POSIX-Layer Data Structures

The resource manager library defines several key structures that are related to the POSIX-layer support routines:

iofunc_ocb_t (Open Control Block) structure: Contains per-open data, such as the current position into a file (the lseek() offset).
iofunc_attr_t (attribute) structure: Since a resource manager may be responsible for more than one device (e.g. devc-ser* may be responsible for /dev/ser1, /dev/ser2, /dev/ser3, etc.), the attributes structure holds data on a per-name basis. This structure contains such items as the user and group ID of the owner of the device, the last modification time, etc.
iofunc_mount_t (mount) structure: Contains per-mountpoint data items that are global to the entire mount device. Filesystem (block I/O device) managers use this structure; a resource manager for a device typically won't have a mount structure.

This picture may help explain their interrelationships:

Three data structures of a resource manager

A resource manager is responsible for three data structures.

If three clients open two paths associated with a resource manager, the data structures are linked like this:

Figure showing 3 clients with 3 OCBs

Multiple clients with multiple OCBs, all linked to one mount structure.

The `iofunc_ocb_t` (Open Control Block) structure

The Open Control Block (OCB) maintains the state information about a particular session involving a client and a resource manager. It's created during open() handling and exists until a close() is performed.

This structure is used by the iofunc layer helper functions. (In the Extending the POSIX-Layer Data Structures chapter, we'll show you how to extend this to include your own data).

The OCB structure contains at least the following:

typedef struct _iofunc_ocb {
    IOFUNC_ATTR_T   *attr;
    int32_t         ioflag;
    off_t           offset;
    uint16_t        sflag;
    uint16_t        flags;
} iofunc_ocb_t;

where the values represent:

attr

A pointer to the attribute structure (see below).

ioflag

Contains the mode (e.g. reading, writing, blocking) that the resource was opened with. This information is inherited from the io_connect_t structure that's available in the message passed to the io_open handler. The open modes (as passed to open() on the client side) are converted to the ioflag values as follows:

Open mode	ioflag value
O_RDONLY	_IO_FLAG_RD
O_RDWR	_IO_FLAG_RD \| _IO_FLAG_WR
O_WRONLY	_IO_FLAG_WR

offset

The read/write offset into the resource (e.g. our current lseek() position within a file). Your resource manager can modify this member.

sflag

Defines the sharing mode. This information is inherited from the io_connect_t structure that's available in the message passed to the io_open handler.

flags

When the IOFUNC_OCB_PRIVILEGED bit is set, a privileged process (i.e. root) performed the open(). Additionally, you can use flags in the range IOFUNC_OCB_FLAGS_PRIVATE (see <sys/iofunc.h>) for your own purposes. Your resource manager can modify this member.

The `iofunc_attr_t` (attribute) structure

The iofunc_attr_t structure defines the characteristics of the device that you're supplying the resource manager for. This is used in conjunction with the OCB structure.

The attribute structure contains at least the following:

typedef struct _iofunc_attr {
    IOFUNC_MOUNT_T            *mount;
    uint32_t                  flags;
    int32_t                   lock_tid;
    uint16_t                  lock_count;
    uint16_t                  count;
    uint16_t                  rcount;
    uint16_t                  wcount;
    uint16_t                  rlocks;
    uint16_t                  wlocks;
    struct _iofunc_mmap_list  *mmap_list;
    struct _iofunc_lock_list  *lock_list;
    void                      *list;
    uint32_t                  list_size;
    off_t                     nbytes;
    ino_t                     inode;
    uid_t                     uid;
    gid_t                     gid;
    time_t                    mtime;
    time_t                    atime;
    time_t                    ctime;
    mode_t                    mode;
    nlink_t                   nlink;
    dev_t                     rdev;
} iofunc_attr_t;

where the values represent:

mount

A pointer to the mount structure (see below).

flags

The bit-mapped flags member can contain the following flags:

