path: root/ecos/packages/io/fileio/current/doc/fileio.sgml

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<part id="fileio">
<title>File System Support Infrastructure</title>

<!-- {{{ Introduction -->

<chapter id="fileio-intro">
<title>Introduction</title>

<para>
This document describes the filesystem infrastructure provided in
eCos. This is implemented by the FILEIO package and provides POSIX
compliant file and IO operations together with the BSD socket
API. These APIs are described in the relevant standards and original
documentation and will not be described here. See <xref
linkend="posix-standard-support"> for details of which parts of the
POSIX standard are supported.
</para>

<para>
This document is concerned with the interfaces presented to client
filesystems and network protocol stacks.
</para>

<para>
The FILEIO infrastructure consist mainly of a set of tables containing
pointers to the primary interface functions of a file system. This
approach avoids problems of namespace pollution (for example several
filesystems can have a function called <function>read()</function>, so long as they are
static). The system is also structured to eliminate the need for
dynamic memory allocation.
</para>

<para>
New filesystems can be written directly to the interfaces described
here. Existing filesystems can be ported very easily by the
introduction of a thin veneer porting layer that translates FILEIO
calls into native filesystem calls. 
</para>

<para>
The term filesystem should be read fairly loosely in this
document. Object accessed through these interfaces could equally be
network protocol sockets, device drivers, fifos, message queues or any
other object that can present a file-like interface.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ File System Table -->

<chapter id="fileio-fstab">
<title>File System Table</title>

<para>
The filesystem table is an array of entries that describe each
filesystem implementation that is part of the system image. Each
resident filesystem should export an entry to this table using the
<literal>FSTAB_ENTRY()</literal> macro.
</para>

<note>
<title>Note</title>
<para>
At present we do not support dynamic addition or removal of table
entries. However, an API similar to <function>mount()</function> would
allow new entries to be added to the table.
</para>
</note>

<para>
The table entries are described by the following structure:
</para>

<programlisting>
struct cyg_fstab_entry
{
    const char          *name;          // filesystem name
    CYG_ADDRWORD        data;           // private data value
    cyg_uint32          syncmode;       // synchronization mode
    
    int     (*mount)    ( cyg_fstab_entry *fste, cyg_mtab_entry *mte );
    int     (*umount)   ( cyg_mtab_entry *mte );
    int     (*open)     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          int mode,  cyg_file *fte );
    int     (*unlink)   ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
    int     (*mkdir)    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
    int     (*rmdir)    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
    int     (*rename)   ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
                          cyg_dir dir2, const char *name2 );
    int     (*link)     ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
                          cyg_dir dir2, const char *name2, int type );
    int     (*opendir)  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          cyg_file *fte );
    int     (*chdir)    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          cyg_dir *dir_out );
    int     (*stat)     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          struct stat *buf);
    int     (*getinfo)  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          int key, char *buf, int len );
    int     (*setinfo)  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                          int key, char *buf, int len );
};
</programlisting>

<para>
The <structfield>name</structfield> field points to a string that
identifies this filesystem implementation. Typical values might be
&quot;romfs&quot;, &quot;msdos&quot;, &quot;ext2&quot; etc.
</para>

<para>
The <structfield>data</structfield> field contains any private data
that the filesystem needs, perhaps the root of its data structures.
</para>

<para>
The <structfield>syncmode</structfield> field contains a description of
the locking protocol to be used when accessing this filesystem. It
will be described in more detail in <xref linkend="fileio-synchronization">.
</para>

<para>
The remaining fields are pointers to functions that implement
filesystem operations that apply to files and directories as whole
objects. The operation implemented by each function should be obvious
from the names, with a few exceptions:
</para>

<para>
The <function>opendir()</function> function pointer opens a directory
for reading. See <xref linkend="fileio-directories"> for details.
</para>

<para>
The <function>getinfo()</function> and
<function>setinfo()</function> function pointers provide support for
various minor control and information functions such as
<function>pathconf()</function> and <function>access()</function>.
</para>

<para>
With the exception of the <function>mount()</function> and
<function>umount()</function> functions, all of these functions
take three standard arguments, a pointer to a mount table entry (see
later) a directory pointer (also see later) and a file name relative
to the directory. These should be used by the filesystem to locate the
object of interest.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Mount Table -->

<chapter id="fileio-mount-table">
<title>Mount Table</title>

<para>
The mount table records the filesystems that are actually active.
These can be seen as being analogous to mount points in Unix systems.
</para>

