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.

Caching functions
-----------------

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

 -- Function: bool bfd_cache_init (bfd *abfd);
     Add a newly opened BFD to the cache.

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

 -- Function: bool bfd_cache_close (bfd *abfd);
     Remove the BFD ABFD from the cache. If the attached file is open, then close it too.

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

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

 -- Function: bool bfd_cache_close_all (void);
     Remove all BFDs from the cache. If the attached file is open, then close it too.  Note - despite its name this function will close a BFD even if it is not marked as being cacheable, ie even if bfd_get_cacheable() returns false.

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

bfd_cache_size
..............

 -- Function: unsigned bfd_cache_size (void);
     Return the number of open files in the cache.

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

 -- Function: FILE* bfd_open_file (bfd *abfd);
     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.