Re: extern

On Thu, Jul 25, 2013 at 10:39:09PM -0400, Ian Chapman wrote:
>      I'm using -  msp430-gcc (GCC) 4.6.3 20120301 (mspgcc LTS 20120406 
> unpatched)- and it fails on
> 
> extern unsigned short int ContactThreshold;
> 
> giving this error
> 
> msp430-gcc -Os -Wall -g -mmcu=msp430f5438a main.cpp
> /tmp/cciyW9nS.o: In function `SetSensitivity(unsigned short)':
> /home/stan/msp430/Ovi_Stuff/main.cpp:258: undefined reference to 
> `ContactThreshold'
> 
> If I junk extern it compiles and once I get there I expect a linker 
> error with ContactThreshold defined in two places.
> 
> Any suggestions appreciated.  Ian.  Oh and it did compile with windows.  
> Maybe I should change to straight c in place of cpp.

Hi Ian,

The "extern" keyword means that ContactThreshold is defined in another
compilation unit. That is:

    unsigned short int ContactTheshold;

...defines a variable called ContactThreshold. If you prefix it with
"extern", you aren't defining it -- you're telling the compiler that it
exists and is defined somewhere else.

Since you appear to be compiling only a single file, "extern" is
probably not what you want.

- Daniel

-- 
Daniel Beer <[email protected]>    www.dlbeer.co.nz
IRC: inittab (Freenode)    PGP key: 2048D/160A553B

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