File caching
============

The file caching mechanism is embedded within BFD and allows the application to open as many BFDs as it wants without regard to the underlying operating system's file descriptor limit (often as low as 20 open files).  The module in cache.c maintains a least recently used list of BFD_CACHE_MAX_OPEN files, and exports the name bfd_cache_lookup, which runs around and makes sure that the required BFD is open. If not, then it chooses a file to close, closes it and opens the one wanted, returning its file handle.

BFD_CACHE_MAX_OPEN macro
........................

Description

The maximum number of files which the cache will keep open at one time.

     #define BFD_CACHE_MAX_OPEN 10

bfd_last_cache
..............

Synopsis

     extern bfd *bfd_last_cache;

Description

Zero, or a pointer to the topmost BFD on the chain.  This is used by the bfd_cache_lookup macro in libbfd.h to determine when it can avoid a function call.

bfd_cache_lookup
................

Description

Check to see if the required BFD is the same as the last one looked up. If so, then it can use the stream in the BFD with impunity, since it can't have changed since the last lookup; otherwise, it has to perform the complicated lookup function.

     #define bfd_cache_lookup(x) \
         ((x) == bfd_last_cache ? \
           (FILE *) (bfd_last_cache->iostream): \
            bfd_cache_lookup_worker (x))

bfd_cache_init
..............

Synopsis

     bfd_boolean bfd_cache_init (bfd *abfd);

Description

Add a newly opened BFD to the cache.

bfd_cache_close
...............

Synopsis

     bfd_boolean bfd_cache_close (bfd *abfd);

Description

Remove the BFD ABFD from the cache. If the attached file is open, then close it too.

Returns

FALSE is returned if closing the file fails, TRUE is returned if all is well.

bfd_cache_close_all
...................

Synopsis

     bfd_boolean bfd_cache_close_all (void);

Description

Remove all BFDs from the cache. If the attached file is open, then close it too.

Returns

FALSE is returned if closing one of the file fails, TRUE is returned if all is well.

bfd_open_file
.............

Synopsis

     FILE* bfd_open_file (bfd *abfd);

Description

Call the OS to open a file for ABFD.  Return the FILE * (possibly NULL) that results from this operation.  Set up the BFD so that future accesses know the file is open. If the FILE * returned is NULL, then it won't have been put in the cache, so it won't have to be removed from it.

bfd_cache_lookup_worker
.......................

Synopsis

     FILE *bfd_cache_lookup_worker (bfd *abfd);

Description

Called when the macro bfd_cache_lookup fails to find a quick answer.  Find a file descriptor for ABFD.  If necessary, it open it.  If there are already more than BFD_CACHE_MAX_OPEN files open, it tries to close one first, to avoid running out of file descriptors.  It will abort rather than returning NULL if it is unable to (re)open the ABFD.