On 21 Sep 2009, at 21:19, Frederick C. Lee wrote:
I'm trying to replace #define directives with c datatype
directives.Example:
(1) #define x 123
versus...
(2) const unsigned x = 123;
I place this at the top of the .h (or .m) file, outside of any class
or
method as stand alone.
However, every time I try to use the (2) declaration (within a .h
file) the
linker gives the error: 'duplicate symbol _x in ....'
I tried placing the (2) declaration at the top of a .m or .c file,
but I get
the same linker error.
I've declared global constants in another application without a
problem.
I've checked the compiler type within the build: gcc 4.2
What am I doing wrong?
You should define the constant in ONE .m file e.g.
foo.m
----
const unsigned int x = 123;
----
and declare it extern in a header file e.g.
foo.h
---
extern const unsigned int x;
---
This is basic C usage but it often confuses people. Back in the 90's
I came across a bug where somebody had done exactly the opposite (with
a variable, not a constant) i.e. put the extern declaration in every C
file and the definition in a header file and the linker was too
primitive to complain about the duplicate symbols (or it only emitted
a warning that was being ignored). When the code was running,
different C functions chose different seemingly random versions of the
variable to read/write.
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