XVFS Build Walkthrough

xvfs-create

The script "xvfs-create" which is referenced below creates a C source file containing a local static structure and an Xvfs_<name>_Init() symbol. The C source file contains all the data from the specified directory, file names, contents, sizes, directory children. This C file can be compiled in a few different ways ("Client", "Standalone", and "Flexible") covered below.

"Server"

Description

The "Server" mode is a single in-process handler for "//xvfs:/" and additional clients can call Xvfs_Register() to register their filesystem with that handler. This has the advantage of being more efficient since there are not one Tcl_Filesystem per extension which have to all be searched through but instead a single handler which uses a Tcl_HashTable to map the namespace to things which have called Xvfs_Register().

The "Server" mode thus has 2 symbols:

1. Xvfs_Init() which initializes the Tcl_Filesystem for all of "//xvfs:/" as well as setup the empty Tcl_HashTable
2. Xvfs_Register() which is for other extensions to call to register their name under "//xvfs:/"

Build Output

$ make xvfs.so
cc -I. -DUSE_TCL_STUBS=1 -DXVFS_DEBUG -I/usr/include  -DXVFS_MODE_SERVER -fPIC -g3 -ggdb3 -Wall  -o xvfs.o -c xvfs-core.c
cc -fPIC -g3 -ggdb3 -Wall   -shared -o xvfs.so xvfs.o -L/usr/lib64 -ltclstub8.6

$ nm -D xvfs.so | grep ' [TU] '
0000000000004a07 T Xvfs_Init
0000000000004c6a T Xvfs_Register
                 U fprintf
                 U memcmp
                 U memcpy
                 U stderr
                 U strchr

Here we can see that indeed the only defined symbols for relocation are Xvfs_Init() and Xvfs_Register(), as well as some undefined symbols which we rely on. The symbols fprintf() and stderr are only used for debugging and would not be required in a production build.

"Client"

Description

The "Client" mode is very simple. It converts the specified directory ("example") into a C source file where all of the files contents and all directories children are structures of a local static structure. Then a Xvfs_example_Init() function is created which call Xvfs_Register() pointing to this structure (actually it points to some handlers so that the structure can change over time without relying on the core to be adaptable).

The "Client" mode thus exports a single symbol, Xvfs_<name>_Init(), and relies on a single symbol from the "Server" mode, Xvfs_Register().

Once you [load] this extension the Xvfs_example_Init() function is called and the VFS is registered under //xvfs:/example for all interpreters in this process.

Build Output

$ make example-client.so
./xvfs-create --directory example --name example > example.c.new
mv example.c.new example.c

cc -I. -DUSE_TCL_STUBS=1 -DXVFS_DEBUG -I/usr/include  -DXVFS_MODE_CLIENT -fPIC -g3 -ggdb3 -Wall  -o example-client.o -c example.c
cc -fPIC -g3 -ggdb3 -Wall   -shared -o example-client.so example-client.o -L/usr/lib64 -ltclstub8.6

$ nm -D example-client.so | grep ' [TU] '
                 U Xvfs_Register
0000000000000ed5 T Xvfs_example_Init
                 U memcmp
                 U strlen

"Standalone"

Description

The "Standalone" mode is a logically combined "Client" and "Server" mode. However, it does not check for anything already handling "//xvfs:/" and always registers its own Tcl_Filesystem handling only //xvfs:/<name>. It does not export an Xvfs_Register() function and is completely standalone (hence the name).

It will work and can be [load]d on any version of Tcl, the same as "Client" mode but with no external dependencies on the "Server" mode.

The "Standalone" mode thus exports a single symbol, Xvfs_<name>_Init().

Build Output

$ make example-standalone.so
./xvfs-create --directory example --name example > example.c.new
mv example.c.new example.c

cc -I. -DUSE_TCL_STUBS=1 -DXVFS_DEBUG -I/usr/include  -DXVFS_MODE_STANDALONE -fPIC -g3 -ggdb3 -Wall  -o example-standalone.o -c example.c
cc -fPIC -g3 -ggdb3 -Wall   -shared -o example-standalone.so example-standalone.o -L/usr/lib64 -ltclstub8.6
$ nm -D example-standalone.so | grep ' [TU] '
0000000000004d26 T Xvfs_example_Init
                 U fprintf
                 U memcmp
                 U memcpy
                 U stderr
                 U strlen

"Flexible"

Description

The "Flexible" mode is a combination of "Standalone" and "Client modes. It will search for an existing handler of "//xvfs:/" and if so call that handler's Xvfs_Register() function. If no such handler exists it will invoke the same handler as "Standalone" mode and register its own Tcl_Filesystem.

Because we cannot be linked with an undefined symbol to Xvfs_Register() the "Flexible" mode does not use the run-time linker to find the symbol for Xvfs_Register() but rather works with the Tcl VFS layer to locate that symbol dynamically.

The "Flexible" mode thus exports a single symbol, Xvfs_<name>_Init() and has no hard requirements on dependent symbols, however the "Server" mode's Xvfs_Register() will be called if its Xvfs_Init() function has been called previously to register a handler for //xvfs:/.

Build Output

$ make example-flexible.so
./xvfs-create --directory example --name example > example.c.new
mv example.c.new example.c

cc -I. -DUSE_TCL_STUBS=1 -DXVFS_DEBUG -I/usr/include  -DXVFS_MODE_FLEXIBLE -fPIC -g3 -ggdb3 -Wall  -o example-flexible.o -c example.c
cc -fPIC -g3 -ggdb3 -Wall   -shared -o example-flexible.so example-flexible.o -L/usr/lib64 -ltclstub8.6

$ nm -D example-flexible.so | grep ' [TU] '
00000000000050c8 T Xvfs_example_Init
                 U fprintf
                 U memcmp
                 U memcpy
                 U stderr
                 U strlen