[ ghc-Bugs-775969 ] warning: implicit declaration of function foo

Bugs item #775969, was opened at 2003-07-22 23:17
Message generated for change (Comment added) made by simonmar
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=775969&group_id=8032

Category: Compiler (FFI)
Group: 6.0
>Status: Closed
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: warning: implicit declaration of function foo

Initial Comment:
Hi,

when I compile a simple program with a function 
imported from C using -fffi -O, I get the following:
warning: implicit declaration of function 'foo'

the imported function simply returns its argument 
multiplied by two.

if I compile it without -O option, everything is fine, but 
with the -O option it gives the warning and when I run 
the program it gives the wrong result ( 1.0 for any 
number)

Thanks.

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>Comment By: Simon Marlow (simonmar)
Date: 2003-07-23 13:49

Message:
Logged In: YES 
user_id=48280

-O implies -fvia-C.  When compiling FFI code via C, the C 
compiler needs to have prototypes in scope for each of the 
functions you call using the FFI: this is why the C compiler 
emits the warning about an implicit declaration.

The GHC User's Guide describes how to bring the correct 
prototypes into scope:

http://www.haskell.org/ghc/docs/latest/html/users_guide/sec
-ffi-ghc.html#GLASGOW-FOREIGN-HEADERS


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