Re: c/3165: ANSI C standard violation

Re: c/3165: ANSI C standard violation

[jsm28]

Synopsis: ANSI C standard violation

State-Changed-From-To: open->closed
State-Changed-By: jsm28
State-Changed-When: Wed Jun 13 06:19:09 2001
State-Changed-Why:
    Not a bug.  Re-read the C standard, in particular the rules
    on sequence points.  Your program has undefined behavior.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=3165&database=gcc

c/3165: ANSI C standard violation

[tczarnecki]

>Number:         3165
>Category:       c
>Synopsis:       ANSI C standard violation
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Wed Jun 13 05:36:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Tomasz Czarnecki
>Release:        unknown-1.0
>Organization:
>Environment:
Sun Solaris 2.6 (Sparc)
>Description:
The order of expression evaluation is bad. The example code produces output like that:

1, a=1, b=1, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
11, a=3, b=2, c=1
1, a=1, b=1, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
22, a=2, b=1, c=1
1, a=1, b=1, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
33, a=1, b=1, c=1

But it should look like:

3, a=1, b=1, c=1
2, a=1, b=1, c=1
1, a=1, b=2, c=1
11, a=3, b=2, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
1, a=1, b=1, c=1
22, a=2, b=1, c=1
3, a=1, b=1, c=1
2, a=1, b=1, c=1
1, a=1, b=1, c=1
33, a=1, b=1, c=1

It looks like exressions are evaluated first and then operators are applied. But I don't think it should work that way. Microsoft VC 6.0 compiler has some problems too  but only += like operators case an error. In this case output looks like:

1, a=1, b=1, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
11, a=3, b=2, c=1
2, a=1, b=1, c=1
3, a=1, b=1, c=1
1, a=1, b=1, c=1
22, a=2, b=1, c=1
3, a=1, b=1, c=1
2, a=1, b=1, c=1
1, a=1, b=1, c=1
33, a=1, b=1, c=1

>How-To-Repeat:
/*Compile this file with no options*/
#include <stdio.h>

foo(int n, int a, int b, int c)
{
  printf("%d, a=%d, b=%d, c=%d\n", n, a, b, c);
}

int main()
{
  int a=1, b=1, c=1;
  *(foo(1, a, b, c), &a) += *(foo(2, a, b, c), &b) += *(foo(3, a, b, c), &c);
  foo(11, a, b, c);
  a=1, b=1, c=1;
  *(foo(1, a, b, c), &a) = *(foo(2, a, b, c), &b) + *(foo(3, a, b, c), &c);
  foo(22, a, b, c);
  a=1, b=1, c=1;
  *(foo(1, a, b, c), &a) = *(foo(2, a, b, c), &b) = *(foo(3, a, b, c), &c);
  foo(33, a, b, c);
  return 0;
}
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: