fcntl.2

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.IP F_GETLK 13
When \fIrequest\fR is set to F_GETLK:
.sp
The 
.PN fcntl
call gets the lock information for a read or write locked region.
In the call, you pass a lock description in a 
variable of type
.I struct flock 
pointed to by
.I arg.
.sp
If the region defined in the
.I flock
structure is already locked by a process other than the
caller, a description of the existing lock
is returned in the 
.I flock 
structure.
If no lock is found that would prevent this lock from being created,
then the structure is passed back unchanged except for the lock type
which is set to F_UNLCK.
.sp
The
.I flock 
structure is defined as follows:
.EX
struct flock {
        short    l_type;
        short    l_whence;
        long     l_start;
        long     l_len;
        int      l_pid;
};
.EE
\fBData Passed in \fIflock\fR:
.sp
In the data you pass in
.I flock,
the 
.I l_type
value defines the lock type to be tested for: F_RDLCK for
a read lock and F_WRLCK for a write lock.  
.sp
The
.I l_whence 
value defines the point from which the starting byte of the region is to be
measured.  If
.I l_whence 
is 0, the value in
.I l_start
is taken as the starting byte of the region.
If
.I l_whence 
is 1,
the current file offset plus the value of
.I l_start
is taken as the starting point.
If
.I l_whence
is 2,
the file size plus the value of
.I l_start
is taken as the starting point.
.sp
The
.I l_len
value is the length of the region to be tested, in bytes.
If
.I l_len 
is zero, the length to be tested extends to the end of file.
If 
.I l_len
is zero and
.I l_start
is zero, the whole file is to be tested.
If
.I l_len
is negative, the area affected starts at \fIl_start\fP + \fIl_len\fP
and ends at \fIl_start\fP - 1.
.sp
The
.I l_pid
value has no significance in the data passed.
.sp
\fBData Returned in \fIflock\fR:
.sp
The 
.I l_type 
value can be F_RDLCK if the region passed is under a read lock.
F_WRLCK means that the region passed is under a write lock.
F_UNLCK means that the region is not currently locked by any
process that would prevent this lock from being created;
for example, the region might be locked by the caller.
.sp
The 
.IR l_whence ,
.IR l_start ,
and
.IR l_len
values have similar meanings as discussed under Data Passed,
except that they define the region currently under
read or write lock.
.sp
The 
.I l_pid 
value is only used with F_GETLK to return the value for a
blocking lock.  An example of a blocking lock is a write lock currently
set by a process other than the calling process.
.IP F_SETLK 13
When \fIrequest\fR is set to F_SETLK:
.sp
You set or clear a file region lock according to the variable of 
.I l_type
in the
.I struct flock
pointed to by 
.IR arg .
(The 
.I flock
structure is shown under the description of F_GETLK,
preceding.)
.sp
The 
.I l_type 
value
is used to establish read (F_RDLCK) and write (F_WRLCK) locks,
as well as remove either type of lock (F_UNLCK).
If a read or write lock cannot be set, 
.PN fcntl
will return immediately with an
error value of \-1.
.IP F_SETLKW 13
When \fIrequest\fR is set to F_SETLKW:
.sp
The 
.PN fcntl
call takes the same action as for
F_SETLK,
except that if a read or write lock is blocked by other locks,
the process sleeps until the segment is free to be locked.
.PP
Files and region locking are supported over the Network File
System (NFS) services if you have enabled the NFS locking
service.
.SH Return Values
.NXR "fcntl system call" "return value"
Upon successful completion, the value returned depends upon
the
.I request
argument
as follows:
.sp .5v
.nf
.ta .0i 1i
	F_DUPFD	A new file descriptor.
	F_GETFD	Value of flag (only the low-order bit is defined).
	F_GETFL	Value of flags.
	F_GETOWN	Value of file descriptor owner.
	other	Value other than \-1.
.fi
.sp .5v
Otherwise, a value of \-1 is returned and
.I errno
is set to indicate the error.
.SH Diagnostics
.NXR "fcntl system call" "diagnostics"
The
.PN fcntl
fails if under the following conditions: 
.TP 15
[EBADF]
The 
.I fildes
argument is not a valid open file descriptor.
.TP
[EBADF]
The environment defined is POSIX, the \fIrequest\fP argument is F_SETLK 
or F_SETLKW, the type of lock, \fIl_type\fP, is a shared lock (F_RDLCK),
and \fIfildes\fP is not a valid file descriptor open for reading, or the
type of lock, \fIl_type\fP, is an exclusive lock (F_WRLCK), and
\fIfildes\fP is not a valid file descriptor open for writing.
.TP 15
[EFAULT]
The 
.I arg
is pointing to an address outside the process's allocated space.
.TP 15
[EINVAL]
The
.I request
argument
is F_DUPFD, and
.I arg
is negative or greater than the maximum allowable number.
For further information, see 
.MS getdtablesize 2 .
.TP 15
[EINVAL]
The
.I request
argument
is F_SETSYN, to change the write mode of a file to
synchronous, and this operation is not valid 
for the file descriptor.  For example, the file
was opened for read-only operations.
.TP 15
[EINVAL]
The
.I request
argument is
F_GETLK,F_SETLK, or SETLKW and the data  
.I arg
points to is not valid.
.TP
[EINVAL] 
The
.I request
argument is invalid.
.TP
[EINVAL] 
The \fIfildes\fP argument refers to a file that does not support
locking.
.TP 15
[EACCES]
The
.I request
argument
is
F_SETLK, 
the type of lock (
.I l_type
)
is a read
(F_RDLCK) or write (F_WRLCK) lock, and the region 
of the
file to be locked is already write locked by another process.
Or, the type is a write lock and the region
of the file to be locked is already read or write
locked by another process.
Or,
the file is remotely mounted and the NFS locking service is not enabled.
.TP 15
[EMFILE]
The
.I request
argument is
F_DUPFD, and the maximum allowed number of file descriptors is currently
open, or no file descriptors greater than or equal to \fIarg\fP are
available.
.TP 15
[ENOSPC]
The
.I request
argument is
F_SETLK
or
F_SETLKW,
the type of lock is a read or write lock,
and there are no more file locking headers available
(too many files have segments locked).
Or, there are no more record locks available
(too many file segments locked).
.TP 15
[EDEADLK]
The
.I request
argument is
F_SETLKW,
and the lock is blocked by some lock from another process
that is sleeping (waiting) for that lock to become free.
This detection avoids a deadlock situation.
.TP 15
[EOPNOTSUPP]
Attempting an operation that is not valid for
the file descriptor.  This can occur if the file descriptor
argument, \fIfd\fR, points
to a socket address, and the
.I request
argument is only valid for files.
.TP
[EINTR]
The \fIrequest\fP argument is F_SETLKW and the function was
interrupted by a signal.
.SH Environment
The 
.PN fcntl
description differs from the POSIX and XPG3 definitions in that ENOLCK is
not a possible error condition.
.SH See Also
close(2), execve(2), getdtablesize(2), open(2), sigvec(2), lockd(8c)