xref: /freebsd/lib/libsys/lockf.3 (revision cc697d22c4a43d6d40744799695a65a1076bf3e8)

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.Dd September 11, 2013
.Dt LOCKF 3
.Os
.Sh NAME
.Nm lockf
.Nd record locking on files
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn lockf "int fd" "int function" "off_t size"
.Sh DESCRIPTION
The
.Fn lockf
function allows sections of a file to be locked with advisory-mode locks.
Calls to
.Fn lockf
from other processes which attempt to lock the locked file section will
either return an error value or block until the section becomes unlocked.
All the locks for a process are removed when the process terminates.
.Pp
The argument
.Fa fd
is an open file descriptor.
The file descriptor must have been opened either for write-only
.Dv ( O_WRONLY )
or read/write
.Dv ( O_RDWR )
operation.
.Pp
The
.Fa function
argument is a control value which specifies the action to be taken.
The permissible values for
.Fa function
are as follows:
.Bl -tag -width F_ULOCKXX -compact -offset indent
.It Sy Function
.Sy Description
.It Dv F_ULOCK
unlock locked sections
.It Dv F_LOCK
lock a section for exclusive use
.It Dv F_TLOCK
test and lock a section for exclusive use
.It Dv F_TEST
test a section for locks by other processes
.El
.Pp
.Dv F_ULOCK
removes locks from a section of the file;
.Dv F_LOCK
and
.Dv F_TLOCK
both lock a section of a file if the section is available;
.Dv F_TEST
detects if a lock by another process is present on the specified section.
.Pp
The
.Fa size
argument is the number of contiguous bytes to be locked or
unlocked.
The section to be locked or unlocked starts at the current
offset in the file and extends forward for a positive size or backward
for a negative size (the preceding bytes up to but not including the
current offset).
However, it is not permitted to lock a section that
starts or extends before the beginning of the file.
If
.Fa size
is 0, the section from the current offset through the largest possible
file offset is locked (that is, from the current offset through the
present or any future end-of-file).
.Pp
The sections locked with
.Dv F_LOCK
or
.Dv F_TLOCK
may, in whole or in part, contain or be contained by a previously
locked section for the same process.
When this occurs, or if adjacent
locked sections would occur, the sections are combined into a single
locked section.
If the request would cause the number of locks to
exceed a system-imposed limit, the request will fail.
.Pp
.Dv F_LOCK
and
.Dv F_TLOCK
requests differ only by the action taken if the section is not
available.
.Dv F_LOCK
blocks the calling process until the section is available.
.Dv F_TLOCK
makes the function fail if the section is already locked by another
process.
.Pp
File locks are released on first close by the locking process of any
file descriptor for the file.
.Pp
.Dv F_ULOCK
requests release (wholly or in part) one or more locked sections
controlled by the process.
Locked sections will be unlocked starting
at the current file offset through
.Fa size
bytes or to the end of file if size is 0.
When all of a locked section
is not released (that is, when the beginning or end of the area to be
unlocked falls within a locked section), the remaining portions of
that section are still locked by the process.
Releasing the center
portion of a locked section will cause the remaining locked beginning
and end portions to become two separate locked sections.
If the
request would cause the number of locks in the system to exceed a
system-imposed limit, the request will fail.
.Pp
An
.Dv F_ULOCK
request in which size is non-zero and the offset of the last byte of
the requested section is the maximum value for an object of type
off_t, when the process has an existing lock in which size is 0 and
which includes the last byte of the requested section, will be treated
as a request to unlock from the start of the requested section with a
size equal to 0.
Otherwise an
.Dv F_ULOCK
request will attempt to unlock only the requested section.
.Pp
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock the locked region of
another process.
This implementation detects that sleeping until a
locked region is unlocked would cause a deadlock and fails with an
.Er EDEADLK
error.
.Pp
The
.Fn lockf ,
.Xr fcntl 2 ,
and
.Xr flock 2
locks are compatible.
Processes using different locking interfaces can cooperate
over the same file safely.
However, only one of such interfaces should be used within
the same process.
If a file is locked by a process through
.Xr flock 2 ,
any record within the file will be seen as locked
from the viewpoint of another process using
.Xr fcntl 2
or
.Fn lockf ,
and vice versa.
.Pp
Blocking on a section is interrupted by any signal.
.Sh RETURN VALUES
.Rv -std lockf
In the case of a failure, existing locks are not changed.
.Sh ERRORS
The
.Fn lockf
function
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The argument
.Fa function
is
.Dv F_TLOCK
or
.Dv F_TEST
and the section is already locked by another process.
.It Bq Er EBADF
The argument
.Fa fd
is not a valid open file descriptor.
.Pp
The argument
.Fa function
is
.Dv F_LOCK
or
.Dv F_TLOCK ,
and
.Fa fd
is not a valid file descriptor open for writing.
.It Bq Er EDEADLK
The argument
.Fa function
is
.Dv F_LOCK
and a deadlock is detected.
.It Bq Er EINTR
The argument
.Fa function
is F_LOCK
and
.Fn lockf
was interrupted by the delivery of a signal.
.It Bq Er EINVAL
The argument
.Fa function
is not one of
.Dv F_ULOCK ,
.Dv F_LOCK ,
.Dv F_TLOCK
or
.Dv F_TEST .
.Pp
The argument
.Fa fd
refers to a file that does not support locking.
.It Bq Er ENOLCK
The argument
.Fa function
is
.Dv F_ULOCK ,
.Dv F_LOCK
or
.Dv F_TLOCK ,
and satisfying the lock or unlock request would result in the number
of locked regions in the system exceeding a system-imposed limit.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr flock 2
.Sh STANDARDS
The
.Fn lockf
function conforms to
.St -xpg4.2 .