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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 

linkend="posix-standard-support"> for details of which parts of the

POSIX standard are supported.

This document is concerned with the interfaces presented to client

filesystems and network protocol stacks.

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 read(), so long as they are

static). The system is also structured to eliminate the need for

dynamic memory allocation.

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.

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.

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

FSTAB_ENTRY() macro.

At present we do not support dynamic addition or removal of table

entries. However, an API similar to mount() would

allow new entries to be added to the table.

The table entries are described by the following structure:

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 );

};

The name field points to a string that

identifies this filesystem implementation. Typical values might be

"romfs", "msdos", "ext2" etc.

The data field contains any private data

that the filesystem needs, perhaps the root of its data structures.

The syncmode field contains a description of

the locking protocol to be used when accessing this filesystem. It

will be described in more detail in .

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:

The opendir() function pointer opens a directory

for reading. See  for details.

The getinfo() and

setinfo() function pointers provide support for

various minor control and information functions such as

pathconf() and access().

With the exception of the mount() and

umount() 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.

The mount table records the filesystems that are actually active.

These can be seen as being analogous to mount points in Unix systems.

There are two sources of mount table entries. Filesystems (or other

components) may export static entries to the table using the

MTAB_ENTRY() macro. Alternatively, new entries may

be installed at run time using the mount()

function. Both types of entry may be unmounted with the

umount() function.

A mount table entry has the following structure:

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

};

The name field identifies the mount

point. This is used to direct rooted filenames (filenames that

begin with "/") to the correct filesystem. When a file

name that begins with "/" is submitted, it is matched

against the name fields of all valid mount

table entries. The entry that yields the longest match terminating

before a "/", 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 root field as the directory

pointer.

For example, consider a mount table that contains the following

entries:

        { "/",    "msdos", "/dev/hd0", ... }

        { "/fd",  "msdos", "/dev/fd0", ... }

        { "/rom", "romfs", "", ... }

        { "/tmp", "ramfs", "", ... }

        { "/dev", "devfs", "", ... }

An attempt to open "/tmp/foo" would be directed to the RAM

filesystem while an open of "/bar/bundy" would be directed

to the hard disc MSDOS filesystem. Opening "/dev/tty0" would

be directed to the device management filesystem for lookup in the

device table.

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.

The fsname field points to a string that

should match the name field of the

implementing filesystem. During initialization the mount table is

scanned and the fsname 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 fs pointer installed.

The devname 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.

The data field is a private data value. This

may be installed either statically when the table entry is defined, or

may be installed during the mount() operation.

The valid field indicates whether this mount

point has actually been mounted successfully. Entries with a false

valid field are ignored when searching for a

name match.

The fs field is installed after a successful

mount() operation to point to the implementing

filesystem.

The root field contains a directory pointer

value that the filesystem can interpret as the root of its directory

tree. This is passed as the dir argument of

filesystem functions that operate on rooted filenames. This field must

be initialized by the filesystem's mount()

function.

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.

A file table entry has the following structure:

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            */

};

The f_flag field contains some FILEIO

control bits and some bits propagated from the

flags argument of the

open() call (defined by

CYG_FILE_MODE_MASK).

The f_ucount 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.

The f_type field indicates the type of the

underlying file object. Some of the possible values here are

CYG_FILE_TYPE_FILE,

CYG_FILE_TYPE_SOCKET or CYG_FILE_TYPE_DEVICE.

The f_syncmode field is copied from the

syncmode field of the implementing

filesystem. Its use is described in .

The f_offset field records the current file

position. It is the responsibility of the file operation functions to

keep this field up to date.

The f_data 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.

The f_xops 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.

The f_mte 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.

The f_ops field contains a pointer to a

table of file I/O operations. This has the following structure:

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 );

};

It should be obvious from the names of most of these functions what

their responsibilities are. The fo_getinfo()

and fo_setinfo() function pointers, like their

counterparts in the filesystem structure, implement minor control and

info functions such as fpathconf().

The second argument to the fo_read() and

fo_write() function pointers is a pointer to a

UIO structure:

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. */

};

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.

When a file is opened (or a file object created by some other means,

such as socket() or accept()) it is the

responsibility of the filesystem open operation to initialize all the

fields of the object except the f_ucount,

f_syncmode and

f_mte fields. Since the

f_flag field will already contain bits belonging to the FILEIO

infrastructure, any changes to it must be made with the appropriate

logical operations.

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.

The chdir() filesystem function pointer has two

modes of use. When passed a pointer in the

dir_out argument, it should locate the named

directory and place a directory pointer there. If the

dir_out argument is NULL then the

dir argument is a previously generated

directory pointer that can now be disposed of. When the infrastructure

is implementing the chdir() function it makes two

calls to filesystem chdir() 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.