<para>
There are two sources of mount table entries. Filesystems (or other
components) may export static entries to the table using the
<literal>MTAB_ENTRY()</literal> macro. Alternatively, new entries may
be installed at run time using the <function>mount()</function>
function. Both types of entry may be unmounted with the
<function>umount()</function> function.
</para>

<para>
A mount table entry has the following structure:
</para>

<programlisting>
struct cyg_mtab_entry
{
    const char          *name;          // name of mount point
    const char          *fsname;        // name of implementing filesystem
    const char          *devname;       // name of hardware device
    CYG_ADDRWORD        data;           // private data value
    cyg_bool            valid;          // Valid entry?
    cyg_fstab_entry     *fs;            // pointer to fstab entry
    cyg_dir             root;           // root directory pointer
};
</programlisting>

<para>
The <structfield>name</structfield> field identifies the mount
point. This is used to direct rooted filenames (filenames that
begin with &quot;/&quot;) to the correct filesystem. When a file
name that begins with &quot;/&quot; is submitted, it is matched
against the <structfield>name</structfield> fields of all valid mount
table entries. The entry that yields the longest match terminating
before a &quot;/&quot;, or end of string, wins and the appropriate
function from the filesystem table entry is then passed the remainder
of the file name together with a pointer to the table entry and the
value of the <structfield>root</structfield> field as the directory
pointer.
</para>

<para>
For example, consider a mount table that contains the following
entries:
</para>

<programlisting>
	{ "/",    "msdos", "/dev/hd0", ... }
	{ "/fd",  "msdos", "/dev/fd0", ... }
	{ "/rom", "romfs", "", ... }
	{ "/tmp", "ramfs", "", ... }
	{ "/dev", "devfs", "", ... }
</programlisting>

<para>
An attempt to open &quot;/tmp/foo&quot; would be directed to the RAM
filesystem while an open of &quot;/bar/bundy&quot; would be directed
to the hard disc MSDOS filesystem. Opening &quot;/dev/tty0&quot; would
be directed to the device management filesystem for lookup in the
device table.
</para>

<para>
Unrooted file names (those that do not begin with a '/') are passed
straight to the filesystem that contains the current directory. The
current directory is represented by a pair consisting of a mount table
entry and a directory pointer.
</para>

<para>
The <structfield>fsname</structfield> field points to a string that
should match the <structfield>name</structfield> field of the
implementing filesystem. During initialization the mount table is
scanned and the <structfield>fsname</structfield> entries looked up in
the filesystem table. For each match, the filesystem's _mount_
function is called and if successful the mount table entry is marked
as valid and the <structfield>fs</structfield> pointer installed.
</para>

<para>
The <structfield>devname</structfield> field contains the name of the
device that this filesystem is to use. This may match an entry in the
device table (see later) or may be a string that is specific to the
filesystem if it has its own internal device drivers.
</para>

<para>
The <structfield>data</structfield> field is a private data value. This
may be installed either statically when the table entry is defined, or
may be installed during the <function>mount()</function> operation.
</para>

<para>
The <structfield>valid</structfield> field indicates whether this mount
point has actually been mounted successfully. Entries with a false
<structfield>valid</structfield> field are ignored when searching for a
name match.
</para>

<para>
The <structfield>fs</structfield> field is installed after a successful
<function>mount()</function> operation to point to the implementing
filesystem.
</para>

<para>
The <structfield>root</structfield> field contains a directory pointer
value that the filesystem can interpret as the root of its directory
tree. This is passed as the <parameter>dir</parameter> argument of
filesystem functions that operate on rooted filenames. This field must
be initialized by the filesystem's <function>mount()</function>
function.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ File Table -->

<chapter id="fileio-file-table">
<title>File Table</title>

<para>
Once a file has been opened it is represented by an open file
object. These are allocated from an array of available file
objects. User code accesses these open file objects via a second array
of pointers which is indexed by small integer offsets. This gives the
usual Unix file descriptor functionality, complete with the various
duplication mechanisms.
</para>

<para>
A file table entry has the following structure:
</para>

<programlisting>
struct CYG_FILE_TAG
{
    cyg_uint32	                f_flag;		/* file state                   */
    cyg_uint16                  f_ucount;       /* use count                    */
    cyg_uint16                  f_type;		/* descriptor type              */
    cyg_uint32                  f_syncmode;     /* synchronization protocol     */
    struct CYG_FILEOPS_TAG      *f_ops;         /* file operations              */
    off_t       	        f_offset;       /* current offset               */
    CYG_ADDRWORD	        f_data;		/* file or socket               */
    CYG_ADDRWORD                f_xops;         /* extra type specific ops      */
    cyg_mtab_entry              *f_mte;         /* mount table entry            */
};
</programlisting>