IOFUNC_ATTR_ATIME: The access time is no longer valid. Typically set on a read from the resource.
IOFUNC_ATTR_CTIME: The change of status time is no longer valid. Typically set on a file info change.
IOFUNC_ATTR_DIRTY_NLINK: The number of links has changed.
IOFUNC_ATTR_DIRTY_MODE: The mode has changed.
IOFUNC_ATTR_DIRTY_OWNER: The uid or the gid has changed.
IOFUNC_ATTR_DIRTY_RDEV: The rdev member has changed, e.g. mknod().
IOFUNC_ATTR_DIRTY_SIZE: The size has changed.
IOFUNC_ATTR_DIRTY_TIME: One or more of mtime, atime, or ctime has changed.
IOFUNC_ATTR_MTIME: The modification time is no longer valid. Typically set on a write to the resource.

Since your resource manager uses these flags, you can tell right away which fields of the attribute structure have been modified by the various iofunc-layer helper routines. That way, if you need to write the entries to some medium, you can write just those that have changed. The user-defined area for flags is IOFUNC_ATTR_PRIVATE (see <sys/iofunc.h>).

For details on updating your attribute structure, see the section on “Updating the time for reads and writes” in the Handling Read and Write Messages chapter.

lock_tid and lock_count

To support multiple threads in your resource manager, you'll need to lock the attribute structure so that only one thread at a time is allowed to change it. The resource manager layer automatically locks the attribute (using iofunc_attr_lock()) for you when certain handler functions are called (i.e. IO_*). The lock_tid member holds the thread ID; the lock_count member holds the number of times the thread has locked the attribute structure. For more information, see the iofunc_attr_lock() and iofunc_attr_unlock() functions in the QNX Neutrino Library Reference.)

count, rcount, wcount, rlocks and wlocks

Several counters are stored in the attribute structure and are incremented/decremented by some of the iofunc layer helper functions. Both the functionality and the actual contents of the message received from the client determine which specific members are affected.

This counter:	Tracks the number of:
count	OCBs using this attribute in any manner. When this count goes to zero, it means that no one is using this attribute.
rcount	OCBs using this attribute for reading.
wcount	OCBs using this attribute for writing.
rlocks	read locks currently registered on the attribute.
wlocks	write locks currently registered on the attribute.

These counts aren't exclusive. For example, if an OCB has specified that the resource is opened for reading and writing, then count, rcount, and wcount will all be incremented. (See the iofunc_attr_init(), iofunc_lock_default(), iofunc_lock(), iofunc_ocb_attach(), and iofunc_ocb_detach() functions.)

mmap_list and lock_list

To manage their particular functionality on the resource, the mmap_list member is used by the iofunc_mmap() and iofunc_mmap_default() functions; the lock_list member is used by the iofunc_lock_default() function. Generally, you shouldn't need to modify or examine these members.

list

Reserved for future use.

list_size

The size of the reserved list area; reserved for future use.

nbytes

The number of bytes in the resource. Your resource manager can modify this member. For a file, this would contain the file's size. For special devices (e.g. /dev/null) that don't support lseek() or have a radically different interpretation for lseek(), this field isn't used (because you wouldn't use any of the helper functions, but would supply your own instead.) In these cases, we recommend that you set this field to zero, unless there's a meaningful interpretation that you care to put to it.

inode

This is a mountpoint-specific inode that must be unique per mountpoint. You can specify your own value, or 0 to have the process manager fill it in for you. For filesystem resource managers, this may correspond to some on-disk structure. In any case, the interpretation of this field is up to you.

uid and gid

The user ID and group ID of the owner of this resource. These fields are updated automatically by the chown() helper functions (e.g. iofunc_chown_default()) and are referenced in conjunction with the mode member for access-granting purposes by the open() help functions (e.g. iofunc_open_default()).

mtime, atime, and ctime

The three POSIX time members:

mtime — modification time (write() updates this)
atime — access time (read() updates this)
ctime — change of status time (write(), chmod(), and chown() update this)

One or more of the time members may be invalidated as a result of calling an iofunc-layer function. This is to avoid having each and every I/O message handler go to the kernel and request the current time of day, just to fill in the attribute structure's time member(s).

