D 2019-09-17T22:05:09.081
L XVFS\sBuild\sWalkthrough
N text/x-markdown
P feaa1925d1ca28023330afcb1626340b01a9a75181dd4b4b92dbd9b1b3a16604
U rkeene
W 16902
# 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.

See the final section for what this C source looks like.

Note the generated output will work for any mode (though it's unnecessary for "Server"
mode, because "Server" mode does not supply any actual VFS data).

## "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 is 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
    
    $ ls -l xvfs.so
    -rwxr-xr-x 1 rkeene users 14896 Sep 17 14:54 xvfs.so*

    $ 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
    
    $ ls -l example-client.so
    -rwxr-xr-x 1 rkeene users 22584 Sep 17 14:54 example-client.so*
    
    $ 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
    
    $ ls -l example-standalone.so
    -rwxr-xr-x 1 rkeene users 27680 Sep 17 14:54 example-standalone.so*
    
    $ 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
    
    $ ls -l example-flexible.so
    -rwxr-xr-x 1 rkeene users 27728 Sep 17 14:54 example-flexible.so*
    
    $ nm -D example-flexible.so | grep ' [TU] '
    00000000000050c8 T Xvfs_example_Init
                     U fprintf
                     U memcmp
                     U memcpy
                     U stderr
                     U strlen

## Generated Output

    $ make example.c
    ./xvfs-create --directory example --name example > example.c.new
    mv example.c.new example.c
    
    $ ls -l example.c
    -rw-r--r-- 1 rkeene users 47539 Sep 17 14:58 example.c

    $ cat example.c
    #include <xvfs-core.h>
    #include <sys/stat.h>
    #include <unistd.h>
    #include <string.h>
    #include <tcl.h>

    #define XVFS_NAME_LOOKUP_ERROR (-1)
    #define XVFS_FILE_BLOCKSIZE 1024

    /*
     * XXX:TODO: Determine this automatically rather than
     *           by heuristics
     */
    #define HAVE_STRUCT_STAT_ST_BLKSIZE 1
    #define HAVE_STRUCT_STAT_ST_BLOCKS  1
    #ifdef WIN32
    #  undef HAVE_STRUCT_STAT_ST_BLKSIZE
    #  undef HAVE_STRUCT_STAT_ST_BLOCKS
    #endif

    #define MIN(a, b) (((a) < (b)) ? (a) : (b))

    typedef enum {
        XVFS_FILE_TYPE_REG,
        XVFS_FILE_TYPE_DIR
    } xvfs_file_type_t;

    typedef Tcl_WideInt xvfs_size_t;

    struct xvfs_file_data {
        const char          *name;
        xvfs_file_type_t    type;
        xvfs_size_t         size;
        union {
                const unsigned char *fileContents;
                const char          **dirChildren;
        } data;
    };

    static struct xvfs_file_data xvfs_example_data[] = {
        {
                .name = "main.tcl",
                .type = XVFS_FILE_TYPE_REG,
                .size = 8730,
                .data.fileContents = (const unsigned char *) "\x23\x21\x20\x2f\x75\x73\x72\x2f\x62\x69"...
        },
        {
                .name = "foo",
                .type = XVFS_FILE_TYPE_REG,
                .size = 12,
                .data.fileContents = (const unsigned char *) "\x46\x6f\x6f\x20\x42\x61\x72\x20\x42\x61"...
        },
        {
                .name = "lib/hello/hello.tcl",
                .type = XVFS_FILE_TYPE_REG,
                .size = 77,
                .data.fileContents = (const unsigned char *) "\x6e\x61\x6d\x65\x73\x70\x61\x63\x65\x20"...
        },
        {
                .name = "lib/hello/pkgIndex.tcl",
                .type = XVFS_FILE_TYPE_REG,
                .size = 66,
                .data.fileContents = (const unsigned char *) "\x70\x61\x63\x6b\x61\x67\x65\x20\x69\x66"...
        },
        {
                .name = "lib/hello",
                .type = XVFS_FILE_TYPE_DIR,
                .size = 2,
                .data.dirChildren  = (const char *[]) {"hello.tcl", "pkgIndex.tcl"}
        },
        {
                .name = "lib",
                .type = XVFS_FILE_TYPE_DIR,
                .size = 1,
                .data.dirChildren  = (const char *[]) {"hello"}
        },
        {
                .name = "",
                .type = XVFS_FILE_TYPE_DIR,
                .size = 3,
                .data.dirChildren  = (const char *[]) {"main.tcl", "lib", "foo"}
        },
    };

    static long xvfs_example_nameToIndex(const char *path) {
        unsigned int pathHash;
        size_t pathLen;

        if (path == NULL) {
                return(XVFS_NAME_LOOKUP_ERROR);
        }

        pathLen = strlen(path);
        pathHash = Tcl_ZlibAdler32(0, (const unsigned char *) path, pathLen);
        switch (pathHash) {
                case 0:
                        if (pathLen == 0 && memcmp(path, xvfs_example_data[6].name, pathLen) == 0) {
                                return(6);
                        }
                        break;
                case 41419063:
                        if (pathLen == 3 && memcmp(path, xvfs_example_data[5].name, pathLen) == 0) {
                                return(5);
                        }
                        break;
                case 41877828:
                        if (pathLen == 3 && memcmp(path, xvfs_example_data[1].name, pathLen) == 0) {
                                return(1);
                        }
                        break;
                case 233898774:
                        if (pathLen == 8 && memcmp(path, xvfs_example_data[0].name, pathLen) == 0) {
                                return(0);
                        }
                        break;
                case 285410170:
                        if (pathLen == 9 && memcmp(path, xvfs_example_data[4].name, pathLen) == 0) {
                                return(4);
                        }
                        break;
                case 1197082414:
                        if (pathLen == 19 && memcmp(path, xvfs_example_data[2].name, pathLen) == 0) {
                                return(2);
                        }
                        break;
                case 1596983380:
                        if (pathLen == 22 && memcmp(path, xvfs_example_data[3].name, pathLen) == 0) {
                                return(3);
                        }
                        break;