<para>
The <structfield>f_flag</structfield> field contains some FILEIO
control bits and some bits propagated from the
<parameter>flags</parameter> argument of the
<function>open()</function> call (defined by
<literal>CYG_FILE_MODE_MASK</literal>).
</para>

<para>
The <structfield>f_ucount</structfield> field contains a use count that
controls when a file will be closed. Each duplicate in the file
descriptor array counts for one reference here. It is also
incremented around each I/O operation to ensure that the file cannot
be closed while it has current I/O operations.
</para>

<para>
The <structfield>f_type</structfield> field indicates the type of the
underlying file object. Some of the possible values here are
<literal>CYG_FILE_TYPE_FILE</literal>,
<literal>CYG_FILE_TYPE_SOCKET</literal> or <literal>CYG_FILE_TYPE_DEVICE</literal>.
</para>

<para>
The <structfield>f_syncmode</structfield> field is copied from the
<structfield>syncmode</structfield> field of the implementing
filesystem. Its use is described in <xref linkend="fileio-synchronization">.
</para>

<para>
The <structfield>f_offset</structfield> field records the current file
position. It is the responsibility of the file operation functions to
keep this field up to date.
</para>

<para>
The <structfield>f_data</structfield> field contains private data
placed here by the underlying filesystem. Normally this will be a
pointer to, or handle on, the filesystem object that implements this
file.
</para>

<para>
The <structfield>f_xops</structfield> field contains a pointer to any
extra type specific operation functions. For example, the socket I/O
system installs a pointer to a table of functions that implement the
standard socket operations.
</para>

<para>
The <structfield>f_mte</structfield> field contains a pointer to the
parent mount table entry for this file. It is used mainly to implement
the synchronization protocol. This may contain a pointer to some other
data structure in file objects not derived from a filesystem.
</para>

<para>
The <structfield>f_ops</structfield> field contains a pointer to a
table of file I/O operations. This has the following structure:
</para>

<programlisting>
struct CYG_FILEOPS_TAG
{
        int	(*fo_read)      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
        int	(*fo_write)     (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
        int     (*fo_lseek)     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
        int	(*fo_ioctl)     (struct CYG_FILE_TAG *fp, CYG_ADDRWORD com,
                                 CYG_ADDRWORD data);
        int	(*fo_select)    (struct CYG_FILE_TAG *fp, int which, CYG_ADDRWORD info);
        int     (*fo_fsync)     (struct CYG_FILE_TAG *fp, int mode );        
        int	(*fo_close)     (struct CYG_FILE_TAG *fp);
        int     (*fo_fstat)     (struct CYG_FILE_TAG *fp, struct stat *buf );
        int     (*fo_getinfo)   (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
        int     (*fo_setinfo)   (struct CYG_FILE_TAG *fp, int key, char *buf, int len );
};
</programlisting>

<para>
It should be obvious from the names of most of these functions what
their responsibilities are. The <function>fo_getinfo()</function>
and <function>fo_setinfo()</function> function pointers, like their
counterparts in the filesystem structure, implement minor control and
info functions such as <function>fpathconf()</function>.
</para>

<para>
The second argument to the <function>fo_read()</function> and
<function>fo_write()</function> function pointers is a pointer to a
UIO structure:
</para>

<programlisting>
struct CYG_UIO_TAG
{
    struct CYG_IOVEC_TAG *uio_iov;	/* pointer to array of iovecs */
    int	                 uio_iovcnt;	/* number of iovecs in array */
    off_t       	 uio_offset;	/* offset into file this uio corresponds to */
    ssize_t     	 uio_resid;	/* residual i/o count */
    enum cyg_uio_seg     uio_segflg;    /* see above */
    enum cyg_uio_rw      uio_rw;        /* see above */
};

struct CYG_IOVEC_TAG
{
    void           *iov_base;           /* Base address. */
    ssize_t        iov_len;             /* Length. */
};
</programlisting>

<para>
This structure encapsulates the parameters of any data transfer
operation. It provides support for scatter/gather operations and
records the progress of any data transfer. It is also compatible with
the I/O operations of any BSD-derived network stacks and filesystems.
</para>

