NAME
lockf - record locking on files
SYNOPSIS
#include <unistd.h>
int lockf(int fildes, int function, off_t size);
DESCRIPTION
The lockf() function shall lock sections of a file with advisory-mode
locks. Calls to lockf() from other threads which attempt to lock the
locked file section shall either return an error value or block until
the section becomes unlocked. All the locks for a process are removed
when the process terminates. Record locking with lockf() shall be
supported for regular files and may be supported for other files.
The fildes argument is an open file descriptor. To establish a lock
with this function, the file descriptor shall be opened with write-only
permission (O_WRONLY) or with read/write permission (O_RDWR).
The function argument is a control value which specifies the action to
be taken. The permissible values for function are defined in <unistd.h>
as follows:
Function Description
F_ULOCK Unlock locked sections.
F_LOCK Lock a section for exclusive use.
F_TLOCK Test and lock a section for exclusive use.
F_TEST Test a section for locks by other processes.
F_TEST shall detect if a lock by another process is present on the
specified section.
F_LOCK and F_TLOCK shall both lock a section of a file if the section
is available.
F_ULOCK shall remove locks from a section of the file.
The 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). If size is 0, the section from the current offset
through the largest possible file offset shall be locked (that is, from
the current offset through the present or any future end-of-file). An
area need not be allocated to the file to be locked because locks may
exist past the end-of-file.
The sections locked with F_LOCK or 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 shall be combined into a single locked section. If the
request would cause the number of locks to exceed a system-imposed
limit, the request shall fail.
F_LOCK and F_TLOCK requests differ only by the action taken if the
section is not available. F_LOCK shall block the calling thread until
the section is available. F_TLOCK shall cause the function to fail if
the section is already locked by another process.
File locks shall be released on first close by the locking process of
any file descriptor for the file.
F_ULOCK requests may release (wholly or in part) one or more locked
sections controlled by the process. Locked sections shall be unlocked
starting at the current file offset through size bytes or to the end-
of-file if size is (off_t)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
shall remain locked by the process. Releasing the center portion of a
locked section shall 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 shall fail.
A potential for deadlock occurs if the threads of a process controlling
a locked section are blocked by accessing another process’ locked
section. If the system detects that deadlock would occur, lockf() shall
fail with an [EDEADLK] error.
The interaction between fcntl() and lockf() locks is unspecified.
Blocking on a section shall be interrupted by any signal.
An 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, shall be
treated as a request to unlock from the start of the requested section
with a size equal to 0. Otherwise, an F_ULOCK request shall attempt to
unlock only the requested section.
Attempting to lock a section of a file that is associated with a
buffered stream produces unspecified results.
RETURN VALUE
Upon successful completion, lockf() shall return 0. Otherwise, it shall
return -1, set errno to indicate an error, and existing locks shall not
be changed.
ERRORS
The lockf() function shall fail if:
EBADF The fildes argument is not a valid open file descriptor; or
function is F_LOCK or F_TLOCK and fildes is not a valid file
descriptor open for writing.
EACCES or EAGAIN
The function argument is F_TLOCK or F_TEST and the section is
already locked by another process.
EDEADLK
The function argument is F_LOCK and a deadlock is detected.
EINTR A signal was caught during execution of the function.
EINVAL The function argument is not one of F_LOCK, F_TLOCK, F_TEST, or
F_ULOCK; or size plus the current file offset is less than 0.
EOVERFLOW
The offset of the first, or if size is not 0 then the last, byte
in the requested section cannot be represented correctly in an
object of type off_t.
The lockf() function may fail if:
EAGAIN The function argument is F_LOCK or F_TLOCK and the file is
mapped with mmap().
EDEADLK or ENOLCK
The function argument is F_LOCK, F_TLOCK, or F_ULOCK, and the
request would cause the number of locks to exceed a system-
imposed limit.
EOPNOTSUPP or EINVAL
The implementation does not support the locking of files of the
type indicated by the fildes argument.
The following sections are informative.
EXAMPLES
Locking a Portion of a File
In the following example, a file named /home/cnd/mod1 is being
modified. Other processes that use locking are prevented from changing
it during this process. Only the first 10000 bytes are locked, and the
lock call fails if another process has any part of this area locked
already.
#include <fcntl.h>
#include <unistd.h>
int fildes;
int status;
...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
APPLICATION USAGE
Record-locking should not be used in combination with the fopen(),
fread(), fwrite(), and other stdio functions. Instead, the more
primitive, non-buffered functions (such as open()) should be used.
Unexpected results may occur in processes that do buffering in the user
address space. The process may later read/write data which is/was
locked. The stdio functions are the most common source of unexpected
buffering.
The alarm() function may be used to provide a timeout facility in
applications requiring it.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
alarm() , chmod() , close() , creat() , fcntl() , fopen() , mmap() ,
open() , read() , write() , the Base Definitions volume of
IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .