TH STAT 2
SH NAME
stat, fstat, wstat, fwstat, dirstat, dirfstat, dirwstat, dirfwstat, nulldir \- get and put file status
SH SYNOPSIS
B #include <u.h>
br
B #include <libc.h>
PP
B
int stat(char *name, uchar *edir, int nedir)
PP
B
int fstat(int fd, uchar *edir, int nedir)
PP
B
int wstat(char *name, uchar *edir, int nedir)
PP
B
int fwstat(int fd, uchar *edir, int nedir)
PP
B
Dir* dirstat(char *name)
PP
B
Dir* dirfstat(int fd)
PP
B
int dirwstat(char *name, Dir *dir)
PP
B
int dirfwstat(int fd, Dir *dir)
PP
B
void nulldir(Dir *d)
SH DESCRIPTION
Given a file's
IR name ,
or an open file descriptor
IR fd ,
these routines retrieve or modify file status information.
IR Stat ,
IR fstat ,
IR wstat ,
and
I fwstat
are the system calls; they deal with machine-independent
IR "directory entries" .
Their format is defined by
IR stat (5).
I Stat
and
I fstat
retrieve information about
I name
or
I fd
into
IR edir ,
a buffer of length
IR nedir ,
defined in
BR <libc.h> .
I Wstat
and
I fwstat
write information back, thus changing file attributes according to the contents of
IR edir .
The data returned from the kernel includes its leading 16-bit length field
as described in
IR intro (5).
For symmetry, this field must also be present when passing data to the kernel in a call to
I wstat
and
IR fwstat ,
but its value is ignored.
PP
IR Dirstat ,
IR dirfstat ,
IR dirwstat ,
and
I dirfwstat
are similar to their counterparts, except that they
operate on
I Dir
structures:
IP
EX
ta 6n +\w'ulong 'u +\w'mtime;   'u
typedef
struct Dir {
       /* system-modified data */
       uint    type;   /* server type */
       uint    dev;    /* server subtype */
       /* 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;
EE
PP
The returned structure is allocated by
IR malloc (2);
freeing it also frees the associated strings.
PP
This structure and
the
B Qid
structure
are defined in
BR <libc.h> .
If the file resides on permanent storage and is not a directory,
the length returned by
I 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.
PP
Each file is the responsibility of some
IR server :
it could be a file server, a kernel device, or a user process.
B Type
identifies the server type, and
B dev
says which of a group of servers of the same type is the one
responsible for this file.
B Qid
is a structure containing
B path
and
B vers
fields:
B path
is guaranteed to be
unique among all path names currently on the file server, and
B vers
changes each time the file is modified.
The
B path
is a
B long
B long
(64 bits,
BR vlong )
and the
B vers
is an
B unsigned long
(32 bits,
BR ulong ).
Thus, if two files have the same
BR type ,
BR dev ,
and
B qid
they are the same file.
PP
The bits in
B mode
are defined by
PP
ta 6n +\w'\fL0x80000000   'u
nf
\fL     0x80000000\fP   directory
\fL     0x40000000\fP   append only
\fL     0x20000000\fP   exclusive use (locked)

\fL           0400\fP   read permission by owner
\fL           0200\fP   write permission by owner
\fL           0100\fP   execute permission (search on directory) by owner
\fL           0070\fP   read, write, execute (search) by group
\fL           0007\fP   read, write, execute (search) by others
fi
PP
There are constants defined in
B <libc.h>
for these bits:
BR DMDIR ,
BR DMAPPEND ,
and
B DMEXCL
for the first three; and
BR DMREAD ,
BR DMWRITE ,
and
B DMEXEC
for the read, write, and execute bits for others.
PP
The two time fields are measured in seconds since the epoch
(Jan 1 00:00 1970 GMT).
B Mtime
is the time of the last change of content.
Similarly,
B atime
is set whenever the contents are accessed;
also, it is set whenever
B mtime
is set.
PP
B Uid
and
B gid
are the names of the owner and group of the file;
B muid
is the name of the user that last modified the file (setting
BR mtime ).
Groups are also users, but each server is free to associate
a list of users with any user name
IR g ,
and that list is the
set of users in the group
IR 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
B mode
is used to check permissions.
PP
Only some of the fields may be changed with the
I wstat
calls.
The
B name
can be changed by anyone with write permission in the parent directory.
The
B mode
and
B mtime
can be changed by the owner or the group leader of the file's current
group.
The
I 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
IR intro (5)
for more information about permissions and
IR users (6)
for users and groups).
The
B length
can be changed by anyone with write permission, provided the operation
is implemented by the server.
(See
IR intro (5)
for permission information, and
IR users (6)
for user and group information).
PP
Special values in the fields of the
B Dir
passed to
I wstat
indicate that the field is not intended to be changed by the call.
The values are the maximum unsigned integer of appropriate size
for integral values (usually
BR ~0 ,
but beware of conversions and size mismatches
when comparing values) and the empty or nil string for string values.
The routine
I nulldir
initializes all the elements of
I d
to these ``don't care'' values.
Thus one may change the mode, for example, by using
I nulldir
to initialize a
BR Dir ,
then setting the mode, and then doing
IR wstat ;
it is not necessary to use
I stat
to retrieve the initial values first.
SH SOURCE
TF /sys/src/libc/9syscall
TP
B /sys/src/libc/9syscall
for the
RB non- dir
routines
TP
B /sys/src/libc/9sys
for the routines prefixed
B dir
SH SEE ALSO
IR intro (2),
IR fcall (2),
IR dirread (2),
IR stat (5)
SH DIAGNOSTICS
The
I dir
functions return a pointer to the data for a successful call, or
B nil
on error.
The others
return the number of bytes copied on success, or \-1 on error.
All set
IR errstr .
PP
If the buffer for
I stat
or
I fstat
is too short for the returned data, the return value will be
B BIT16SZ
(see
IR fcall (2))
and the two bytes
returned will contain the initial count field of the
returned data;
retrying with
B nedir
equal to
that value plus
B BIT16SZ
(for the count itself) should succeed.

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