<para>
When a file is opened (or a file object created by some other means,
such as <function>socket()</function> or <function>accept()</function>) it is the
responsibility of the filesystem open operation to initialize all the
fields of the object except the <structfield>f_ucount</structfield>,
<structfield>f_syncmode</structfield> and
<structfield>f_mte</structfield> fields. Since the
<structfield>f_flag</structfield> field will already contain bits belonging to the FILEIO
infrastructure, any changes to it must be made with the appropriate
logical operations.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Directories -->

<chapter id="fileio-directories">
<title>Directories</title>

<para>
Filesystem operations all take a directory pointer as one of their
arguments.  A directory pointer is an opaque handle managed by the
filesystem. It should encapsulate a reference to a specific directory
within the filesystem. For example, it may be a pointer to the data
structure that represents that directory (such as an inode), or a
pointer to a pathname for the directory.
</para>

<para>
The <function>chdir()</function> filesystem function pointer has two
modes of use. When passed a pointer in the
<parameter>dir_out</parameter> argument, it should locate the named
directory and place a directory pointer there. If the
<parameter>dir_out</parameter> argument is NULL then the
<parameter>dir</parameter> argument is a previously generated
directory pointer that can now be disposed of. When the infrastructure
is implementing the <function>chdir()</function> function it makes two
calls to filesystem <function>chdir()</function> functions. The first
is to get a directory pointer for the new current directory. If this
succeeds the second is to dispose of the old current directory
pointer.
</para>

<para>
The <function>opendir()</function> function is used to open a
directory for reading. This results in an open file object that can be
read to return a sequence of <structname>struct dirent</structname>
objects. The only operations that are allowed on this file are
<function>read</function>, <function>lseek</function> and
<function>close</function>. Each read operation on this file should
return a single <structname>struct dirent</structname> object. When
the end of the directory is reached, zero should be returned. The only
seek operation allowed is a rewind to the start of the directory, by
supplying an offset of zero and a <parameter>whence</parameter>
specifier of <literal>SEEK_SET</literal>.
</para>

<para>
Most of these considerations are invisible to clients of a filesystem
since they will access directories via the POSIX
<function>opendir()</function>, <function>readdir()</function> and
<function>closedir()</function> functions. The <structname> struct
dirent</structname> object returned by <function>readdir()</function>
will always contain <structname>d_name</structname> as required by
POSIX. When <literal>CYGPKG_FILEIO_DIRENT_DTYPE</literal> is enabled
it will also contain <structname>d_type</structname>, which is not
part of POSIX, but often implemented by OSes. Currently only the
FATFS, RAMFS, ROMFS and JFFS2 filesystem sets this value. For other
filesystems a value of 0 will be returned in the member.</para>

<para>
Support for the <function>getcwd()</function> function is provided by
three mechanisms.  The first is to use the
<literal>FS_INFO_GETCWD</literal> getinfo key on the filesystem to use
any internal support that it has for this. If that fails it falls back
on one of the two other mechanisms. If
<literal>CYGPKG_IO_FILEIO_TRACK_CWD</literal> is set then the current
directory is tracked textually in <function>chdir()</function> and the result of that is
reported in getcwd(). Otherwise an attempt is made to traverse the
directory tree to its root using &quot;..&quot; entries.
</para>

<para>
This last option is complicated and expensive, and relies on the
filesystem supporting &quot;.&quot; and &quot;..&quot;  entries. This is not always the
case, particularly if the filesystem has been ported from a
non-UNIX-compatible source. Tracking the pathname textually will
usually work, but might not produce optimum results when symbolic
links are being used.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Synchronization -->

<chapter id="fileio-synchronization">
<title>Synchronization</title>

<para>
The FILEIO infrastructure provides a synchronization mechanism for
controlling concurrent access to filesystems. This allows existing
filesystems to be ported to eCos, even if they do not have their own
synchronization mechanisms. It also allows new filesystems to be
implemented easily without having to consider the synchronization
issues.
</para>

<para>
The infrastructure maintains a mutex for each entry in each of
the main tables: filesystem table, mount table and file table. For
each class of operation each of these mutexes may be locked before the
corresponding filesystem operation is invoked.
</para>

<para>
The synchronization protocol required by a filesystem is described
by the <structfield>syncmode</structfield> field of the filesystem
table entry. This is a combination of the following flags:
</para>

