STAT(2) STAT(2)
NAME
stat, fstat, wstat, fwstat, dirstat, dirfstat, dirwstat,
dirfwstat - get and put file status
SYNOPSIS
int stat(char *name, char *edir)
int fstat(int fd, char *edir)
int wstat(char *name, char *edir)
int fwstat(int fd, char *edir)
int dirstat(char *name, Dir *dir)
int dirfstat(int fd, Dir *dir)
int dirwstat(char *name, Dir *dir)
int dirfwstat(int fd, Dir *dir)
DESCRIPTION
Given a file's name, or an open file descriptor fd, these
routines retrieve or modify file status information. Stat,
fstat, wstat, and fwstat are the system calls; they deal
with machine-independent directory entries. Their format is
defined by stat(5). Stat and fstat retrieve information
about name or fd into edir, a buffer of length DIRLEN,
defined in <libc.h>. Wstat and fwstat write information
back, thus changing file attributes according to edir.
Dirstat, dirfstat, dirwstat, and dirfwstat are the same as
their counterparts, except that they operate on Dir struc-
tures:
typedef
struct Dir {
char name[NAMELEN]; /* last element of path */
char uid[NAMELEN]; /* owner name */
char gid[NAMELEN]; /* group name */
Qid qid; /* unique id from server */
long mode; /* permissions */
long atime; /* last read time */
long mtime; /* last write time */
Length; /* file length: see <u.h> */
short type; /* server type */
short dev; /* server subtype */
} Dir;
This structure, the Qid structure, NAMELEN, and DIRLEN are
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defined in <libc.h>. The Length structure is defined in
<u.h>. Length is an unnamed structure (see 2c(1)), which
means that its fields are directly accessible; if the length
is known to fit in a long, then use length as a field name
to retrieve it. If the file resides on permanent storage
and is not a directory, the length returned by stat is the
number of bytes in the file. For directories, the length
returned is zero. For files that are streams (e.g., pipes
and network connections), the length is the number of bytes
that can be read without blocking.
Each file is the responsibility of some server: it could be
a file server, a kernel device, or a user process. Type
identifies the server type, and dev says which of a group of
servers of the same type is the one responsible for this
file. Qid is a structure containing path and vers fields,
each an unsigned long: path is guaranteed to be unique among
all path names currently on the file server, and vers
changes each time the file is modified. Thus, if two files
have the same type, dev, and qid they are the same file.
The bits in mode are defined by
0x80000000 directory
0x40000000 append only
0x20000000 exclusive use (locked)
0400 read permission by owner
0200 write permission by owner
0100 execute permission (search on directory) by owner
0070 read, write, execute (search) by group
0007 read, write, execute (search) by others
There are constants defined in <libc.h> for these bits:
CHDIR, CHAPPEND, and CHEXCL for the first three; and CHREAD,
CHWRITE, and CHEXEC for the read, write, and execute bits
for others.
The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 local time). Mtime is the time of the
last change of content. Similarly, atime is set whenever
the contents are accessed; also, it is set whenever mtime is
set.
Uid and gid are the names of the owner and group of the
file. Groups are also users, but each server is free to
associate a list of users with any user name g, and that
list is the set of users in the group g. When an initial
attachment is made to a server, the user string in the pro-
cess group is communicated to the server. Thus, the server
knows, for any given file access, whether the accessing pro-
cess is the owner or in the group of the file. This selects
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which sets of three bits in mode is used to check permis-
sions.
Only some of the fields may be changed with the wstat calls.
The name can be changed by anyone with write permission in
the parent directory. The mode can be changed by the owner
or the group leader of the file's current group. The gid
can be changed by the owner if he or she is a member of the
new group. The gid can be changed by the group leader of
the file's current group if he or she is the leader of the
new group. (See intro(5) for permission information, and
users(6) for user and group information).
SEE ALSO
intro(2), fcall(2), dirread(2), stat(5)
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