There be divils here

I reckon a lot of hackers don’t realise how wary they need to
be when using enums. Problems occur frequently because people
(usually unwittingly) make assumptions about either the size,
alignment or signedness of the integer representation for the
enum which the compiler uses.

C99 says Each enumerated type shall be compatible with
an integer type. The choice of type is implementation-defined.
An implementation may delay the choice of which integer type
until all enumeration constants have been seen
. What
this means is that once the compiler has seen all the constants
you have defined for that enum, it will then decide what integer
representation to use and that could be a signed or unsigned
integer of any size.

So, GNOME hackers beware. gconf_enum_to_string() is
nasty. Have a look at the prototype:

gboolean  gconf_string_to_enum (GConfEnumStringPair  lookup_table[],
                                const gchar         *str,
                                gint                *enum_value_retloc);
  

Bugs I’ve seen include:

{
  enum { A, B } test_enum;

  gconf_enum_to_string (enum_to_str_table, str, (int *) &test_enum);
}
  

This is guaranteed to cause crashes on some architectures since
alignment of the enum is different from the expected
alignment of an int. So it should be:

{
  enum { A, B } test_enum;
  int test_int;

  if (gconf_enum_to_string (enum_to_str_table, str, &test_int))
    {
      test_enum = test_int;
    }
}
  

Also, this breaks:

{
enum { A, B } test_enum;
int test_int;

test_enum = -1;
gconf_enum_to_string (enum_to_str_table, str, &test_int);
test_enum = test_int;

if (test_enum
The problem here is that you're assuming the enum has a signed
representation when in fact the compiler may choose an unsigned
representation. If the conversion fails, if (test_enum
will not detect it.

Alex also points out that when adding values to an enum in an API
you need to be careful that you're not unwittingly changing the
ABI because the compiler chooses to use a large integer representation
for the enum than was previously used.

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