<variablelist>
<varlistentry>
<term><literal>CYG_SYNCMODE_FILE_FILESYSTEM</literal></term>
<listitem>
<para>
Lock the filesystem table entry mutex
during all filesystem level operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_FILE_MOUNTPOINT</literal></term>
<listitem>
<para>
Lock the mount table entry mutex
during all filesystem level operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_IO_FILE</literal></term>
<listitem>
<para>
Lock the file table entry mutex during all
I/O operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_IO_FILESYSTEM</literal></term>
<listitem>
<para>
Lock the filesystem table entry mutex during all I/O operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_IO_MOUNTPOINT</literal></term>
<listitem><para>
Lock the mount table entry mutex during all I/O operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_SOCK_FILE</literal></term>
<listitem>
<para>
Lock the file table entry mutex during all socket operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_SOCK_NETSTACK</literal></term>
<listitem>
<para>
Lock the network stack table entry mutex during all socket operations.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><literal>CYG_SYNCMODE_NONE</literal></term>
<listitem>
<para>
Perform no locking at all during any operations.
</para>
</listitem>
</varlistentry>

</variablelist>

<para>
The value of the <structfield>syncmode</structfield> field in the
filesystem table entry will be copied by the infrastructure to the
open file object after a successful <function>open()</function> operation.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Initialization and Mounting -->

<chapter id="fileio-mounting">
<title>Initialization and Mounting</title>

<para>
As mentioned previously, mount table entries can be sourced from two
places. Static entries may be defined by using the
<literal>MTAB_ENTRY()</literal> macro. Such entries will be
automatically mounted on system startup.  For each entry in the mount
table that has a non-null <structfield>name</structfield> field the
filesystem table is searched for a match with the
<structfield>fsname</structfield> field. If a match is found the
filesystem's <structfield>mount</structfield> entry is called and if
successful the mount table entry marked valid and the
<structfield>fs</structfield> field initialized. The
<function>mount()</function> function is responsible for initializing
the <structfield>root</structfield> field.
</para>


<para>
The size of the mount table is defined by the configuration value
<literal>CYGNUM_FILEIO_MTAB_MAX</literal>. Any entries that have not
been statically defined are available for use by dynamic mounts.
</para>

<para>
A filesystem may be mounted dynamically by calling <function>mount()</function>. This
function has the following prototype:
</para>

<programlisting width=72>
int mount( const char *devname,
           const char *dir,
	   const char *fsname);
</programlisting>

<para>
The <parameter>devname</parameter> argument identifies a device that
will be used by this filesystem and will be assigned to the
<structfield>devname</structfield> field of the mount table entry.
</para>

<para>
The <parameter>dir</parameter> argument is the mount point name, it
will be assigned to the <structfield>name</structfield> field of the
mount table entry.
</para>

<para>
The <parameter>fsname</parameter> argument is the name of the
implementing filesystem, it will be assigned to the
<structfield>fsname</structfield> entry of the mount table entry.
</para>

<para>
The process of mounting a filesystem dynamically is as follows. First
a search is made of the mount table for an entry with a NULL
<structfield>name</structfield> field to be used for the new mount
point. The filesystem table is then searched for an entry whose name
matches <structfield>fsname</structfield>. If this is successful then
the mount table entry is initialized and the filesystem's
<function>mount()</function> operation called. If this is successful,
the mount table entry is marked valid and the
<structfield>fs</structfield> field initialized.
</para>

<para>
Unmounting a filesystem is done by the <function>umount()</function>
function. This can unmount filesystems whether they were mounted
statically or dynamically.
</para>

<para>
The <function>umount()</function> function has the following prototype:
</para>

<programlisting width=72>
int umount( const char *name );
</programlisting>

<para>
The mount table is searched for a match between the
<parameter>name</parameter> argument and the entry
<structfield>name</structfield> field. When a match is found the
filesystem's <function>umount()</function> operation is called and if
successful, the mount table entry is invalidated by setting its
<structfield>valid</structfield> field false and the
<structfield>name</structfield> field to NULL.
</para>

<!-- 


-->

</chapter>

<!-- }}} -->
<!-- {{{ Sockets -->

<chapter id="fileio-sockets">
<title>Sockets</title>

<para>
If a network stack is present, then the FILEIO infrastructure also
provides access to the standard BSD socket calls.
</para>

<para>
The netstack table contains entries which describe the network
protocol stacks that are in the system image. Each resident stack
should export an entry to this table using the
<literal>NSTAB_ENTRY()</literal> macro.
</para>

