STAT(2) STAT(2)
NAME
stat, fstat, wstat, fwstat, dirstat, dirfstat, dirwstat,
dirfwstat, nulldir - get and put file status
SYNOPSIS
#include <u.h>
#include <libc.h>
int stat(char *name, uchar *edir, int nedir)
int fstat(int fd, uchar *edir, int nedir)
int wstat(char *name, uchar *edir, int nedir)
int fwstat(int fd, uchar *edir, int nedir)
Dir* dirstat(char *name)
Dir* dirfstat(int fd)
int dirwstat(char *name, Dir *dir)
int dirfwstat(int fd, Dir *dir)
void nulldir(Dir *d)
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 nedir,
defined in <libc.h>. Wstat and fwstat write information
back, thus changing file attributes according to the con-
tents of edir. The data returned from the kernel includes
its leading 16-bit length field as described in intro(5).
For symmetry, this field must also be present when passing
data to the kernel in a call to wstat and fwstat, but its
value is ignored.
Dirstat, dirfstat, dirwstat, and dirfwstat are similar to
their counterparts, except that they operate on Dir struc-
tures:
typedef
struct Dir {
/* system-modified data */
uint type; /* server type */
uint dev; /* server subtype */
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STAT(2) STAT(2)
/* file data */
Qid qid; /* unique id from server */
ulong mode; /* permissions */
ulong atime; /* last read time */
ulong mtime; /* last write time */
vlong length; /* file length: see <u.h> */
char *name; /* last element of path */
char *uid; /* owner name */
char *gid; /* group name */
char *muid; /* last modifier name */
} Dir;
The returned structure is allocated by malloc(2); freeing it
also frees the associated strings.
This structure and the Qid structure are defined in
<libc.h>. 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 net-
work 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:
path is guaranteed to be unique among all path names cur-
rently on the file server, and vers changes each time the
file is modified. The path is a long long (64 bits, vlong)
and the vers is an unsigned long (32 bits, ulong). 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)
0x04000000 non-backed-up files
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:
DMDIR, DMAPPEND, DMEXCL, and DMTMP for the first four; and
DMREAD, DMWRITE, and DMEXEC for the read, write, and execute
bits for others.
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The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 GMT). 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; muid is the name of the user that last modified the
file (setting mtime). 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 process group is communicated to the server.
Thus, the server knows, for any given file access, whether
the accessing process is the owner of, or in the group of,
the file. This selects which sets of three bits in mode is
used to check permissions.
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 and mtime 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 also a member of the
new group; or by the group leader of the file's current
group if also leader of the new group (see intro(5) for more
information about permissions and users(6) for users and
groups). The length can be changed by anyone with write
permission, provided the operation is implemented by the
server. (See intro(5) for permission information, and
users(6) for user and group information).
Special values in the fields of the Dir passed to wstat
indicate that the field is not intended to be changed by the
call. The values are the maximum unsigned integer of appro-
priate size for integral values (usually ~0, but beware of
conversions and size mismatches when comparing values) and
the empty or nil string for string values. The routine
nulldir initializes all the elements of d to these ``don't
care'' values. Thus one may change the mode, for example,
by using nulldir to initialize a Dir, then setting the mode,
and then doing wstat; it is not necessary to use stat to
retrieve the initial values first.
SOURCE
/sys/src/libc/9syscall for the non-dir routines
/sys/src/libc/9sys for the routines prefixed dir
SEE ALSO
intro(2), fcall(2), dirread(2), stat(5)
DIAGNOSTICS
The dir functions return a pointer to the data for a
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STAT(2) STAT(2)
successful call, or nil on error. The others return the
number of bytes copied on success, or -1 on error. All set
errstr.
If the buffer for stat or fstat is too short for the
returned data, the return value will be BIT16SZ (see
fcall(2)) and the two bytes returned will contain the ini-
tial count field of the returned data; retrying with nedir
equal to that value plus BIT16SZ (for the count itself)
should succeed.
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