2.2.t

早期freebsd实现

The \fIstat\fP structure includes the file
type, protection, ownership, access times,
size, and a count of hard links.
If the file is a symbolic link, then the status of the link
itself (rather than the file the link references)
may be found using the \fIlstat\fP call:
.DS
lstat(path, stb);
char *path; result struct stat *stb;
.DE
.PP
Newly created files are assigned the user id of the
process that created it and the group id of the directory
in which it was created.  The ownership of a file may
be changed by either of the calls
.DS
chown(path, owner, group);
char *path; int owner, group;

fchown(fd, owner, group);
int fd, owner, group;
.DE
.PP
In addition to ownership, each file has three levels of access
protection associated with it.  These levels are owner relative,
group relative, and global (all users and groups).  Each level
of access has separate indicators for read permission, write
permission, and execute permission.
The protection bits associated with a file may be set by either
of the calls:
.DS
chmod(path, mode);
char *path; int mode;

fchmod(fd, mode);
int fd, mode;
.DE
where \fImode\fP is a value indicating the new protection
of the file, as listed in section 2.2.3.2.
.PP
Finally, the access and modify times on a file may be set by the call:
.DS
utimes(path, tvp)
char *path; struct timeval *tvp[2];
.DE
This is particularly useful when moving files between media, to
preserve relationships between the times the file was modified.
.NH 3
Links and renaming
.PP
Links allow multiple names for a file
to exist.  Links exist independently of the file linked to.
.PP
Two types of links exist, \fIhard\fP links and \fIsymbolic\fP
links.  A hard link is a reference counting mechanism that
allows a file to have multiple names within the same file
system.  Symbolic links cause string substitution
during the pathname interpretation process.
.PP
Hard links and symbolic links have different
properties.  A hard link insures the target
file will always be accessible, even after its original
directory entry is removed; no such guarantee exists for a symbolic link.
Symbolic links can span file systems boundaries.
.PP
The following calls create a new link, named \fIpath2\fP,
to \fIpath1\fP:
.DS
link(path1, path2);
char *path1, *path2;

symlink(path1, path2);
char *path1, *path2;
.DE
The \fIunlink\fP primitive may be used to remove
either type of link. 
.PP
If a file is a symbolic link, the ``value'' of the
link may be read with the \fIreadlink\fP call,
.DS
len = readlink(path, buf, bufsize);
result int len; result char *path, *buf; int bufsize;
.DE
This call returns, in \fIbuf\fP, the null-terminated string
substituted into pathnames passing through \fIpath\fP\|.
.PP
Atomic renaming of file system resident objects is possible
with the \fIrename\fP call:
.DS
rename(oldname, newname);
char *oldname, *newname;
.DE
where both \fIoldname\fP and \fInewname\fP must be
in the same file system.
If \fInewname\fP exists and is a directory, then it must be empty.
.NH 3
Extension and truncation
.PP
Files are created with zero length and may be extended
simply by writing or appending to them.  While a file is
open the system maintains a pointer into the file
indicating the current location in the file associated with
the descriptor.  This pointer may be moved about in the
file in a random access fashion.
To set the current offset into a file, the \fIlseek\fP
call may be used,
.DS
oldoffset = lseek(fd, offset, type);
result off_t oldoffset; int fd; off_t offset; int type;
.DE
where \fItype\fP is given in \fI<sys/file.h>\fP as one of:
.DS
._d
#define	L_SET	0	/* set absolute file offset */
#define	L_INCR	1	/* set file offset relative to current position */
#define	L_XTND	2	/* set offset relative to end-of-file */
.DE
The call ``lseek(fd, 0, L_INCR)''
returns the current offset into the file.
.PP
Files may have ``holes'' in them.  Holes are void areas in the
linear extent of the file where data has never been
written.  These may be created by seeking to
a location in a file past the current end-of-file and writing.
Holes are treated by the system as zero valued bytes.
.PP
A file may be truncated with either of the calls:
.DS
truncate(path, length);
char *path; int length;

ftruncate(fd, length);
int fd, length;
.DE
reducing the size of the specified file to \fIlength\fP bytes.
.NH 3
Checking accessibility
.PP
A process running with
different real and effective user ids
may interrogate the accessibility of a file to the
real user by using
the \fIaccess\fP call:
.DS
accessible = access(path, how);
result int accessible; char *path; int how;
.DE
Here \fIhow\fP is constructed by or'ing the following bits, defined
in \fI<sys/file.h>\fP:
.DS
._d
#define	F_OK	0	/* file exists */
#define	X_OK	1	/* file is executable */
#define	W_OK	2	/* file is writable */
#define	R_OK	4	/* file is readable */
.DE
The presence or absence of advisory locks does not affect the
result of \fIaccess\fP\|.
.NH 3
Locking
.PP
The file system provides basic facilities that allow cooperating processes
to synchronize their access to shared files.  A process may
place an advisory \fIread\fP or \fIwrite\fP lock on a file,
so that other cooperating processes may avoid interfering
with the process' access.  This simple mechanism
provides locking with file granularity.  More granular
locking can be built using the IPC facilities to provide a lock
manager.
The system does not force processes to obey the locks;
they are of an advisory nature only.
.PP
Locking is performed after an \fIopen\fP call by applying the
\fIflock\fP primitive,
.DS
flock(fd, how);
int fd, how;
.DE
where the \fIhow\fP parameter is formed from bits defined in \fI<sys/file.h>\fP:
.DS
._d
#define	LOCK_SH	1	/* shared lock */
#define	LOCK_EX	2	/* exclusive lock */
#define	LOCK_NB	4	/* don't block when locking */
#define	LOCK_UN	8	/* unlock */
.DE
Successive lock calls may be used to increase or
decrease the level of locking.  If an object is currently
locked by another process when a \fIflock\fP call is made,
the caller will be blocked until the current lock owner
releases the lock; this may be avoided by including LOCK_NB
in the \fIhow\fP parameter.
Specifying LOCK_UN removes all locks associated with the descriptor.
Advisory locks held by a process are automatically deleted when
the process terminates.
.NH 3
Disk quotas
.PP
As an optional facility, each file system may be requested to
impose limits on a user's disk usage.
Two quantities are limited: the total amount of disk space which
a user may allocate in a file system and the total number of files
a user may create in a file system.  Quotas are expressed as
\fIhard\fP limits and \fIsoft\fP limits.  A hard limit is
always imposed; if a user would exceed a hard limit, the operation
which caused the resource request will fail.  A soft limit results
in the user receiving a warning message, but with allocation succeeding.
Facilities are provided to turn soft limits into hard limits if a
user has exceeded a soft limit for an unreasonable period of time.
.PP
To enable disk quotas on a file system the \fIsetquota\fP call
is used:
.DS
setquota(special, file)
char *special, *file;
.DE
where \fIspecial\fP refers to a structured device file where
a mounted file system exists, and
\fIfile\fP refers to a disk quota file (residing on the file
system associated with \fIspecial\fP) from which user quotas
should be obtained.  The format of the disk quota file is 
implementation dependent.
.PP
To manipulate disk quotas the \fIquota\fP call is provided:
.DS
#include <sys/quota.h>

quota(cmd, uid, arg, addr)
int cmd, uid, arg; caddr_t addr;
.DE
The indicated \fIcmd\fP is applied to the user ID \fIuid\fP.
The parameters \fIarg\fP and \fIaddr\fP are command specific.
The file \fI<sys/quota.h>\fP contains definitions pertinent to the
use of this call.