c enum with 32 bit values

Question

i need some unsigned 32bit enum values for my software, therefore i implemented this (simple) enum:

enum{
   val1    = 0xFFFFFFFFu,
   val2    = 0xFFFFFFFEu,
   val3    = 0xFFFFFFF0
};

The problem: Everytime i run the compiler, Eclipse aborts compiling and marks the enum with the following error:

In my oppinion, a 32 int value should not be a problem for enums, but obviously it seems to be. I would be grateful about some input :)

[Update 1:] I will try to find the problem in the compiler settings, i will keep you up to date

Answer 1

An enumeration constant (val1 in your example) is required to be of type int by the C standard. This is a signed type and on a 32 bit system, the value FFFFFFFF will not fit inside it. This value will therefore get converted to a signed integer in some implementation-defined (compiler-specific) manner. If this can't be done, you'll get an implementation-defined signal.

Writing code which relies on this is bad, since it is non-portable and unpredictable. There is no compiler setting that can fix this, because this is by language design.

I believe the gcc flag -pedantic/-pedantic-errors could be removed in order to get rid of the warning, but that's a bad idea, as you will no longer follow standard C. gcc in its default, non-standard "skunk mode" -std=gnu90 or -std=gnu11 will compile the code, as will any -std=cxx without -pedantic-errors.

This is why enums are unsuitable for any form of bit masks or bit-wise operations.

The best solution is to get rid of the enum and use either #define or const uint32_t, whichever is most convenient for your specific scenario.

Answer 2

Did you try this?:

enum {
   val1    = (int)0xFFFFFFFFu,
   val2    = (int)0xFFFFFFFEu,
   val3    = (int)0xFFFFFFF0
};

Edit: I just installed gcc on cygwin and tried this out.

test-enum.c is the original version, test-enum-int.c the one with explicit casting:

$ cc -std=c11 -pedantic-errors -c test-enum.c
test-enum.c:2:8: error: ISO C restricts enumerator values to range of 'int' [-Wpedantic]
   val1 = 0xFFFFFFFFu,
          ^
test-enum.c:3:8: error: ISO C restricts enumerator values to range of 'int' [-Wpedantic]
   val2 = 0xFFFFFFFEu,
          ^
test-enum.c:4:8: error: ISO C restricts enumerator values to range of 'int' [-Wpedantic]
   val3 = 0xFFFFFFF0u
          ^

$ cc -std=c11 -pedantic-errors -c test-enum-int.c

(no complains)

$ cc --version
cc (GCC) 5.4.0
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