POSIX states that these times must be valid when the fstat() is performed, but they don't have to reflect the actual time that the associated change occurred. Also, the times must change between fstat() invocations if the associated change occurred between fstat() invocations. If the associated change never occurred between fstat() invocations, then the time returned should be the same as returned last time. Furthermore, if the associated change occurred multiple times between fstat() invocations, then the time need only be different from the previously returned time.

There's a helper function that fills the members with the correct time; you may wish to call it in the appropriate handlers to keep the time up-to-date on the device — see the iofunc_time_update() function.

mode

Contains the resource's mode (e.g. type, permissions). Valid modes may be selected from the S_* series of constants in <sys/stat.h>.

nlink

The number of links to this particular name. For names that represent a directory, this value must be at least 2 (one for the directory itself, one for the ./ entry in it). Your resource manager can modify this member.

rdev

Contains the device number for a character special device and the rdev number for a named special device.

The optional `iofunc_mount_t` (mount) structure

The members of the mount structure, specifically the conf and flags members, modify the behavior of some of the iofunc layer functions. This optional structure contains at least the following:

typedef struct _iofunc_mount {
    uint32_t            flags;
    uint32_t            conf;
    dev_t               dev;
    int32_t             blocksize;
    iofunc_funcs_t      *funcs;
} iofunc_mount_t;

The variables are:

flags

Contains one relevant bit (manifest constant IOFUNC_MOUNT_32BIT), which indicates that the offsets used by this resource manager are 32-bit (as opposed to the extended 64-bit offsets). The user-modifiable mount flags are defined as IOFUNC_MOUNT_FLAGS_PRIVATE (see <sys/iofunc.h>).

conf

Contains several bits:

IOFUNC_PC_CHOWN_RESTRICTED: Causes the default handler for the _IO_CHOWN message to behave in a manner defined by POSIX as “chown-restricted”.
IOFUNC_PC_NO_TRUNC: Has no effect on the iofunc layer libraries, but is returned by the iofunc layer's default _IO_PATHCONF handler.
IOFUNC_PC_SYNC_IO: If not set, causes the default iofunc layer _IO_OPEN handler to fail if the client specified any one of O_DSYNC, O_RSYNC, or O_SYNC.
IOFUNC_PC_LINK_DIR: Controls whether or not root is allowed to link and unlink directories.

Note that the options mentioned above for the conf member are returned by the iofunc layer _IO_PATHCONF default handler.

dev

Contains the device number for the filesystem. This number is returned to the client's stat() function in the struct stat st_dev member.

blocksize

Contains the block size of the device. On filesystem types of resource managers, this indicates the native blocksize of the disk, e.g. 512 bytes.

funcs

Contains the following structure:

struct _iofunc_funcs {
   unsigned     nfuncs;
   IOFUNC_OCB_T *(*ocb_calloc) (resmgr_context_t *ctp,
                                IOFUNC_ATTR_T *attr);
   void         (*ocb_free) (IOFUNC_OCB_T *ocb);
};

where:

nfuncs: Indicates the number of functions present in the structure; it should be filled with the manifest constant _IOFUNC_NFUNCS.
ocb_calloc() and ocb_free(): Allows you to override the OCBs on a per-mountpoint basis. (See the section titled “Extending the OCB and attribute structures” in the Extending the POSIX-Layer Data Structures chapter.) If these members are NULL, then the default library versions are used. You must specify either both or neither of these functions — they operate as a matched pair.

POSIX-Layer Data Structures

The iofunc_ocb_t (Open Control Block) structure

The iofunc_attr_t (attribute) structure

The optional iofunc_mount_t (mount) structure

The `iofunc_ocb_t` (Open Control Block) structure

The `iofunc_attr_t` (attribute) structure

The optional `iofunc_mount_t` (mount) structure