The opendir() 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 struct dirent

objects. The only operations that are allowed on this file are

read, lseek and

close. Each read operation on this file should

return a single struct dirent 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 whence

specifier of SEEK_SET.

Most of these considerations are invisible to clients of a filesystem

since they will access directories via the POSIX

opendir(), readdir() and

closedir() functions.

Support for the getcwd() function is provided by

three mechanisms.  The first is to use the

FS_INFO_GETCWD 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

CYGPKG_IO_FILEIO_TRACK_CWD is set then the current

directory is tracked textually in chdir() and the result of that is

reported in getcwd(). Otherwise an attempt is made to traverse the

directory tree to its root using ".." entries.

This last option is complicated and expensive, and relies on the

filesystem supporting "." and ".."  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.

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.

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.

The synchronization protocol required by a filesystem is described

by the syncmode field of the filesystem

table entry. This is a combination of the following flags:

CYG_SYNCMODE_FILE_FILESYSTEM

Lock the filesystem table entry mutex

during all filesystem level operations.

CYG_SYNCMODE_FILE_MOUNTPOINT

Lock the mount table entry mutex

during all filesystem level operations.

CYG_SYNCMODE_IO_FILE

Lock the file table entry mutex during all

I/O operations.

CYG_SYNCMODE_IO_FILESYSTEM

Lock the filesystem table entry mutex during all I/O operations.

CYG_SYNCMODE_IO_MOUNTPOINT

Lock the mount table entry mutex during all I/O operations.

CYG_SYNCMODE_SOCK_FILE

Lock the file table entry mutex during all socket operations.

CYG_SYNCMODE_SOCK_NETSTACK

Lock the network stack table entry mutex during all socket operations.

CYG_SYNCMODE_NONE

Perform no locking at all during any operations.

The value of the syncmode field in the

filesystem table entry will be copied by the infrastructure to the

open file object after a successful open() operation.

As mentioned previously, mount table entries can be sourced from two

places. Static entries may be defined by using the

MTAB_ENTRY() macro. Such entries will be

automatically mounted on system startup.  For each entry in the mount

table that has a non-null name field the

filesystem table is searched for a match with the

fsname field. If a match is found the

filesystem's mount entry is called and if

successful the mount table entry marked valid and the

fs field initialized. The

mount() function is responsible for initializing

the root field.

The size of the mount table is defined by the configuration value

CYGNUM_FILEIO_MTAB_MAX. Any entries that have not

been statically defined are available for use by dynamic mounts.

A filesystem may be mounted dynamically by calling mount(). This

function has the following prototype:

int mount( const char *devname,

           const char *dir,

           const char *fsname);

The devname argument identifies a device that

will be used by this filesystem and will be assigned to the

devname field of the mount table entry.

The dir argument is the mount point name, it

will be assigned to the name field of the

mount table entry.

The fsname argument is the name of the

implementing filesystem, it will be assigned to the

fsname entry of the mount table entry.

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

name field to be used for the new mount

point. The filesystem table is then searched for an entry whose name

matches fsname. If this is successful then

the mount table entry is initialized and the filesystem's

mount() operation called. If this is successful,

the mount table entry is marked valid and the

fs field initialized.

Unmounting a filesystem is done by the umount()

function. This can unmount filesystems whether they were mounted

statically or dynamically.

The umount() function has the following prototype:

int umount( const char *name );

The mount table is searched for a match between the

name argument and the entry

name field. When a match is found the

filesystem's umount() operation is called and if

successful, the mount table entry is invalidated by setting its

valid field false and the

name field to NULL.

If a network stack is present, then the FILEIO infrastructure also

provides access to the standard BSD socket calls.

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

NSTAB_ENTRY() macro.

Each table entry has the following structure:

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 );

};

This table is analogous to a combination of the filesystem and mount

tables.

The valid field is set

true if the stack's init()

function returned successfully and the

syncmode field contains the

CYG_SYNCMODE_SOCK_* bits described above.

The name field contains the name of the

protocol stack.

The devname field names the device that the stack is using. This may

reference a device under "/dev", or may be a name that is only

meaningful to the stack itself.

The init() function pointer is called during

system initialization to start the protocol stack running. If it

returns non-zero the valid field is set

false and the stack will be ignored subsequently.

The socket() function is called to attempt to create a socket in the

stack. When the socket() API function is called the netstack table is

scanned and for each valid entry the socket()

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.

The result of a successful socket call is an initialized file object

with the f_xops field pointing to the

following structure:

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 );

};

It should be obvious from the names of these functions which API calls

they provide support for. The getname() function

pointer provides support for both getsockname()

and getpeername() while the

sendmsg() and recvmsg()

function pointers provide support for send(),

sendto(), sendmsg(),

recv(), recvfrom() and

recvmsg() as appropriate.

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.

The main part of the mechanism is the select()

API call. This processes its arguments and calls the

fo_select() function pointer on all file objects