        }

        return(XVFS_NAME_LOOKUP_ERROR);
    }

    static const char **xvfs_example_getChildren(const char *path, Tcl_WideInt *count) {
        struct xvfs_file_data *fileInfo;
        long inode;

        /*
         * Validate input parameters
         */
        if (count == NULL) {
                return(NULL);
        }

        /*
         * Get the inode from the lookup function
         */
        inode = xvfs_example_nameToIndex(path);
        if (inode == XVFS_NAME_LOOKUP_ERROR) {
                *count = XVFS_RV_ERR_ENOENT;
                return(NULL);
        }

        fileInfo = &xvfs_example_data[inode];

        /*
         * Ensure this is a directory
         */
        if (fileInfo->type != XVFS_FILE_TYPE_DIR) {
                *count = XVFS_RV_ERR_ENOTDIR;
                return(NULL);
        }

        *count = fileInfo->size;
        return(fileInfo->data.dirChildren);
    }

    static const unsigned char *xvfs_example_getData(const char *path, Tcl_WideInt start, Tcl_WideInt *length) {
        struct xvfs_file_data *fileInfo;
        Tcl_WideInt resultLength;
        long inode;

        /*
         * Validate input parameters
         */
        if (length == NULL) {
                return(NULL);
        }

        if (start < 0) {
                *length = XVFS_RV_ERR_EINVAL;
                return(NULL);
        }

        if (*length < 0) {
                *length = XVFS_RV_ERR_EINVAL;
                return(NULL);
        }

        /*
         * Get the inode from the lookup function
         */
        inode = xvfs_example_nameToIndex(path);
        if (inode == XVFS_NAME_LOOKUP_ERROR) {
                *length = XVFS_RV_ERR_ENOENT;
                return(NULL);
        }

        fileInfo = &xvfs_example_data[inode];

        /*
         * Ensure this is a file that can be read
         */
        if (fileInfo->type != XVFS_FILE_TYPE_REG) {
                *length = XVFS_RV_ERR_EISDIR;
                return(NULL);
        }

        /*
         * Validate the length
         */
        if (start > fileInfo->size) {
                *length = XVFS_RV_ERR_EFAULT;
                return(NULL);
        }

        if (*length == 0) {
                resultLength = fileInfo->size - start;
        } else {
                resultLength = MIN(fileInfo->size - start, *length);
        }
        *length = resultLength;

        /*
         * Return the data
         */
        return(fileInfo->data.fileContents + start);
    }

    static int xvfs_example_getStat(const char *path, Tcl_StatBuf *statBuf) {
        struct xvfs_file_data *fileInfo;
        long inode;

        /*
         * Validate input parameters
         */
        if (!statBuf) {
                return(XVFS_RV_ERR_EINVAL);
        }

        /*
         * Get the inode from the lookup function
         */
        inode = xvfs_example_nameToIndex(path);
        if (inode == XVFS_NAME_LOOKUP_ERROR) {
                return(XVFS_RV_ERR_ENOENT);
        }

        fileInfo = &xvfs_example_data[inode];

        statBuf->st_dev   = 0;
        statBuf->st_rdev  = 0;
        statBuf->st_ino   = inode;
        statBuf->st_uid   = -1;
        statBuf->st_gid   = -1;
        statBuf->st_atime = 0;
        statBuf->st_ctime = 0;
        statBuf->st_mtime = 0;
    #ifdef HAVE_STRUCT_STAT_ST_BLKSIZE
        statBuf->st_blksize = XVFS_FILE_BLOCKSIZE;
    #endif

        if (fileInfo->type == XVFS_FILE_TYPE_REG) {
                statBuf->st_mode   = 0100444;
                statBuf->st_nlink  = 1;
                statBuf->st_size   = fileInfo->size;
    #ifdef HAVE_STRUCT_STAT_ST_BLOCKS
                statBuf->st_blocks = (fileInfo->size + statBuf->st_blksize - 1) / statBuf->st_blksize;
    #endif
        } else if (fileInfo->type == XVFS_FILE_TYPE_DIR) {
                statBuf->st_mode   = 040555;
                statBuf->st_nlink  = fileInfo->size;
                statBuf->st_size   = fileInfo->size;
    #ifdef HAVE_STRUCT_STAT_ST_BLOCKS
                statBuf->st_blocks = 1;
    #endif
        }

        return(0);
    }

    static struct Xvfs_FSInfo xvfs_example_fsInfo = {
        .protocolVersion = XVFS_PROTOCOL_VERSION,
        .name            = "example",
        .getChildrenProc = xvfs_example_getChildren,
        .getDataProc     = xvfs_example_getData,
        .getStatProc     = xvfs_example_getStat
    };

    int Xvfs_example_Init(Tcl_Interp *interp) {
        int register_ret;

    #ifdef USE_TCL_STUBS
        const char *tclInitStubs_ret;
        /* Initialize Stubs */
        tclInitStubs_ret = Tcl_InitStubs(interp, TCL_PATCH_LEVEL, 0);
        if (!tclInitStubs_ret) {
                return(TCL_ERROR);
        }
    #endif

        register_ret = Xvfs_Register(interp, &xvfs_example_fsInfo);
        if (register_ret != TCL_OK) {
                return(register_ret);
        }

        return(TCL_OK);
    }
    #undef XVFS_NAME_LOOKUP_ERROR
    #undef XVFS_FILE_BLOCKSIZE
    $

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