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Man Pages

Manual Reference Pages  -  STAT (3)


stat, fstat, wstat, fwstat, dirstat, dirfstat, dirwstat, dirfwstat, nulldir - get and put file status


See Also


#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)


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:
typedef struct Dir {
      /* system-modified data */
      uint type;  /* server type */
      uint dev;   /* server subtype */
      /* file data */
      Qid  qid;   /* unique id from server */
      ulongmode;  /* permissions */
      ulongatime; /* last read time */
      ulongmtime; /* last write time */
      vlonglength;/* 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(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

      0x80000000 directory
      0x40000000 append only
      0x20000000 exclusive use (locked)
    0x00800000   Unix device file
      0x00400000 symbolic link
      0x00200000 named pipe
      0x00100000 socket

       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, 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.




intro(3), fcall(3), dirread(3), stat(9p)


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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