> - 2011-05-02 13:35 -0400
      Re: Pointers and Sequence Point Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-05-03 17:45 +0300
  Re: Pointers and Sequence Point John Bode <jfbode1029@gmail.com> - 2011-05-02 10:17 -0700
  Re: Pointers and Sequence Point Tim Rentsch <txr@alumni.caltech.edu> - 2011-05-02 10:31 -0700
  Re: Pointers and Sequence Point "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-05-02 20:55 -0400
    Re: Pointers and Sequence Point David Resnick <lndresnick@gmail.com> - 2011-05-03 08:01 -0700

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