<para>
Each table entry has the following structure:
</para>

<programlisting width=72>
struct cyg_nstab_entry
{
    cyg_bool            valid;          // true if stack initialized
    cyg_uint32          syncmode;       // synchronization protocol
    char                *name;          // stack name
    char                *devname;       // hardware device name
    CYG_ADDRWORD        data;           // private data value

    int     (*init)( cyg_nstab_entry *nste );
    int     (*socket)( cyg_nstab_entry *nste, int domain, int type,
		       int protocol, cyg_file *file );
};
</programlisting>

<para>
This table is analogous to a combination of the filesystem and mount
tables.
</para>

<para>
The <structfield>valid</structfield> field is set
<literal>true</literal> if the stack's <function>init()</function>
function returned successfully and the
<structfield>syncmode</structfield> field contains the
<literal>CYG_SYNCMODE_SOCK_*</literal> bits described above.
</para>

<!--


-->

<para>
The <structfield>name</structfield> field contains the name of the
protocol stack.
</para>

<!--


-->

<para>
The <structfield>devname</structfield> field names the device that the stack is using. This may
reference a device under &quot;/dev&quot;, or may be a name that is only
meaningful to the stack itself.
</para>

<!--


-->

<para>
The <function>init()</function> function pointer is called during
system initialization to start the protocol stack running. If it
returns non-zero the <structfield>valid</structfield> field is set
false and the stack will be ignored subsequently.
</para>

<para>
The <function>socket()</function> function is called to attempt to create a socket in the
stack. When the <function>socket()</function> API function is called the netstack table is
scanned and for each valid entry the <function>socket()</function>
function pointer is called. If
this returns non-zero then the scan continues to the next valid stack,
or terminates with an error if the end of the table is reached.
</para>

<para>
The result of a successful socket call is an initialized file object
with the <structfield>f_xops</structfield> field pointing to the
following structure:
</para>

<programlisting width=72>
struct cyg_sock_ops
{
    int (*bind)      ( cyg_file *fp, const sockaddr *sa, socklen_t len );
    int (*connect)   ( cyg_file *fp, const sockaddr *sa, socklen_t len );
    int (*accept)    ( cyg_file *fp, cyg_file *new_fp,
                       struct sockaddr *name, socklen_t *anamelen );
    int (*listen)    ( cyg_file *fp, int len );
    int (*getname)   ( cyg_file *fp, sockaddr *sa, socklen_t *len, int peer );
    int (*shutdown)  ( cyg_file *fp, int flags );
    int (*getsockopt)( cyg_file *fp, int level, int optname,
                       void *optval, socklen_t *optlen);
    int (*setsockopt)( cyg_file *fp, int level, int optname,
                       const void *optval, socklen_t optlen);
    int (*sendmsg)   ( cyg_file *fp, const struct msghdr *m,
                       int flags, ssize_t *retsize );
    int (*recvmsg)   ( cyg_file *fp, struct msghdr *m,
                       socklen_t *namelen, ssize_t *retsize );
};
</programlisting>

<para>
It should be obvious from the names of these functions which API calls
they provide support for. The <function>getname()</function> function
pointer provides support for both <function>getsockname()</function>
and <function>getpeername()</function> while the
<function>sendmsg()</function> and <function>recvmsg()</function>
function pointers provide support for <function>send()</function>,
<function>sendto()</function>, <function>sendmsg()</function>,
<function>recv()</function>, <function>recvfrom()</function> and
<function>recvmsg()</function> as appropriate.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Select -->

<chapter id="fileio-select">
<title>Select</title>

<para>
The infrastructure provides support for implementing a select
mechanism. This is modeled on the mechanism in the BSD kernel, but has
been modified to make it implementation independent.
</para>

<para>
The main part of the mechanism is the <function>select()</function>
API call. This processes its arguments and calls the
<function>fo_select()</function> function pointer on all file objects
referenced by the file descriptor sets passed to it. If the same
descriptor appears in more than one descriptor set, the
<function>fo_select()</function> function will be called separately
for each appearance.
</para>

<para>
The <parameter>which</parameter> argument of the
<function>fo_select()</function> function will either be
<literal>CYG_FREAD</literal> to test for read conditions,
<literal>CYG_FWRITE</literal> to test for write conditions or zero to
test for exceptions. For each of these options the function should
test whether the condition is satisfied and if so return true. If it
is not satisfied then it should call
<function>cyg_selrecord()</function> with the
<parameter>info</parameter> argument that was passed to the function
and a pointer to a <structname>cyg_selinfo</structname> structure.
</para>

