@itemize @bullet
@item
Some way of translating in software from virtual page frames to
-physical page frames. Consider using a hash table (@pxref{Hash
-Table}).
+physical page frames. Pintos provides a hash table that you may find
+useful for this purpose (@pxref{Hash Table}).
It is possible to do this translation without adding a new data
structure, by modifying the code in @file{userprog/pagedir.c}. However,
written back to the file, and pages not written must not be. The pages
are then removed from the process's list of virtual pages.
+If two or more processes map the same file, there is no requirement that
+they see consistent data. Unix handles this by making the two mappings
+share the same physical page, but the @code{mmap} system call also has
+an argument allowing the client to specify whether the page is shared or
+private (i.e.@: copy-on-write).
+
@node Project 3 FAQ
@section FAQ
Yes.
-@item How do I use the hash table provided in @file{lib/kernel/hash.c}?
-@anchor{Hash Table}
-
-First, you need to add a @struct{hash_elem} as a member of the
-object that the hash table will contain. Each @struct{hash_elem} allows
-the object to a member of at most one hash table at a given time. All
-the hash table functions that deal with hash table items actually use
-the address of a @struct{hash_elem}. You can convert a pointer to a
-@struct{hash_elem} member into a pointer to the structure in which
-member is embedded using the @code{hash_entry} macro.
-
-Second, you need to decide on a key type. The key should be something
-that is unique for each object, because a given hash table may not
-contain two objects with equal keys. Then you need to write two
-functions. The first is a @dfn{hash function} that converts a key
-into an integer. Some sample hash functions that you can use or just
-examine are given in @file{lib/kernel/hash.c}. The second needed
-function is a @dfn{comparison function} that compares a pair of objects
-and returns
-true if the first is less than the second. These two functions have
-to be compatible with the prototypes for @code{hash_hash_func} and
-@code{hash_less_func} in @file{lib/kernel/hash.h}.
-
-Here's a quick example. Suppose you want to put @struct{thread}s
-in a hash table. First, add a @struct{hash_elem} to the thread
-structure by adding a line to its definition:
-
-@example
-struct hash_elem h_elem; /* Hash table element. */
-@end example
-
-We'll choose the @code{tid} member in @struct{thread} as the key,
-and write a hash function and a comparison function:
-
-@example
-/* Returns a hash for E. */
-unsigned
-thread_hash (const struct hash_elem *e, void *aux UNUSED)
-@{
- struct thread *t = hash_entry (e, struct thread, h_elem);
- return hash_int (t->tid);
-@}
-
-/* Returns true if A's tid is less than B's tid. */
-bool
-thread_less (const struct hash_elem *a_,
- const struct hash_elem *b_,
- void *aux UNUSED)
-@{
- struct thread *a = hash_entry (a_, struct thread, h_elem);
- struct thread *b = hash_entry (b_, struct thread, h_elem);
- return a->tid < b->tid;
-@}
-@end example
-
-Then we can create a hash table like this:
-
-@example
-struct hash threads;
-
-hash_init (&threads, thread_hash, thread_less, NULL);
-@end example
-
-Finally, if @code{@var{t}} is a pointer to a @struct{thread},
-then we can insert it into the hash table with:
-
-@example
-hash_insert (&threads, &@var{t}->h_elem);
-@end example
-
-The CS109 and CS161 textbooks have chapters on hash tables.
-
-@item Why do the hash table functions have @var{aux} parameters?
-
-In simple cases you won't have any need for the @var{aux} parameters.
-In these cases you can just pass a null pointer to @func{hash_init}
-for @var{aux} and ignore the values passed to the hash function and
-comparison functions. (You'll get a compiler warning if you don't use
-the @var{aux} parameter, but you can turn that off with the
-@code{UNUSED} macro, as shown above, or you can just ignore it.)
-
-@var{aux} is useful when you have some property of the data in the
-hash table that's both constant and needed for hashing or comparisons,
-but which is not stored in the data items themselves. For example, if
-the items in a hash table contain fixed-length strings, but the items
-themselves don't indicate what that fixed length is, you could pass
-the length as an @var{aux} parameter.
-
-@item Can we change the hash table implementation?
-
-You are welcome to modify it. It is not used by any of the code we
-provided, so modifying it won't affect any code but yours. Do
-whatever it takes to make it work the way you want.
-
@item What extra credit is available?
You may implement sharing: when multiple processes are created that use
The @command{mcp} program in @file{src/examples} shows how to copy a
file using memory-mapped I/O.
-@item What if two processes map the same file into memory?
-
-There is no requirement in Pintos that the two processes see
-consistent data. Unix handles this by making the two mappings share the
-same physical page, but the @code{mmap} system call also has an
-argument allowing the client to specify whether the page is shared or
-private (i.e.@: copy-on-write).
-
@item What happens if a user removes a @code{mmap}'d file?
The mapping should remain valid, following the Unix convention.
projects / pintos-anon / blobdiff