[Bug general/5971] New: Associativity of C operators

[Bug general/5971] New: Associativity of C operators

[tthomas at redhat dot com]

Incorrect evaluation for C operators that associate right-to-left.

Frysk:
(fhpd) print i
1
(fhpd) print i++ + ++i + i 
7

But on a GCC compiler:
int i=1;
printf ("%d\n", i++ + ++i + i);

Output: 6


Gdb also seems to have this bug:
(gdb) print i
$2 = 1
(gdb)  print i++ + ++i + i
$3 = 7

-- 
           Summary: Associativity of C operators
           Product: frysk
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: general
        AssignedTo: frysk-bugzilla at sourceware dot org
        ReportedBy: tthomas at redhat dot com
OtherBugsDependingO 2246
             nThis:


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[Bug general/5971] Associativity of C operators

[pmachata at redhat dot com]

------- Additional Comments From pmachata at redhat dot com  2008-03-19 17:03 -------
I believe this hits undefined behavior, because you change "i" twice between two
sequence points.  I would notabug this.

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[Bug general/5971] Associativity of C operators

[tthomas at redhat dot com]

------- Additional Comments From tthomas at redhat dot com  2008-03-19 17:30 -------
Even if I do something like: 

(fhpd) print i = 1
1
(fhpd) print i++ + i
3

frysk and gdb gives result 3, whereas C evaluates it to a 2, which is why I
think its a bug with associativity.


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[Bug general/5971] Associativity of C operators

[pmachata at redhat dot com]

------- Additional Comments From pmachata at redhat dot com  2008-03-19 18:29 -------
But that expression has the same problem.  You can't both change and read the
number in one expression (or, more preciselly, between two sequence points). 
You may want to read the exact wording of the C or C++ standard, but FWIW,
expressions like these always make me feel icky.

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[Bug general/5971] Associativity of C operators

[tthomas at redhat dot com]

------- Additional Comments From tthomas at redhat dot com  2008-03-19 19:17 -------
Closing bug. 

Would be a neat feature to get frysk to warn users of possibly undefined operations.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://sourceware.org/bugzilla/show_bug.cgi?id=5971

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