<para>
The <structname>cyg_selinfo</structname> structure is used to record information about current
select operations. Any object that needs to support select must
contain an instance of this structure.  Separate <structname>cyg_selinfo</structname>
structures should be kept for each of the options that the object can
select on - read, write or exception.
</para>

<para>
If none of the file objects report that the select condition is
satisfied, then the <function>select()</function> API function puts
the calling thread to sleep waiting either for a condition to become
satisfied, or for the optional timeout to expire.
</para>

<para>
A selectable object must have some asynchronous activity that may
cause a select condition to become true - either via interrupts or the
activities of other threads. Whenever a selectable condition is
satisfied, the object should call <function>cyg_selwakeup()</function> with a pointer to
the appropriate <structname>cyg_selinfo</structname> structure. If the thread is still waiting,
this will cause it to wake up and repeat its poll of the file
descriptors. This time around, the object that caused the wakeup
should indicate that the select condition is satisfied, and the
<function>select()</function> API call will return.
</para>

<para>
Note that <function>select()</function> does not exhibit real time
behaviour: the iterative poll of the descriptors, and the wakeup
mechanism mitigate against this. If real time response to device or
socket I/O is required then separate threads should be devoted to each
device of interest and should use blocking calls to wait for a
condition to become ready.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Devices -->

<chapter id="fileio-devices">
<title>Devices</title>

<para>
Devices are accessed by means of a pseudo-filesystem, &quot;devfs&quot;, that is
mounted on &quot;/dev&quot;. Open operations are translated into calls to
<function>cyg_io_lookup()</function> and if successful result in a file object whose
<structfield>f_ops</structfield> functions translate filesystem API functions into calls into
the device API.
</para>

</chapter>

<!-- }}} -->
<!-- {{{ Writing a New Filesystem -->

<chapter id="fileio-writing">
<title>Writing a New Filesystem</title>

<para>
To create a new filesystem it is necessary to define the fstab entry
and the file IO operations. The easiest way to do this is to copy an
existing filesystem: either the test filesystem in the FILEIO package,
or the RAM or ROM filesystem packages.
</para>

<para>
To make this clearer, the following is a brief tour of the FILEIO
relevant parts of the RAM filesystem.
</para>

<para>
First, it is necessary to provide forward definitions of the functions
that constitute the filesystem interface:
</para>

<programlisting width=72>
//==========================================================================
// Forward definitions

// Filesystem operations
static int ramfs_mount    ( cyg_fstab_entry *fste, cyg_mtab_entry *mte );
static int ramfs_umount   ( cyg_mtab_entry *mte );
static int ramfs_open     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             int mode,  cyg_file *fte );
static int ramfs_unlink   ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
static int ramfs_mkdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
static int ramfs_rmdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name );
static int ramfs_rename   ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
                             cyg_dir dir2, const char *name2 );
static int ramfs_link     ( cyg_mtab_entry *mte, cyg_dir dir1, const char *name1,
                             cyg_dir dir2, const char *name2, int type );
static int ramfs_opendir  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             cyg_file *fte );
static int ramfs_chdir    ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             cyg_dir *dir_out );
static int ramfs_stat     ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             struct stat *buf);
static int ramfs_getinfo  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             int key, void *buf, int len );
static int ramfs_setinfo  ( cyg_mtab_entry *mte, cyg_dir dir, const char *name,
                             int key, void *buf, int len );

// File operations
static int ramfs_fo_read      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
static int ramfs_fo_write     (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
static int ramfs_fo_lseek     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
static int ramfs_fo_ioctl     (struct CYG_FILE_TAG *fp, CYG_ADDRWORD com,
                                CYG_ADDRWORD data);
static int ramfs_fo_fsync     (struct CYG_FILE_TAG *fp, int mode );        
static int ramfs_fo_close     (struct CYG_FILE_TAG *fp);
static int ramfs_fo_fstat     (struct CYG_FILE_TAG *fp, struct stat *buf );
static int ramfs_fo_getinfo   (struct CYG_FILE_TAG *fp, int key, void *buf, int len );
static int ramfs_fo_setinfo   (struct CYG_FILE_TAG *fp, int key, void *buf, int len );

