Problem with FFI?

[Simon Peyton-Jones]

Manuel:

Do you agree that something in the FFI spec needs to be fixed?
And hence is on your to-do list?

(Just wanting to be sure this one doesn't fall off the radar.)

Simon

| -----Original Message-----
| From: Alastair Reid [mailto:reid at cs.utah.edu] 
| Sent: 02 June 2002 18:03
| To: John Meacham
| Cc: ffi at haskell.org
| Subject: Re: Problem with FFI?
| 
| 
| 
| [reply redirected to ffi at haskell.org]
| 
| John Meacham <john at repetae.net> writes:
| > I may be missing something obvious here, but from the 
| current FFI spec 
| > it appears that it is impossible to create libraries in 
| haskell which 
| > are meant to be called from c code without running into undefined 
| > behavior. The problem is in the definition of hs_init() and 
| hs_exit() 
| > .  now, it is acceptable to have hs_init and hs_exit called in the 
| > initialization and finalization of your library, but the problem 
| > arrises when you link against more than one library which is 
| > implemented in haskell, suddenly whichever library is initialized 
| > secondly segfaults! (or whatever undefined behaviour means.).  
| > programs could suddenly stop working when a library is 
| changed from a 
| > c implementation to a haskell one, which seems to be a bad thing.
| 
| Hmmm, I see what you mean.
| 
| > proposed fix: allow nested calls to hs_init, hs_exit, a counter is 
| > incremented when hs_init is called more than once, and 
| decremented on 
| > hs_exit. only the last call to hs_exit will actually do 
| whatever needs 
| > to be done.
| 
| The hs_init function takes arguments - what if each call to 
| hs_init specifies a different set of arguments?  How about:
| 
|   hs_set_hs_argv   (int argc, char *argv[]);
|   hs_set_prog_argv (int argc, char *argv[]);
|   hs_init     ();
|   hs_exit     ();
|   
| Where:
| 
| hs_set_hs_argv sets command line parameters for the Haskell 
|   runtime system 
| 
|   It can be called at most once and that call must be before the 
|   first call to hs_init
| 
|   Warning! The flags are not remotely portable between different
|   Haskell runtimes.
| 
| hs_set_prog_argv sets arguments seen by getProgName and getArgs
| 
|   It can be called at most once and that call must be before the 
|   first call to hs_init
| 
| hs_init initializes the Haskell runtime
| 
|   It can be called multiple times and must be called before the first
|   call to any function exported by the Haskell code.
| 
| hs_exit finalizes the Haskell runtime
| 
|   hs_init and hs_exit are required to satisfy the usual 
| bracketing rules:
| 
|   1) At any time, the number of calls that have been made to 
| hs_exit must be
|      no more than the number of calls that have been made to hs_init.
| 
|   2) If the number of calls to hs_exit is equal to the number of calls
|      to hs_init, then no further calls to hs_init may be made.
| 
| 
| By the way, the name 'hs_exit' is a little confusing - I 
| expected it to have an ExitCode argument.  'hs_fini' would 
| better match its actual purpose.
| 
| > note that this cannot be implemented by the programmer 
| himself since 
| > there might be several third party libraries also implemented in 
| > haskell which an app wishes to link against.
| 
| You can implement the modified API (or your modified 
| semantics) quite readily as a little library which you link 
| against your program.  All you have to do is tweak the names 
| a little (e.g., s/hs/HS/) to avoid name clashes.
| 
| -- 
| Alastair Reid        reid at cs.utah.edu        
| http://www.cs.utah.edu/~reid/
| 
| 
| 
| 
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