Question

Forward declaration of enums in C does not work for me.

I searched the Internet and Stack Overflow, but all of the questions regarding forward declarations of enumerators refer to C++. What do you do for declaring enumerators in C?

Put them at the top of each file (or in a header), so that all functions in the file can access them?

Answer 1

Put them in a header so that all files that need them can access the header and use the declarations from it.

When compiled with the options:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -c enum.c

GCC 4.2.1 (on Mac OS X 10.7.1 (Lion)) accepts the following code:

enum xyz;

struct qqq { enum xyz *p; };

enum xyz { abc, def, ghi, jkl };

Add -pedantic and it warns:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -pedantic -c enum.c
enum.c:1: warning: ISO C forbids forward references to ‘enum’ types
enum.c:5: warning: ISO C forbids forward references to ‘enum’ types

Thus, you are not supposed to try using forward declarations of enumerated types in C; GCC allows it as an extension when not forced to be pedantic.