Derived from: (none)
Declared in: be/StorageKit/BStatable.h
Library: libbe.so
BStatable is a pure abstract class that provides functionality for its two derived class, BEntry and BNode. The BStatable functions let you get and set "statistical" information about an entry in the file system. You can...
The BStatable functions are covers for the POSIX stat() call. stat() retrieves a file-specific stat structure, which records the statistics listed above (and then some). Although BStatable was designed to hide stat details, you can get the stat structure through the GetStat() function. The stat structure is described in The stat Structure at the end of this specification.
stat() is notorious for being expensive. Furthermore, the stat structure is stale as soon as it gets back from the stat() call. So...
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If you're concerned with efficiency, be aware that every BStatable function (the
"setters" as well as the "getters") performs a stat(). For example, calling
GetOwner() and then GetGroup() results in two stat()
calls. If you want to look at lot of fields (within the same stat structure) all at once, you might consider using BStatable's
GetStat() function.
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As for integrity, BStatable info-getting functions are obviously in the same boat as the stat() call itself: The retrieved data isn't guaranteed to be in synch with the actual state of the stat()'d item. |
The BDirectory class also defines a stat-retrieving function that, in some cases, can be more efficient than the GetStat() function defined here:
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The BDirectory::GetStatFor() function retrieves the stat structure for a named entry within a directory. If you're interested in getting stat information for a series of entries within the same directory, you should use this function. You have to call it iteratively (once for each entry), but the accumulation of the iterated calls will be faster than the GetStat() calls made on the analogous BEntry objects. |
BStatable isn't thwarted by file permissions: If you can construct a valid BEntry to an item, then you can invoke any of the info-getting BStatable functions on that object:
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The BStatable functions aren't denied even if the entry that you're looking at is
read-protected. However, you can only invoke the info-setting functions if the entry
allows writing.
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Similarly, you can get stat info for a locked file, but you won't be able to write the info (through functions such as SetOwner() ) unless your object holds the lock. See BNode for more on locking. |
Be aware that...
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Currently, permissions are not enforced as strongly as they could be . If you're looking at a read-only volume, then you won't be able to modify any stat information. Beyond that (in other words, on writable volumes), the only stat- modifying operation that's denied on a read-only file is SetSize(): You can't modify the data in a file that's read-protected against you. |
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You rarely set stat information. |
In practice, you rarely use BStatable's info-setting functions. Setting information such as when a file was created, who owns it, or how big it is, is the responsibility of the system and the privilege of the user. For example, when you Write() to a BFile object, the system automatically updates the size and modification date for the file.
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The BStatable functions only apply to existing entries. |
As explained in BEntry, it's possible to create "abstract" BEntry objects; in other words, objects that don't correspond to actual files on the disk. However, you can't get (or set) BStatable information for abstract entries. The BStatable functions return EINVAL if the invoked-upon entry is abstract.
status_t GetCreationTime(time_t *ctime) const status_t SetCreationTime(time_t ctime) status_t GetModificationTime(time_t *mtime) const status_t SetModificationTime(time_t mtime) status_t GetAccessTime(time_t *atime) const status_t SetAccessTime(time_t atime)
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Access time is currently unused. |
These function let you get and set the time at which the item was created, last modified, and last accessed (opened). The measure of time is given as seconds since (the beginning of ) January 1, 1970.
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Note that the time quanta that stat uses is seconds; the rest of the BeOS measures time in microseconds (bigtime_t). |
RETURN VALUES
status_t GetNodeRef(node_ref *nref) const
Copies the item's node_ref structure into the nref argument, which must be allocated.
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Typically, you use an entry's node_ref as a key to the Node Monitor by passing the
node_ref structure to the watch_node() function. The Node Monitor watches the node for specific changes; see
The Node Monitor for details.
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As a convenience, you can use a node_ref structure to initialize a BDirectory object (through the constructor or BDirectory::SetTo() function). |
RETURN VALUES
status_t GetOwner(uid_t *owner) const status_t SetOwner(uid_t owner) status_t GetGroup(gid_t *group) const status_t SetGroup(gid_t group) status_t GetPermissions(mode_t *perms) const status_t SetPermissions(mode_t perms)
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In AADR9 owner, group, and permissions are mostly ignored by the Be file system. In particular, all users are "root" and there's only one group. |
These functions set and get the owner, group, and read/write/execute permissions for the entry:
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The uid_t, gid_t, and mode_t types used here are standard POSIX types. All three are 32-bit unsigned integers and are defined in headers/posix/sys/types.h. |
The owner and group encodings must match values found in the system's user and group database files [which are as yet unspecified].
The permissions value is a combination of the following bitfield constants (defined in headers/posix/sys/stat.h):
For example:
/* Is a file readable by everybody? */ mode_t perms; if (entry.GetPermissions(&perms) < B_NO_ERROR) /* handle the error... */ if (perms & S_ISROTH) // Yes it is else // No it isn't
RETURN VALUES
status_t GetSize(off_t *size) const status_t SetSize(off_t size) const
These functions get and set the size of the entry's data portion (in bytes). Only the "used" portions of the entry's file blocks is counted; the amount of storage the entry actually requires (i.e. the number of blocks the entry consumes) may be larger than the size given here.
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The size measurement doesn't include the entry's attributes. |
SetSize() really does modify the file--it adds new storage to the end, or removes data from the end of the file, as needed. You can only set the size of a plain file or symbolic link (as opposed to a directory).
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Setting the size of a symbolic link is a very odd thing to do. |
RETURN VALUES
virtual status_t GetStat(struct stat *st) const
GetStat() returns the stat structure for the item. The structure is copied into the st argument, which must be allocated. The stat structure is described in The stat Structure, below. The BStatable object does not cache the stat structure; every time you call GetStat(), fresh stat information is retrieved.
RETURN VALUES
Declared in: posix/sys/stat.h
The stat structure looks like this:
typedef struct stat { dev_t st_dev; ino_t st_ino; mode_t st_mode; nlink_t st_nlink; uid_t st_uid; gid_t st_gid; off_t st_size; dev_t st_rdev; size_t st_blksize; time_t st_atime; time_t st_mtime; time_t st_ctime; } stat;
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The structure isn't typdef'd in AADR9. |
And the fields are...
By combining st_dev and st_ino you can roll your own node_ref:
node_ref nref; stat st; if (file.GetStat(&st) == B_NO_ERROR) { nref.dev = st.st_dev; nref.node = st.st_ino; }
Meanwhile...
if (S_ISREG(st.st_mode)) /* it's a "regular" file */ else if (S_ISDIR(st.st_mode)) /* it's a directory */ else if (S_ISLINK(st.st_mode)) /* it's a symbolic link */
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