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| -rw-r--r-- | hurd.mdwn | 1 | ||||
| -rw-r--r-- | hurd/translator_short-circuiting.mdwn | 59 |
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@@ -92,6 +92,7 @@ in the *unstable* branch of the Debian archive. * [[libhello_example]] -- Hurd library example * [[libnetfs]] -- short introductory material * [[IO_Path]] +* [[Translator short-circuiting]] * [[Porting]] * [[Debugging]] * [Hurd Sourcecode Reference](http://www.htu.tugraz.at/~past/hurd/global/): Searchable and browsable index of the code. diff --git a/hurd/translator_short-circuiting.mdwn b/hurd/translator_short-circuiting.mdwn new file mode 100644 index 00000000..01bd6ccb --- /dev/null +++ b/hurd/translator_short-circuiting.mdwn @@ -0,0 +1,59 @@ +[[!meta copyright="Copyright © 2009 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled +[[GNU Free Documentation License|/fdl]]."]]"""]] + +# Translator short-circuiting + +In traditional Unix filesystems contain special files. These are: +symbolic links, character devices, block devices, named pipes, and +named sockets. Naturally the Hurd also support these. + +However, if you take a look at `hurd/io.defs` and `hurd/fs.defs`, you'll +find that there are no RPCs that deal specifically with these types. +Sure, you can get the type of the file through `io_stat` (among other +things), but there are none that e.g. lets you create a symbolic link. + +If you take a look at how glibc implements `symlink`, you'll notice +that all it does is create a new file and sets its passive translator to +`/hurd/symlink DEST`. You can verify this yourself by creating a symlink +with `ln -s foo bar` and print its translator with `showtrans bar`. + +This is how the other special files are implemented as well. The header +`hurd/paths.h` contains a list of paths that are used to implement +special files: + +* `/hurd/symlink` +* `/hurd/chrdev` +* `/hurd/blkdev` +* `/hurd/fifo` +* `/hurd/ifsock` + +So all special files implemented through special purpose translators, +right? Well, actually there are no implementations of either `chrdev` or +`blkdev` in the Hurd. Instead these are implemented purely by /translator +short-circuiting/. + +Translator short-circuiting is when a translator, instead of starting a +passive translator, implements the functionality itself. For instance, +by continuing a file name look up by simply appending the path of a +symlink without returning to the client. + +In fact the translators that are implemented are only used as a default +implementation if the underlying translator does not implement the +functionality itself. + +To make sure that you use one of these translators, you can start it as +an active translator, or you can use a different path from the one in +`hurd/path.h`, e.g. `settrans bar /hurd/./symlink foo`. + +The best example of how short-circuiting is implemented can be found +in `libdiskfs`. Notice how it detects if a translator is a special +file in `diskfs_S_file_set_translator` and marks the node's `stat` +structure. And how it later checks the node's `stat` structure in +`diskfs_S_dir_lookup` and handles special file types appropriately. |