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NAME
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stat, fstat, wstat, fwstat, dirstat, dirfstat, dirwstat, dirfwstat,
nulldir – get and put file status
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SYNOPSIS
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#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)
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DESCRIPTION
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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(9p). 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 contents of edir. The data returned from the
kernel includes its leading 16-bit length field as described in
intro(9p). 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 structures:
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typedef
struct Dir {
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/* 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 */
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} Dir;
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The returned structure is allocated by malloc(3); 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 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: path is guaranteed to be unique
among all path names currently 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
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0x80000000 directory
0x40000000 append only
0x20000000 exclusive use (locked)
0x00800000 Unix device file
0x00400000 symbolic link
0x00200000 named pipe
0x00100000 socket
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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
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There are constants defined in <libc.h> for these bits: DMDIR, DMAPPEND,
and DMEXCL for the first three; and DMREAD, DMWRITE, and DMEXEC
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 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(9p) for
more information about permissions, users, and groups). The length
can be changed by anyone with write permission, provided the operation
is implemented by the server. (See intro(9p) for permission, 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 appropriate 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.
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SOURCE
SEE ALSO
DIAGNOSTICS
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The dir functions return a pointer to the data for a 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(3)) and the
two bytes returned will contain the initial 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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