[LLVMbugs] [Bug 13766] New: Bounds checking an array of size 1 at end of union in struct

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13766

             Bug #: 13766
           Summary: Bounds checking an array of size 1 at end of union in
                    struct
           Product: clang
           Version: 3.1
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: realnc at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Normally, Clang emits warnings when it sees array accesses that are out of
bounds.  Unless the array has a size of 1 and is the last member of a struct:

  struct Foo {
      int d;
      struct Foo *array[1];
  };

Then this code won't produce warnings:

  struct Foo *foo = malloc(sizeof(struct Foo) * 3);
  foo->array[1] = NULL;
  foo->array[2] = NULL;

However, this struct:

  struct Bar {
      int d;
      union {
          int n;
          struct Foo *array[1];
      } my_union;
  };

will produce warnings with this code:

  struct Bar *bar = malloc(sizeof(struct Bar) * 3);
  bar->my_union.array[1] = NULL;
  bar->my_union.array[2] = NULL;

Not giving the union a name also doesn't help.

Is this intended?  IMO, if in the first case the compiler stays silent, then it
should also stay silent in the second one.  Because otherwise it means that the
code must be rewritten to not use a union, which is a compromise; using a union
in this case is best way to implement it. Passing "-Wno-array-bounds" isn't a
good solution either, since it's a very helpful warning to have around.

This is C by the way, not C++.

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