// Directory operations
static int ramfs_fo_dirread      (struct CYG_FILE_TAG *fp, struct CYG_UIO_TAG *uio);
static int ramfs_fo_dirlseek     (struct CYG_FILE_TAG *fp, off_t *pos, int whence );
</programlisting>

<para>
We define all of the fstab entries and all of the file IO
operations. We also define alternatives for the
<structfield>fo_read</structfield> and
<structfield>fo_lseek</structfield> file IO operations.
</para>

<para>
We can now define the filesystem table entry. There is a macro,
<literal>FSTAB_ENTRY</literal> to do this:
</para>


<programlisting width=72>
//==========================================================================
// Filesystem table entries

// -------------------------------------------------------------------------
// Fstab entry.
// This defines the entry in the filesystem table.
// For simplicity we use _FILESYSTEM synchronization for all accesses since
// we should never block in any filesystem operations.

FSTAB_ENTRY( ramfs_fste, "ramfs", 0,
             CYG_SYNCMODE_FILE_FILESYSTEM|CYG_SYNCMODE_IO_FILESYSTEM,
             ramfs_mount,
             ramfs_umount,
             ramfs_open,
             ramfs_unlink,
             ramfs_mkdir,
             ramfs_rmdir,
             ramfs_rename,
             ramfs_link,
             ramfs_opendir,
             ramfs_chdir,
             ramfs_stat,
             ramfs_getinfo,
             ramfs_setinfo);
</programlisting>

<para>
The first argument to this macro gives the fstab entry a name, the
remainder are initializers for the field of the structure.
</para>

<para>
We must also define the file operations table that is installed in all
open file table entries:
</para>

<programlisting width=72>
// -------------------------------------------------------------------------
// File operations.
// This set of file operations are used for normal open files.

static cyg_fileops ramfs_fileops =
{
    ramfs_fo_read,
    ramfs_fo_write,
    ramfs_fo_lseek,
    ramfs_fo_ioctl,
    cyg_fileio_seltrue,
    ramfs_fo_fsync,
    ramfs_fo_close,
    ramfs_fo_fstat,
    ramfs_fo_getinfo,
    ramfs_fo_setinfo
};
</programlisting>

<para>
These all point to functions supplied by the filesystem except the
<structfield>fo_select</structfield> field which is filled with a
pointer to <function>cyg_fileio_seltrue()</function>. This is provided
by the FILEIO package and is a select function that always returns
true to all operations.
</para>

<para>
Finally, we need to define a set of file operations for use when
reading directories. This table only defines the
<structfield>fo_read</structfield> and
<structfield>fo_lseek</structfield> operations. The rest are filled
with stub functions supplied by the FILEIO package that just return an
error code.
</para>

<programlisting width=72>
// -------------------------------------------------------------------------
// Directory file operations.
// This set of operations are used for open directories. Most entries
// point to error-returning stub functions. Only the read, lseek and
// close entries are functional.

static cyg_fileops ramfs_dirops =
{
    ramfs_fo_dirread,
    (cyg_fileop_write *)cyg_fileio_enosys,
    ramfs_fo_dirlseek,
    (cyg_fileop_ioctl *)cyg_fileio_enosys,
    cyg_fileio_seltrue,
    (cyg_fileop_fsync *)cyg_fileio_enosys,
    ramfs_fo_close,
    (cyg_fileop_fstat *)cyg_fileio_enosys,
    (cyg_fileop_getinfo *)cyg_fileio_enosys,
    (cyg_fileop_setinfo *)cyg_fileio_enosys
};
</programlisting>

<para>
If the filesystem wants to have an instance automatically mounted on
system startup, it must also define a mount table entry. This is done
with the <literal>MTAB_ENTRY</literal> macro. This is an example from
the test filesystem of how this is used:
</para>

<programlisting width=72>
MTAB_ENTRY( testfs_mte1,
                   "/",
                   "testfs",
                   "",
                   0);
</programlisting>

<para>
The first argument provides a name for the table entry. The following
arguments provide initialization for the
<structfield>name</structfield>, <structfield>fsname</structfield>,
<structfield>devname</structfield> and <structfield>data</structfield>
fields respectively.
</para>

<para>
These definitions are adequate to let the new filesystem interact
with the FILEIO package. The new filesystem now needs to be fleshed
out with implementations of the functions defined above. Obviously,
the exact form this takes will depend on what the filesystem is
intended to do. Take a look at the RAM and ROM filesystems for
examples of how this has been done.
</para>

</chapter>

<!-- }}} -->

</part>