Document ambiguity between devices and paths in API.

[libguestfs.git] / src / guestfs.pod

diff --git a/src/guestfs.pod b/src/guestfs.pod
index 97b9282..d034c8e 100644 (file)
--- a/src/guestfs.pod
+++ b/src/guestfs.pod
@@ -747,6 +747,37 @@ fuse/guestmount.c.
 In libguestfs 1.5.4, the protocol was changed so that the
 Linux errno is sent back from the daemon.
 
+=item Ambiguity between devices and paths
+
+There is a subtle ambiguity in the API between a device name
+(eg. C</dev/sdb2>) and a similar pathname.  A file might just happen
+to be called C<sdb2> in the directory C</dev> (consider some non-Unix
+VM image).
+
+In the current API we usually resolve this ambiguity by having two
+separate calls, for example L</guestfs_checksum> and
+L</guestfs_checksum_device>.  Some API calls are ambiguous and
+(incorrectly) resolve the problem by detecting if the path supplied
+begins with C</dev/>.
+
+To avoid both the ambiguity and the need to duplicate some calls, we
+could make paths/devices into structured names.  One way to do this
+would be to use a notation like grub (C<hd(0,0)>), although nobody
+really likes this aspect of grub.  Another way would be to use a
+structured type, equivalent to this OCaml type:
+
+ type path = Path of string | Device of int | Partition of int * int
+
+which would allow you to pass arguments like:
+
+ Path "/foo/bar"
+ Device 1            (* /dev/sdb, or perhaps /dev/sda *)
+ Partition (1, 2)    (* /dev/sdb2 (or is it /dev/sda2 or /dev/sdb3?) *)
+ Path "/dev/sdb2"    (* not a device *)
+
+As you can see there are still problems to resolve even with this
+representation.  Also consider how it might work in guestfish.
+
 =back
 
 =head2 PROTOCOL LIMITS
@@ -1549,6 +1580,9 @@ Only use the handle from a single thread.  Either use the handle
 exclusively from one thread, or provide your own mutex so that two
 threads cannot issue calls on the same handle at the same time.
 
+See the graphical program guestfs-browser for one possible
+architecture for multithreaded programs using libvirt and libguestfs.
+
 =head1 QEMU WRAPPERS
 
 If you want to compile your own qemu, run qemu from a non-standard
@@ -1681,9 +1715,9 @@ has the same effect as calling C<guestfs_set_trace (g, 1)>.
 
 Location of temporary directory, defaults to C</tmp>.
 
-If libguestfs was compiled to use the supermin appliance then each
-handle will require rather a large amount of space in this directory
-for short periods of time (~ 80 MB).  You can use C<$TMPDIR> to
+If libguestfs was compiled to use the supermin appliance then the
+real appliance is cached in this directory, shared between all
+handles belonging to the same EUID.  You can use C<$TMPDIR> to
 configure another directory to use in case C</tmp> is not large
 enough.